Ohm's Law & Power Wheel S1
Interactive Power Wheel — fill any 2 of 4
Type a value into ANY two boxes (V, I, R, or P) at the wheel's center. The other two auto-solve. Used formula highlights gold on the wheel.
POWER WHEEL · 12 FORMULAS · 4 VARIABLES
V
I (A)
P (W)
R (Ω)
KNOWN
SOLVED
FORMULA USED
Enter any 2 values to auto-solve.
HOW TO FIGURE IT - METHODOLOGY
The Power Wheel - 12 Formulas
Every Ohm's Law problem reduces to one of these. Memorize the wheel, never look up another formula.
V = I×R :: V = P/I :: V = √(P×R)
I = V/R :: I = P/V :: I = √(P/R)
R = V/I :: R = V²/P :: R = P/I²
P = V×I :: P = I²×R :: P = V²/R
Walk-Through (the "style of figuring it out")
- List what you know. Write down the two given values with units.
- List what you need. Identify the unknown(s).
- Pick the formula from the wheel that takes ONLY your known inputs and produces the unknown directly. Don't chain formulas if one will do it.
- Plug numbers, don't lose units. Volts in volts, amps in amps, mA→A by dividing 1000.
- Sanity check the answer. Order of magnitude right? Is power positive? Is current under the OCPD rating?
Example - sizing a 24VDC alarm circuit: 24V supply, 0.250A load. R = V/I = 24/0.25 = 96Ω. P = V×I = 24×0.25 = 6W. Pick a power supply with at least 6W (typically size 25-30% over: 8W min).
Common gotchas: Confusing mA with A (380mA = 0.380A). Using line-to-line voltage on a single-leg load. Forgetting that resistance is temperature-dependent (copper rises ~0.4%/°C). For AC loads with reactance, use impedance Z, not pure R.
NEC / Industry Tie-Ins
- NEC 220 - load calculations roll up from individual P=V×I values.
- NFPA 72 - alarm/standby current per device feeds Battery Sizing tab.
- Power supply continuous-load rating: NEC 210.20(A) - size at 125% of P/V.
Power Wheel cheat: P = V×I = I²×R = V²/R. Use whichever pair of inputs you have. For sizing power supplies, fuses, and battery loads, this is the entry point - everything else is downstream of it.
Series & Parallel Resistance S1
Configuration
Add resistor values. Pick SERIES (sum) or PARALLEL (reciprocal sum). Useful for EOL networks, motion detector tamper loops, balanced supervisory circuits, multi-strike door locks on one supply.
HOW TO FIGURE IT - METHODOLOGY
Series - same current, voltages add
Rt = R1 + R2 + R3 + ...
Current flows through each resistor sequentially. Drop the supply through each in proportion to its R value.
Parallel - same voltage, currents add
1/Rt = 1/R1 + 1/R2 + 1/R3 + ...
For two only: Rt = (R1 × R2) / (R1 + R2)
Walk-Through
- Identify the topology - is current splitting (parallel) or flowing through each sequentially (series)?
- Series: just add. Three 1k resistors in series = 3k.
- Parallel: total R is ALWAYS less than the smallest individual R. Three 1k in parallel = 333Ω.
- Mixed networks: collapse from inside out. Solve innermost parallel groups first to a single equivalent R, then add the surrounding series.
Example - dual EOL on a tamper loop: Two 5.6k EOLs in parallel = (5600×5600)/(5600+5600) = 2,800Ω. Panel sees 2.8k. If the loop opens, panel sees one 5.6k = trouble. If shorted, 0Ω = alarm.
Gotcha: Mag locks wired in parallel share voltage but stack current. Two 500mA locks in parallel = 1A draw. Don't undersize the supply or transformer.
Field tip: EOL supervision relies on a known total R value. Adding/removing devices in series shifts that value - always recalc before commissioning.
Watts / VA / Power Factor S1
Convert Between W, VA, and PF
Resistive loads (heaters, incandescents): W = VA. Inductive loads (motors, transformers, locks): W < VA. Power factor (PF) = W/VA. Sizing UPS, transformers, generators always uses VA, not W.
HOW TO FIGURE IT - METHODOLOGY
The Three Formulas
W = VA × PF
VA = W / PF
PF = W / VA
Typical PF Values to Memorize
- Resistive (heaters, halogen): 1.00
- LED drivers (good): 0.90 - 0.95
- Switch-mode supplies (older): 0.65 - 0.75
- Mag locks / strikes / transformers: 0.70 - 0.85
- Single-phase induction motors: 0.65 - 0.85
Walk-Through
- Read the nameplate. If it lists Watts AND Amps, divide W/(V×A) = PF.
- If only Watts: assume worst-case PF for that load type (mag lock = 0.75).
- Sum VA, not W, when sizing transformers and UPSs.
- Generators are sized in kW AND kVA - both must be checked against load.
Example - sizing a UPS for a head-end: NVR = 150W, switch = 60W, FACP = 80W. PF assumed 0.85. VA = (150+60+80)/0.85 = 341 VA. Pick a 500 VA UPS minimum (50% derate for runtime).
Gotcha: Inrush. Mag locks and motors pull 3-7x rated VA at energization. UPS and transformer must handle the surge, not just steady-state.
Why this matters: Order a 1000W UPS for a 1000VA load with 0.7 PF and you'll trip it on day one. Always size the apparent power (VA), not the real power (W).
IPv4 Subnet / CIDR S1
Subnet Resolver
Enter any host IP and a CIDR (/8 to /30). Returns network address, broadcast, usable host range, mask, and host count. Foundation for camera VLAN design and access-control IP planning.
HOW TO FIGURE IT - METHODOLOGY
The Concept
An IPv4 address is 32 bits. CIDR /N means "the first N bits are the network, the rest are hosts." /24 = 24 network bits + 8 host bits = 256 addresses (254 usable, minus network and broadcast).
The Math (binary, fast version)
- Mask = N ones followed by (32-N) zeros, dotted-decimal form. /24 = 11111111.11111111.11111111.00000000 = 255.255.255.0.
- Network address = IP AND mask (bitwise).
- Broadcast = network OR (NOT mask).
- Usable hosts = 2^(32-N) - 2.
Quick Reference
| CIDR | Mask | Hosts | Use |
|---|---|---|---|
| /30 | 255.255.255.252 | 2 | Point-to-point links |
| /29 | 255.255.255.248 | 6 | Tiny LAN, 1 access panel |
| /28 | 255.255.255.240 | 14 | Small camera VLAN |
| /27 | 255.255.255.224 | 30 | Medium camera VLAN |
| /26 | 255.255.255.192 | 62 | Branch office |
| /24 | 255.255.255.0 | 254 | Standard LAN |
| /23 | 255.255.254.0 | 510 | Large camera campus |
| /22 | 255.255.252.0 | 1022 | Enterprise |
Example - 192.168.10.45/24: Network 192.168.10.0, broadcast 192.168.10.255, range .1 - .254, mask 255.255.255.0, 254 usable hosts.
Gotcha: Cameras on the same VLAN as workstations is a security violation per most ISO 27001 / NIST baselines. Always carve a dedicated /24 (or smaller) for cameras and ACLs blocking workstation access.
Common ranges to memorize: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 (RFC1918 private). Don't NAT or route these to the internet without a firewall.
Wire Ampacity Lookup S1
Ampacity by Wire Size - 75°C Column
NEC Table 310.16, copper conductors at 75°C, not more than 3 current-carrying conductors in raceway, ambient ≤30°C.
| AWG | CU 75°C | AL 75°C | Common Use |
|---|---|---|---|
| 14 | 20A* | - | Lighting circuits |
| 12 | 25A* | - | 20A receptacles, general |
| 10 | 35A* | 25A* | 30A appliances, dryers |
| 8 | 50A | 40A | 40A range circuits |
| 6 | 65A | 50A | 50A subfeed, AC units |
| 4 | 85A | 65A | 70A subfeed |
| 2 | 115A | 90A | 100A subfeed |
| 1 | 130A | 100A | - |
| 1/0 | 150A | 120A | 150A service |
| 2/0 | 175A | 135A | - |
| 3/0 | 200A | 155A | 200A service |
| 4/0 | 230A | 180A | - |
| 250 kcmil | 255A | 205A | - |
| 350 kcmil | 310A | 250A | 300A service |
| 500 kcmil | 380A | 310A | 400A service |
HOW TO FIGURE IT - METHODOLOGY
The Process
- Calculate the load. Continuous load (3+ hr) is sized at 125% per NEC 210.20(A). 16A continuous -> 20A circuit minimum.
- Look up ampacity in 310.16 75°C column for the wire material (CU vs AL).
- Apply derates for ambient temp (Table 310.15(B)(1)) and number of CCCs (Table 310.15(C)(1)).
- Check OCPD limit per 240.4(D): 14 AWG = 15A max, 12 AWG = 20A max, 10 AWG = 30A max regardless of ampacity.
Derate Formulas
Adjusted ampacity = Table ampacity × ambient_factor × CCC_factor
Example: 12 AWG CU in 105°F (40°C) attic with 6 CCCs. Base 25A × 0.88 (40°C derate) × 0.80 (4-6 CCC) = 17.6A. OCPD limited to 15A or 20A per 240.4(D); use 15A breaker since derated ampacity < 20A.
Asterisked sizes (14/12/10): NEC 240.4(D) restricts max OCPD: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A even though raw ampacity is higher. Don't get clever.
* Asterisked sizes: NEC 240.4(D) restricts to maximum OCPD: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A, even though ampacity is higher.
Derate triggers: Adjustment factors apply for >3 CCCs in raceway and ambients >30°C.
Derate triggers: Adjustment factors apply for >3 CCCs in raceway and ambients >30°C.
Voltage Drop - Single Phase S2
VD = (2 × K × I × L) / CMA
Single-phase 2-wire drop. Branch circuit limit 3%, total feeder+branch 5% per NEC 210.19(A) Informational Note 4.
HOW TO FIGURE IT - METHODOLOGY
The Formula
VD = (2 × K × I × L) / CMA
K= 12.9 for copper, 21.2 for aluminum (resistivity in ohm-CM/ft)I= current in ampsL= ONE-WAY length in feet (the ×2 in the formula handles round-trip)CMA= circular mil area of the conductor
Walk-Through
- Measure or estimate one-way run length.
- Get continuous load amperage (or 125% of nameplate per 210.20).
- Pick K based on material - aluminum drops ~64% more voltage than copper at same size.
- Calculate VD. Divide by source V for percentage.
- Compare to limit - 3% branch, 5% total. Over -> upsize wire.
Example: 120V circuit, 15A continuous, 200ft one-way, 12 AWG CU. VD = (2×12.9×15×200)/6530 = 11.85V drop = 9.9%. FAIL. Upsize to 8 AWG: VD = (2×12.9×15×200)/16510 = 4.69V = 3.9%. Still over branch limit. Need 6 AWG: VD = 2.95V = 2.5%. PASS.
Gotcha: "Length" is one-way, not round-trip. The ×2 in the formula already accounts for return path. Doubling your length input gives you the wrong (overstated) drop.
NEC limits: 3% on branch circuits, 5% total (feeder + branch) per 210.19(A) Informational Note 4 and 215.2(A) Informational Note 2. Not enforceable, but performance/code-officials reference it.
Voltage Drop - 3 Phase S2
VD = (√3 × K × I × L) / CMA
3-phase balanced load. Use line-to-line voltage. The √3 (1.732) factor replaces the 2 in single-phase, and length is one-way.
HOW TO FIGURE IT - METHODOLOGY
The Formula
VD = (1.732 × K × I × L) / CMA
The 1.732 (√3) factor accounts for the phase relationship in a balanced 3-phase system. You don't double for return path because each phase serves as the others' return.
When to Use Which
- Single-phase 1F or 1F/3W: Use VD1 tab. Factor of 2.
- 3-phase delta or wye, balanced load: This tab. Factor of 1.732.
- 3-phase to single-phase L-N load (lighting): Use VD1 with the L-N voltage (208/√3 = 120V or 480/√3 = 277V).
Example: 480V 3F, 100A motor, 500ft, 4 AWG CU. VD = (1.732×12.9×100×500)/41740 = 26.77V = 5.6%. Marginal. 2 AWG: VD = (1.732×12.9×100×500)/66360 = 16.83V = 3.5%. PASS.
Gotcha: Mixing line voltages. 480V 3F has 277V L-N. 208V 3F has 120V L-N. Using L-N voltage in this calc gives wrong drop %.
3-phase efficiency: For the same power, 3F draws less current than 1F at the same voltage, so wire can typically be smaller. Always run the calc - don't assume.
NAC Voltage Drop S2
End-of-Line Voltage on Notification Circuit
Sum the alarm current of all NAC devices. Calculate drop to last device. Compare end-of-line V to the device's minimum operating voltage (UL 1971 listing).
HOW TO FIGURE IT - METHODOLOGY
Worst-Case-First Method
NFPA 72 requires the LAST device on the circuit to operate within its UL listing at the END-OF-LINE voltage. The trick is calculating that worst-case point.
Walk-Through
- Add up all alarm currents on the circuit. Use ALARM, not standby.
- Determine the run length to the LAST device.
- Calculate VD using single-phase formula: VD = (2 × 12.9 × I × L) / CMA. Copper only - NAC wire is always copper.
- EOL Voltage = Source Voltage - VD.
- Compare EOL to the device's UL minimum (typically 16V for 24V devices, 10.2V for 12V).
Better Method - Per-Segment Drop
For accurate calc on a long T-tap circuit: walk segment by segment, accumulating current at each branch and summing the drop on each leg. The simple formula here assumes lumped load at the end (worst case - over-estimates drop, safe).
Example: 24V FACP, 0.450A total alarm, 250ft, 14 AWG CU, min op 16.0V. VD = (2×12.9×0.45×250)/4110 = 2,902.5/4110 = 0.71V drop. EOL = 24 - 0.71 = 23.29V. PASS by 7.29V margin. (Worst-case using regulated 20.4V FACP terminal: EOL = 20.4 - 0.71 = 19.69V, still PASS by 3.69V.)
Gotcha: Manufacturers' "regulated" 24V FACP is actually 20.4V minimum at the panel terminals during a battery-backup event. Use 20.4V (not 24V) as your starting point for worst-case design.
UL minimums (24V devices): Most strobes/horns 16.0V. Sync modules sometimes 16.5V. CHECK the spec sheet for each model. 12V devices: Typically 10.2V min, 13.5V max.
Battery Sizing - Fire Alarm S2
Standby + Alarm Capacity (with NFPA Derate)
Sum standby Ah (current × standby hours) and alarm Ah (current × alarm minutes/60). Multiply by 1.20 derate per NFPA 72 to account for end-of-life capacity loss.
HOW TO FIGURE IT - METHODOLOGY
The NFPA 72 Equation
Required Ah = [(I_standby × H_standby) + (I_alarm × M_alarm/60)] × 1.20
Standby Hours
- 24 hours - protected premises, central station supervised
- 60 hours - residential / unsupervised systems
- 72 hours - some AHJ requires for high-rise / emergency communication
Alarm Minutes
- 5 minutes - standard alarm signaling
- 15 minutes - voice evac / mass notification
Walk-Through
- Sum total standby current of every panel-powered device (use FACP Load tab).
- Sum total alarm current including all NAC appliances.
- Multiply current by time, convert min to hours.
- Add standby Ah + alarm Ah.
- Multiply by 1.20 derate.
- Round UP to next standard battery size: 7, 12, 18, 26, 33, 55, 75, 100 Ah.
Example: 0.200A standby × 24h = 4.80 Ah. 2.5A alarm × (5/60)h = 0.208 Ah. Subtotal = 5.008 Ah. ×1.20 = 6.01 Ah. Round to 7 Ah battery.
Gotcha: Two parallel batteries DO add Ah, but they must be the same age, same brand, same model. Don't mix old + new - the new one will discharge into the old one. Replace as a matched pair.
Standby duration map: Most protected premises = 24h. Residential = 60h. Always verify with AHJ. Alarm duration: 5 min standard, 15 min for voice evac.
Fire Alarm Power Load S2
Total Standby + Alarm Current
Sum standby and alarm draw of all FACP-powered devices. Use mfr spec sheet "Standby Current" and "Alarm Current" values.
HOW TO FIGURE IT - METHODOLOGY
The Process
- List EVERY device the FACP powers - smoke detectors, heat detectors, pull stations, isolators, modules, addressable interface units, and ALL notification appliances.
- Pull the "Standby Current" and "Alarm Current" values from each spec sheet (almost always in mA).
- Multiply each by quantity.
- Sum standby column - this is total continuous draw on battery during normal operation.
- Sum alarm column - this is the surge during an alarm event.
- Feed both totals into Battery Sizing tab.
Where Standby vs Alarm Differ
- Smoke detectors: standby = quiescent draw (often microamps). Alarm = sounder/LED active = milliamps.
- Strobes: standby = ~0mA (no draw until activated). Alarm = full rated draw (60-180mA per strobe at 24V).
- Modules/relays: most have continuous standby and small alarm increase.
Gotcha: Spec sheets often give "alarm" current as the appliance current draw during signaling, NOT including the panel's coordinator/driver overhead. Some panels add 30-50mA per NAC card just for the supervision circuit. Read the panel installation manual.
Output feeds Battery Sizing: Total standby A and total alarm A from this calc plug into the Battery tab to compute required Ah.
PoE Budget Calculator S2
Switch PoE Budget vs Connected Loads
Verify your switch's PoE budget can power all connected devices. PoE has cable loss - the device sees less wattage than the switch ports deliver.
HOW TO FIGURE IT - METHODOLOGY
The Two-Layer Problem
PoE has TWO checks: (1) does total PD power fit within the switch's budget, and (2) does each individual port deliver enough watts after cable loss to its device.
Standards Reference
| Standard | Port W | PD W | Common Use |
|---|---|---|---|
| 802.3af | 15.4 | 12.95 | VoIP phones, basic IP cams |
| 802.3at (PoE+) | 30 | 25.5 | PTZ, dual-radio APs |
| 802.3bt T3 (PoE++) | 60 | 51 | Heated cams, displays |
| 802.3bt T4 (PoE++) | 90 | 71 | Wi-Fi 6E APs, IP turrets w/heat |
Cable Loss
Loss (W) = I² × R_cable, where R_cable depends on cat type and length
Cat5e at 100m loses ~16% of power. Cat6 ~10%. The standards already factor "worst case" loss into the PD wattage rating.
Walk-Through
- Read the switch spec - "PoE Budget" or "Total PoE Power" (typically 60-740W on enterprise gear).
- List every PD with its actual wattage (NOT the standard's max - the camera might be 7W af, not 12.95W).
- Sum required wattage at PD level.
- If sum > budget × 0.9 (10% margin) -> swap for higher-budget switch or split across two switches.
Example: 24-port af switch with 200W budget. 18 cameras × 7W each = 126W. PASS. Add 4 PTZ cameras (at, 22W each) = 88W more. New total 214W > 200W. FAIL - need higher-budget switch.
Gotcha: Switch budgets are spec'd at 50°C ambient. In a hot closet (40°C+), the switch may auto-derate PoE budget. Check the temperature curves in the datasheet.
Cable matters: Use Cat5e minimum, Cat6 for runs over 50m. Don't run PoE+ on stranded patch cable - solid conductor only for the long-run portion.
Camera Storage Calculator S2
Total Storage Required
Estimate raw storage needed given camera count, average bitrate, recording hours/day, and retention days. Add 10-15% for indexing/overhead.
HOW TO FIGURE IT - METHODOLOGY
The Formula
TB = (Cams × Mbps × 3600 s/hr × hr/day × days) / (8 bits/byte × 10^12) × margin
Simplified mental math:
TB ≈ (Cams × Mbps × hr × days) / 2,222 × margin
Typical Bitrates by Codec / Resolution
| Resolution | H.264 Mbps | H.265 Mbps |
|---|---|---|
| 720p @ 15fps | 1.5 - 2.5 | 0.7 - 1.2 |
| 1080p @ 15fps | 2.5 - 4.0 | 1.2 - 2.0 |
| 1080p @ 30fps | 4.0 - 6.0 | 2.0 - 3.0 |
| 4MP @ 15fps | 5.0 - 7.0 | 2.5 - 3.5 |
| 4K (8MP) @ 15fps | 10 - 14 | 5 - 8 |
Walk-Through
- Pick bitrate per camera based on codec, resolution, frame rate, and scene complexity.
- Decide hours per day - 24/7 vs business hours, motion-only.
- Set retention based on AHJ / contract requirement (30 days standard, 90 for some, 1 yr for cannabis/casino).
- Add overhead margin.
- Spec drives - RAID 5 means usable = (N-1) × drive size.
Example: 16 cams, 4Mbps avg, 24h, 30 days retention, 10% overhead. Storage = (16 × 4 × 24 × 30)/2,222 × 1.10 = 22.81 TB. Spec a 24TB NVR or 4×8TB RAID 5 = 24TB usable. (Or use motion-only recording / lower bitrate to reduce.)
Gotcha: Variable bitrate (VBR) cameras spike to 2-3x the average during motion. If the retention spec is "no gaps," size for peak, not average. Or use motion-only recording.
Smart codec features (Hikvision H.265+, Dahua Smart Codec, Axis Zipstream) reduce bitrate 30-50% on low-motion scenes. Useful but verify worst-case in busy scenes.
Camera Bandwidth Calculator S2
Total Network Bandwidth
Sum of all camera bitrates plus overhead. Used for switch uplink sizing, WAN/cloud ingestion, and remote-view planning.
HOW TO FIGURE IT - METHODOLOGY
The Formula
Total Mbps = Cams × Bitrate × viewers_factor + overhead
Viewer Multipliers
- Local record only: 1.0x - cameras to NVR.
- 1 remote viewer of all cams: 2.0x - cam->NVR + NVR->viewer.
- 3 remote viewers: 4.0x - one record stream + three view streams.
- Cloud archive (all cams): 1.0x upload bandwidth, sustained.
Walk-Through
- Use same bitrate input as Camera Storage tab.
- Multiply by camera count = LAN bandwidth at NVR.
- Apply viewer multiplier for remote-view scenarios.
- Add 20% protocol overhead (TCP/IP headers, RTSP keepalives).
- Compare to:
- Switch backplane and uplink (often 1Gb/10Gb).
- Internet upload speed for cloud / remote view.
Example: 16 cams × 4Mbps × 1.0 + 20% = 76.8 Mbps. LAN-only fine on 1Gb. For cloud archive, need 80+ Mbps SUSTAINED upload, which most ISPs sell as a 100/100 fiber plan minimum.
Gotcha: Asymmetric ISP plans (cable, DSL) advertise download speed. Upload is usually 1/10th of download. A "100Mbps" cable plan might be 100/10. Cloud archive will not work.
Switch sizing: Each Gb-port handles ~960 Mbps usable. For 50+ cameras, plan a 10Gb uplink to the NVR or cluster cams across multiple NVRs.
Lock / Transformer Sizing S2
Power Supply / Transformer VA Sizing
Compute total current draw across all locks/strikes, then size the supply at 125% (continuous) or 25% over for inrush headroom.
HOW TO FIGURE IT - METHODOLOGY
The Formula
Required Supply Current = Sum of all device currents × headroom factor
Required VA = Volts × Required Current
Typical Device Currents
| Device | Holding I | Inrush |
|---|---|---|
| 600# mag lock @ 12V | 500 mA | 2.5A (5x) |
| 600# mag lock @ 24V | 250 mA | 1.2A |
| 1200# mag lock @ 24V | 500 mA | 2.5A |
| HES 1006 strike @ 12V | 200 mA | 300 mA |
| HES 5000 strike @ 12V | 150 mA | 250 mA |
| RCI 0162 strike @ 24V | 220 mA | 250 mA |
| Schlage AD-300 @ 12V | 400 mA | 800 mA |
Walk-Through
- List every lock/strike/relay on this supply.
- Sum holding currents (continuous condition).
- Multiply by headroom: 125% for plain mag locks, 150-200% if startup transient or high inrush.
- Multiply by voltage to get VA. Pick supply rated ≥ that.
- Verify supply has FAIL-SAFE behavior - mag locks fail OPEN on power loss (life safety).
Example: Three 600# mag locks @ 24V = 250+250+250 = 750mA continuous. 125% = 0.94A. Need supply rated ≥ 1A @ 24V (24VA). Pick 2A supply (48VA) for headroom.
Gotcha #1: AC vs DC. Many older transformers are 24VAC for strikes. Mag locks are almost always 12 or 24VDC. Don't mix - DC mag lock on AC supply will buzz, vibrate, and burn out.
Gotcha #2: Wire run length adds resistance. A 100ft run of 18AWG to a 500mA mag lock drops the voltage at the lock. Verify with VD calc.
Life safety: Mag locks must fail OPEN on power loss, fire alarm signal, and request-to-exit. Always wire the FACP relay into the supply's enable input, not just the lock.
EOL Resistor Reference S2
EOL Sizing & Loop Verification
EOL resistor establishes the "normal" supervisory current on a zone. Pick value per panel manufacturer. Calc verifies loop voltage at the EOL is within panel's normal/alarm threshold.
HOW TO FIGURE IT - METHODOLOGY
The Concept
A supervised loop has known current at "normal" (loop closed through EOL). Panel measures current and decides: NORMAL / OPEN (trouble) / SHORT (alarm).
Loop Current Math (Ohm's Law)
I_normal = V_loop / R_EOL
For a 2.2k EOL on a 13.8V loop: I = 13.8/2200 = 6.27mA. Panel expects this. If it sees 0mA -> OPEN trouble. If >15mA -> SHORT alarm.
Walk-Through for Field Verification
- Disconnect zone wire from panel.
- Measure resistance at panel terminals across the zone wires.
- Reading should be EOL value (within 5% accounts for wire R).
- If reading is 0 - short. If reading is OL/infinite - open. If reading is 2x EOL - look for two devices in series instead of parallel.
Series vs Parallel Devices
- Normally-closed (NC) contacts wire in SERIES with the EOL. Door/window contacts, motion tampers.
- Normally-open (NO) contacts wire in PARALLEL with the EOL (across it). Smoke detectors, glass break. When activated, NO closes -> shorts out the EOL -> alarm.
Example: 5 NC door contacts in series with 4.7k EOL on 13.8V loop. Loop current = 13.8/4700 = 2.94mA. Open any door -> series broken -> 0mA -> trouble (or alarm depending on panel zone definition).
Gotcha: "Dual EOL" or "End-of-Line and Tamper" loops use TWO resistors to distinguish ALARM from TAMPER from TROUBLE. Most modern panels use 5.6k + 5.6k (or 1k + 1k). DON'T mix values.
EOL placement: Always at the LAST device on the loop, not at the panel. EOL at panel only supervises the panel-to-first-device wire, defeating the purpose.
Wire Size Selector S3
Min Wire Size for Target Drop
Given a max acceptable voltage drop, load, and length - return the SMALLEST wire that meets it. Inverse of the VD calc.
HOW TO FIGURE IT - METHODOLOGY
Inverse Formula
CMA_min = (factor × K × I × L) / VD_max
Where VD_max = source_V × (target_pct/100). Then walk up the AWG table to find the smallest wire whose CMA ≥ CMA_min.
Walk-Through
- Compute target VD in volts: V × pct/100.
- Compute required CMA from inverse formula.
- Find smallest standard size at or above that CMA.
- Verify result by running forward VD calc - should be at or under target %.
- Check NEC 240.4(D) OCPD limit on the chosen size.
- Check ampacity at chosen size also covers continuous load × 125%.
Example: 120V, 12A continuous, 300ft, target 3%, single-phase, CU. VD_max = 120 × 0.03 = 3.6V. CMA_min = (2 × 12.9 × 12 × 300)/3.6 = 25,800 CMA. Closest size at or above: 6 AWG (26,240 CMA). Pick 6 AWG.
Gotcha: Wire size selector returns the smallest wire for VD only. You still must verify ampacity (310.16) and OCPD (240.4(D)) separately. Sometimes ampacity drives a larger size than VD does.
Cable Distance Limits S3
Max Distance by Cable / Protocol
Quick reference for max run lengths before signal degradation, repeater, or media converter is required.
| Cable / Protocol | Max Distance | Notes |
|---|---|---|
| Cat5e - 10/100/1000BASE-T | 100 m / 328 ft | Includes patch cords. Solid for run, stranded for patch. |
| Cat6 - 10GBASE-T | 55 m / 180 ft | Cat6A required for full 100m at 10G. |
| Cat6A - 10GBASE-T | 100 m / 328 ft | Standard for new IDF buildouts. |
| RG-59 baseband video | 228 m / 750 ft | Composite/analog only. Avoid for HD. |
| RG-6 baseband video | 305 m / 1000 ft | Better than RG-59 for distance. |
| HD-SDI on RG-6 | 100 m / 330 ft | HD-SDI degrades fast vs analog. |
| HD over Coax (HDCVI/TVI/AHD) | 300-500 m | Up to 500m with quality RG-59. |
| RS-485 (alarm bus) | 1200 m / 4000 ft | Daisy-chain, 120Ω termination. |
| RS-232 | 15 m / 50 ft | Legacy. Use RS-485 if longer. |
| USB 2.0 | 5 m / 16 ft | Active extender beyond. |
| Multimode fiber OM3 (10G) | 300 m | 10GBASE-SR. |
| Multimode fiber OM4 (10G) | 400 m | 10GBASE-SR. |
| Singlemode OS2 (10G) | 10 km | 10GBASE-LR. Up to 40km with LX optics. |
| Wiegand (access control) | 152 m / 500 ft | Per HID spec, shielded 22 AWG. |
| OSDP (RS-485 access) | 1200 m / 4000 ft | Replacing Wiegand for security. |
| SLC fire alarm bus | 2-3 km | Mfr-specific. Verify with panel manual. |
HOW TO FIGURE IT - METHODOLOGY
Why These Limits Exist
Three things kill signal at distance: (1) attenuation - signal weakens with length, (2) capacitance - high-freq signals slew slower on long cables, (3) reflection - impedance mismatches cause echoes that confuse the receiver.
Walk-Through
- Identify the cable type and protocol.
- Compare planned run length to max in the table.
- If at or near limit -> upsize cable or insert media converter / repeater.
- For PoE on Ethernet, the 100m limit is HARD - power and data both fail at the same point.
Gotcha: Cable run length is the WIRE distance, not the floor distance. Verticals, raceway loops, and slack add 10-30%. Always pull a tape on the actual route or estimate generously.
Beyond Cat6 limits: Use fiber. Don't try to "boost" 10G over copper past spec. The error rate spikes and intermittent failures are nearly impossible to troubleshoot in the field.
Conduit Fill S3
Conduit Fill Calculator
NEC Table 1 fill limits: 53% for 1 conductor, 31% for 2, 40% for 3+. Pick conduit type, size, conductor type, AWG, and quantity.
HOW TO FIGURE IT - METHODOLOGY
NEC Chapter 9 Tables
- Table 1 - fill percentage limits: 53% (1 conductor), 31% (2 conductors), 40% (3+).
- Table 4 - interior dimensions of conduit (sq in).
- Table 5 - approx area of insulated conductors (sq in by AWG and insulation type).
Walk-Through
- Look up conductor area from Table 5 for chosen wire type / AWG.
- Multiply by quantity = total conductor area.
- Look up conduit interior area from Table 4.
- Multiply by fill percentage from Table 1.
- If total conductor area ≤ allowed fill area: PASS.
- Mixed wire types/sizes: sum each individual conductor's area.
Example: 6 #12 THHN in 1/2" EMT. Conductor area = 6 × 0.0133 = 0.0798 sq in. EMT 1/2" 40% area = 0.122 sq in. PASS - 65% used of allowed.
Gotcha: Equipment grounding conductor (EGC) counts as a conductor for fill, even though it's not current-carrying. Don't forget it.
Fuse / Breaker Sizing S3
OCPD Sizing for Continuous Load
NEC 210.20(A) and 215.3 require overcurrent protection sized at 125% of continuous load (3+ hours of operation) plus 100% of non-continuous load.
HOW TO FIGURE IT - METHODOLOGY
The Formula
OCPD min = (Continuous × 1.25) + (Non-continuous × 1.00)
Standard Breaker / Fuse Sizes (NEC 240.6)
15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000.
Walk-Through
- Identify continuous load (3+ hours) - lighting, electric heat, large motors during warm-up.
- Identify non-continuous - intermittent process loads, occasional appliance use.
- Compute formula above.
- Round UP to next standard size (240.6).
- Verify wire ampacity at 75°C column meets or exceeds OCPD value (do not undersize wire to OCPD).
What Counts as Continuous?
- Lighting in offices, retail, warehouses (always 3+ hr).
- Electric water heaters.
- Display freezers/coolers.
- Industrial process motors running continuously.
Example: Bank of LED panels, 16A continuous + 4A non-continuous receptacles on same circuit. OCPD min = 16×1.25 + 4×1.00 = 24A. Round up to 25A or 30A breaker (240.6 standard sizes).
Gotcha: Branch circuits supplying continuous loads cannot exceed 80% of the OCPD's amp rating (inverse of the 125% rule). 20A breaker = max 16A continuous load.
NEC 210.19(A)(1): Conductor ampacity must also be sized at 125% of continuous load. Wire and OCPD both subject to the same 125% factor.
Heat Load (BTU) S4
BTU/hr from Equipment Wattage
Sum total power dissipation of all equipment in the closet. Convert to BTU/hr to size cooling. AC units are rated in BTU/hr (12,000 BTU/hr = 1 ton).
HOW TO FIGURE IT - METHODOLOGY
The Formula
BTU/hr = Watts × 3.412
Cooling Tons = BTU/hr / 12,000
Walk-Through
- List all equipment - switches, NVRs, FACPs, access controllers, UPS losses.
- Get wattage from datasheet OR from "max power consumption" or "thermal output."
- Sum total wattage.
- Multiply by 3.412 to get BTU/hr.
- Add 20% margin for solar gain, occupancy, future expansion.
- Spec AC unit rated for that BTU/hr at the closet's design temperature.
Typical Equipment Wattage
| Equipment | Typical W |
|---|---|
| 24-port managed switch (no PoE) | 25-50 |
| 24-port PoE+ switch (loaded) | 300-450 |
| 1U server / NVR | 150-300 |
| FACP | 30-80 |
| Access controller (loaded) | 15-30 |
| 1500VA UPS (efficiency loss) | 75-150 |
Example: Closet with 1 PoE+ switch (400W) + NVR (250W) + UPS losses (100W) = 750W. BTU/hr = 750 × 3.412 = 2,559. With 20% margin = 3,071 BTU/hr. Need a 3,500-5,000 BTU mini-split or wall AC (1/4 ton).
Gotcha: "Nameplate" wattage is max draw, not typical. Real heat output is usually 60-80% of nameplate. But always size cooling for nameplate worst case - it's the failure mode that thermals out a closet.
Closet temp targets: Maintain <80°F (27°C) for typical IT/security gear. Hotter than that and switch PoE budget derates, NVR drives fail early, batteries lose 50% life per 18°F (10°C) over 77°F.
Fiber Loss Budget S4
Total Link Loss vs Optic Budget
Sum cable loss + connector loss + splice loss. Compare to transceiver's loss budget. Pass requires margin (typ 3 dB) below the optic's max loss spec.
HOW TO FIGURE IT - METHODOLOGY
The Loss Equation
Total Loss = (km × dB/km) + (connectors × 0.5) + (fusion × 0.1) + (mechanical × 0.3)
Standard dB values per element (worst-case industry):
- Connectors: 0.50 dB each (LC/SC/ST patch)
- Fusion splice: 0.10 dB each
- Mechanical splice: 0.30 dB each
Walk-Through
- Identify wavelength and fiber type. dB/km depends on this.
- Multiply length × dB/km = cable attenuation.
- Count connectors at each end and any patch panel passes.
- Count splices.
- Sum all losses.
- Compare to transceiver's max link loss budget. Allow 3 dB margin minimum.
Common Optic Budgets
| Optic | Range (km) | Budget (dB) |
|---|---|---|
| 1G SX (MM) | 0.55 | 7.5 |
| 1G LX (SM) | 10 | 9.0 |
| 10G SR (MM) | 0.3-0.4 | 2.6 |
| 10G LR (SM) | 10 | 6.2 |
| 10G ER (SM) | 40 | 11.3 |
Example: 2.5km SM at 1310nm, 4 connectors, 1 fusion splice, 10G LR optic (6.2dB budget). Loss = (2.5×0.40) + (4×0.5) + (1×0.1) + 0 = 1.0 + 2.0 + 0.1 = 3.1 dB. Margin = 6.2 - 3.1 = 3.1 dB. PASS.
Gotcha: Dirty connectors are the #1 cause of fiber faults. A speck of dust adds 5-15 dB - enough to kill any link. Always clean and inspect with a fiberscope before plugging.
Always test before commissioning with an OTDR or light source/power meter (LSPM). Compare measured loss to calculated. Discrepancies indicate problem connectors or splice issues.
UPS Runtime Calculator S4
Estimated Battery Runtime
Approximate runtime given UPS battery Ah, voltage, and connected load in Watts. Real-world runtime is 70-80% of theoretical due to inverter efficiency and Peukert effect on lead-acid batteries.
HOW TO FIGURE IT - METHODOLOGY
The Formula
Runtime (hours) = (Ah × V × eff) / Load_W
Walk-Through
- Sum all loads in Watts (use VA × PF or measured wattage).
- Get battery Ah from UPS spec or count batteries × per-battery Ah.
- Determine string voltage - 12V, 24V, 48V, 96V, etc.
- Multiply Ah × V = Watt-hours of energy stored.
- Multiply by efficiency factor (typically 0.80).
- Divide by load to get hours.
Peukert Effect (lead-acid only)
SLA batteries deliver less than rated Ah at high discharge rates. A 9 Ah battery rated at 0.45A draw won't deliver 9 Ah at 9A draw - more like 6 Ah. Newer UPS use lithium iron phosphate (LFP) which doesn't suffer this.
Example: APC SmartUPS 1500 has 2 × 12V 9Ah batteries in series = 24V 9Ah. Load = 500W. Runtime = (9 × 24 × 0.80) / 500 = 0.346 hr = 20.7 min. Matches APC's spec sheet of ~17 min at full load.
Gotcha: UPS runtime is graphed by manufacturer at 25%/50%/75%/100% load. The graph is exponential, not linear. At 25% load, runtime is roughly 8x the 100%-load runtime, not 4x.
Sizing tip: For graceful shutdown, target 5-10 min runtime. For ride-through of brief outages, 15+ min. For generator handoff, 1-3 min is enough. For full ride-through of a multi-hour outage, you need an extended battery cabinet, not a standard UPS.