Voltage drop calculator for branch circuits

Q: What is the maximum voltage drop NEC allows?

NEC 210.19(A) and 215.2(A) informational notes recommend a maximum 3% voltage drop on branch circuits and 5% combined on feeder + branch — they are recommendations, not mandates, but most AHJs enforce them.

Q: How do I calculate voltage drop for a long run?

Use Vd = 2 × K × I × D / CM for single-phase (×√3 for three-phase), where K is conductor resistivity (12.9 copper / 21.2 aluminum), I is load current in amps, D is one-way distance in feet, and CM is conductor circular mils. Increase wire size whenever the result exceeds 3% on a branch circuit.

Q: Does voltage drop require upsizing the EGC?

Yes — NEC 250.122(B) requires the equipment grounding conductor to be increased proportionally when the ungrounded conductors are upsized for voltage drop, so the EGC maintains its fault-clearing capacity.

Q: What NEC article covers allowable voltage drop?

Voltage drop is addressed in informational notes: NEC 210.19(A) for branch circuits and 215.2(A) for feeders both recommend a maximum 3% drop on the branch or feeder and 5% combined. Article 647 and 695 (sensitive electronic equipment, fire pumps) carry enforceable limits. The general 3%/5% figures are recommendations, but most AHJs treat them as design requirements.

Q: What does AHJ mean in the National Electrical Code?

AHJ stands for Authority Having Jurisdiction — the office or official empowered to enforce the code and approve equipment and installations, typically the local building or electrical inspector, the state, or a fire marshal. The NEC uses "AHJ" throughout because the code is a model standard; the AHJ is who actually interprets it, grants variances, and signs off on your work.

Q: Can the AHJ override the NEC, and how do I find mine?

Yes — the AHJ has the authority to interpret the NEC, grant or deny variances, and adopt local amendments that are stricter (and occasionally more lenient) than the base code, so their ruling governs your project. Find yours by contacting the building or electrical permit office for the city or county where the work is located; the code edition they enforce and any local amendments define what you must meet.

Q: Does Alaska use the 2020 or 2023 NEC?

Electrical code adoption is set at the state level and updated on its own cycle, so the edition in force in Alaska can lag the newest NEC — always confirm the current adopted edition and effective date with the Alaska state electrical inspection program or your local AHJ before designing. This calculator applies general NEC methods; verify the specific article numbers against the edition your jurisdiction enforces.

Q: What are Alaska’s state amendments to the NEC for residential work?

Like most states, Alaska adopts the NEC with state-specific amendments, and cold-climate and remote-service conditions make local requirements (grounding, service entrance, heat-tape and load calculations) especially important there. Amendments change with each adoption cycle, so pull the current Alaska amendment package from the state electrical inspection office rather than relying on the base NEC alone — then use this calculator to size conductors against those requirements.

Question

Voltage drop calculator for branch circuits

Accepted Answer

This calculator handles branch circuits and feeders for single-phase and three-phase systems. Enter voltage, load current, one-way length, and conductor size/material and it returns the voltage drop in volts and percent, flags the 3% branch / 5% combined recommendations, and can solve for the maximum length or the conductor size needed to stay within limits.