Voltage Drop Calculation for Branch Circuits and Feeders

A conductor that passes the ampacity table can still fail on voltage drop — especially on long runs to perimeter lighting, pumps, or a detached structure. Here is the calculation and the size-selection logic.

Why voltage drop matters

Excessive voltage drop dims lighting, overheats motor windings, trips electronics on brownout, and wastes energy as heat in the cable. The PEC recommends (Fine Print Notes) a maximum drop of 3% on a branch circuit and 5% total from the service to the farthest outlet. Many specifications make these limits mandatory.

The formulas

Using conductor resistance R in ohms per metre (from manufacturer data or R = ρ/A with ρ ≈ 0.0172 Ω·mm²/m for copper):

Where L is the one-way length in metres and I is the load current in amperes. Express the result as a percentage of nominal voltage.

Worked example

A 3-phase, 400 V feeder carries 120 A to a sub-panel 60 m away. Trial conductor: 38 mm² copper, R ≈ 0.00049 Ω/m.

That clears the 3% feeder allowance and leaves 1.5% for the downstream branch circuits within the 5% total. If the run were 150 m, the drop would be 3.8% and you would step up to 60 mm².

Selecting the size

1. Size the conductor for ampacity first (design current, correction factors, termination rating).
2. Calculate %VD for that size at the actual run length.
3. If %VD exceeds the limit, increase one size and re-check until it passes.
4. Keep the equipment grounding conductor proportionate if you significantly upsize the phase conductors.

As a planning rule of thumb for 230 V single-phase copper: expect to upsize once past roughly 25–30 m of run at 80% of the conductor's ampacity.