LowVoltCalc
Tools

Voltage drop calculator

What voltage actually reaches the lock, camera or reader at the end of the run, and whether that is enough.

– at the device

Drop
–
Loss
–
Longest run, this wire
–
Smallest wire that passes
–
Current
–
Loop resistance
–
Power lost in the cable
–

Units
V
V
ft
Conductor
Temperature and doubled conductors
75 °C is the NEC table's basis and the conservative choice.
For example, doubling up spare pairs in a multi-pair cable.

How this works

Current flows out on one conductor and back on the other, so the drop is across both. The calculator multiplies the one-way length by two for you.

Voltage drop = 2 × length × resistance per unit length × current Voltage at device = supply voltage − voltage drop

Resistance comes from NEC Chapter 9, Table 8: DC resistance of uncoated copper at 75 °C, in ohms per 1,000 ft, solid or stranded. Table 8 stops at 18 AWG. For 22 and 24 AWG, common in access control and alarm cable, the calculator uses NBS Handbook 100, the copper wire tables Table 8 is itself calculated from: 19.7 and 31.2 Ω per 1,000 ft at 75 °C. Metric sizes use IEC 60228 (class 2 stranded copper, maximum resistance at 20 °C), corrected to the chosen temperature.

Amps or watts

Enter amps when the datasheet gives a current, as it usually does for locks, strikes and sirens. Enter watts for devices with a switching power supply, such as most IP cameras and many readers and heaters. Those draw more current as their input voltage falls. The calculator solves for that, so a watt-rated load drops more than the same load treated as a fixed current. If the run cannot deliver the power at all, it says so rather than giving a number.

Pass or fail

Enter the device's minimum operating voltage from its datasheet. Typical wording is "12 VDC ±15%", which means a minimum of 10.2 V. The calculator then gives a pass or fail, the longest run that still passes on this wire, and the smallest wire size that passes on this run.

What this does not account for

  • Loads spread along a run, such as several devices daisy-chained. The calculator puts the whole load at the far end, which overstates the drop and so errs on the safe side.
  • AC reactance and power factor. For 24 V AC at these wire sizes, resistance dominates, but this is a DC resistance calculation.
  • Copper-clad aluminium (CCA) cable. Its resistance is higher than copper's, so this calculation would understate the drop.
  • Connector and terminal resistance, and a supply that sags under load. Measure at the device when it matters.

Common questions

What percentage drop is acceptable?
The device decides, not a percentage. The NEC's 3% and 5% figures are informational notes for branch circuits and feeders, not rules for low-voltage wiring. A 12 V lock that works down to 10.2 V can tolerate a 15% drop; a device with a narrow input range cannot. That is why this calculator asks for the device minimum.
Why does the length count twice?
Because the current goes out and comes back. A 100 ft run is 200 ft of conductor.
Should I use 24 V instead of 12 V?
For the same power, doubling the voltage halves the current, so the drop is a quarter as large in proportion. When a device accepts either voltage, a long run is usually easier at 24 V.
Does doubling up conductors help?
Yes. Two conductors in parallel on each leg halve the resistance. Set "Conductors per leg" under the temperature options.

Sources

  1. NFPA 70, National Electrical Code, Chapter 9, Table 8, Conductor Properties. DC resistance at 75 °C, uncoated copper, Ω/kft, and Note 2's temperature formula with α = 0.00323. Values checked against the 2017 and 2011 editions.
  2. NBS Handbook 100, Copper Wire Tables (1966), Table 6: 22 and 24 AWG at 75 °C.
  3. IEC 60228, Conductors of insulated cables: class 2 plain copper, maximum resistance at 20 °C (Table 2 in the 2004 edition, Table 4 in 2023).

Every source used on this site, and how each number is used.