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Voltage drop and wire size

Voltage drop for a copper or aluminum run and the smallest wire that keeps drop under your target.

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Your numbers

ConductorK = 12.9 for copper, 21.2 for aluminum (ohm-cmil/ft).SystemSingle phase uses 2 x length; three phase uses 1.732 x length.System voltage (V)120, 208, 240, 277 or 480 are common.Load current (A)Actual or calculated load current.One-way length (ft)Distance from panel to load, one way.Wire sizeThe conductor you plan to pull.Target maximum drop (%)NEC informational notes suggest 3% for a branch circuit, 5% total.

For the run sheet and payback7 more, filled with typical values

Drop upstream (feeder) (%)Voltage drop already on the feeder that serves this panel. 0 if the panel sits at the service.Equipment tolerance (% under nameplate)How far under its nameplate voltage the equipment still runs properly, from its manual or nameplate.Hours a day at this load (hours)How long the circuit carries this current on a working day.Days a year (days)Days the load runs.Electricity price ($ per kWh)From the utility bill, or the EIA state average.Wire price, planned size ($ per ft)Price per foot of one conductor of the size you plan to pull.Wire price, one size up ($ per ft)Price per foot of one conductor of the next larger size.

Voltage drop and wire sizeForge by Norventus

How it works

Press Calculate to see the voltage drop, the voltage at the load and the smallest wire that meets your target.

The cable run, sized to your wireMeter: percent drop; the band starts at your targetPanel, motor, meter and wire gaugeThe drawing follows your numbers as you type: a longer run, a bigger wire, a bigger load, two or three conductors. Press Calculate and the meter swings to the percent drop worked out by the tool, and the gauge rings the smallest wire that meets your target.

Result

Ready when you are.Enter your numbers and press Calculate.

References

Check the work against these

Total drop with the feeder, the voltage the equipment sees, the energy the run wastes and its cost, a size-by-size table and the upsizing payback, with sources. Numbers in the result and in the plain-words read point here.

1
NFPA 70, National Electrical Code (NEC)Code
National Fire Protection Association
The informational notes on voltage drop (3% for a branch circuit, 5% for feeder and branch together), 250.122(B) (an equipment grounding conductor grows in proportion when the circuit conductors are upsized for voltage drop) and 110.14 terminal ratings.
2
NEC Chapter 9, Table 8 (conductor properties)Code
NFPA free access (read-only, free account)
The circular mils and DC resistance of each conductor size used in the drop formula.
3
Watch Out for that Voltage DropGuide
EC&M
Explains that the NEC's voltage drop figures sit in informational notes after 210.19(A) and for feeders, so they are recommendations, and why following them saves energy.
4
Electric Power Monthly, Table 5.6.A: average price of electricity by state and sectorData
US Energy Information Administration
The average price per kWh in your state, to price the energy the run wastes as heat. Use your own tariff when you have it.
5
Southwire Voltage Drop Calculator (app)Guide
Southwire, on the Apple App Store
A manufacturer's calculator to cross-check the size against both voltage drop and ampacity before you order wire.

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Go deeper

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Voltage drop and wire size

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