Calculators · NEC 215 · CEC 8-102

Voltage drop

The smallest copper or aluminum conductor that keeps the drop inside your limit, for single-phase, three-phase or DC runs. Results update as you type.

Free. No sign-up. Not sealed engineering. By Ali Mazloum, P.Eng.

Inputs

Enter a voltage above 0.
Enter a current above 0.
Enter a distance above 0.
Use a limit between 0.5 and 10 %.
Defaults: 208 V, 50 A, 30 m, copper, 3 %.
Smallest conductor that passes –
Voltage drop–V
Percentage–%

Voltage drop only. Ampacity (CEC Table 2, NEC 310.16), short-circuit withstand and conduit fill are separate checks. Educational use; not sealed engineering.

Formula  VD = K × I × R × L / 1000   K = 2 for single-phase and DC, 1.732 for three-phase. R is DC resistance in Ω/km at 75 °C from NEC Chapter 9, Table 8. L is the one-way length in metres. The tool walks up the standard sizes from 14 AWG to 500 kcmil and stops at the first that passes.

Understanding voltage drop

Five minutes

Voltage drop is the voltage lost along a conductor because the conductor has resistance. Some drop is unavoidable. Too much, and motors run hot, LED drivers flicker, and sensitive equipment misbehaves at the far end of the run.

What the codes say

NEC 210.19(A), Informational Note 4, recommends no more than 3 % on a branch circuit and 5 % for feeder and branch together. For feeders, NEC 215.2(A) carries a matching Informational Note: 3 % on the feeder and 5 % for feeder and branch circuit together. CEC Rule 8-102 sets the same 3 % and 5 % from the service to the point of use. They are recommendations in the NEC and a rule in the CEC, and either way they are what an inspector expects.

How this tool works

It uses the DC resistance of each standard conductor size at 75 °C, calculates the drop for your current and length, and reports the first size that keeps the drop inside your limit. Single-phase and DC use the round-trip factor of 2; three-phase uses the square root of 3. The status line also tells you what one size smaller would give, so you can see the margin.

What it does not do

It does not check ampacity, short-circuit withstand, conduit fill or AC reactance. Above 250 kcmil in steel conduit, reactance matters and you should use AC impedance values. Size the conductor for every requirement, then take the largest.

FAQ

Straight answers
What is an acceptable voltage drop?

3 % on a branch circuit and 5 % total for feeder plus branch, in both the NEC (210.19(A) Informational Note 4) and the CEC (Rule 8-102). Some jurisdictions make the NEC note mandatory by local amendment.

How is three-phase different?

Three-phase uses K = 1.732 (the square root of 3) instead of 2, because the phase relationship means the return current does not flow through a single neutral.

Copper or aluminum?

Copper has about 61 % of the resistance of aluminum at the same size, so it wins on long runs. Aluminum is lighter and cheaper and is common from 4/0 up, where the larger size is offset by cost.

Does it account for conduit fill or temperature?

No. It uses DC resistance at 75 °C from NEC Chapter 9, Table 8. Ampacity derating for ambient temperature and conduit fill (NEC 310, CEC Table 2) is a separate calculation. Above 250 kcmil in steel conduit, AC reactance matters too.

Can I rely on this for a sealed design?

No. It is an educational tool for early checks. A sealed design needs a licensed engineer and the full set of checks.

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