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Three field tools in one — check voltage drop over a run, find the minimum wire size for a load, and estimate a dwelling's service size. Estimates only; always verify against the current NEC and local code.
Voltage drop · wire size · service load — estimates, verify against the current NEC
Rule of thumb: keep branch circuits ≤ 3% and feeder + branch ≤ 5%. Formula: 2 · K · I · L ÷ circular mils (× √3 for three-phase).
Based on NEC 310.16, 75°C column. Does not apply derating for ambient temperature or conduit fill — check those on the job.
NEC standard method (220.42): general load (3 VA/ft² + 1,500 VA per small-appliance & laundry circuit) demanded at 100% of the first 10 kVA + 40% of the rest, plus appliances at 100%. A quick estimate — not a stamped load calc.
See whether a long run will drop too much voltage or what service size a home needs — useful context before hiring a licensed electrician for the real work.
Voltage drop, wire sizing, and a standard-method service estimate from your phone. In ServicePros Hub, wire and service specs drop straight into a proposal line.
Three of the most common field calcs, in one place — voltage drop, wire size, and service load.
Voltage drop uses 2 · K · I · L ÷ circular mils (× √3 for three-phase), where K is the conductor's resistivity (12.9 copper, 21.2 aluminum). Keep branch circuits at or under 3%, and feeder-plus-branch under 5% — the calculator flags when you're over so you can size up or shorten the run.
Wire size returns the smallest conductor whose 75°C ampacity (NEC 310.16) meets your load, for copper or aluminum. Service load runs the NEC standard method for a dwelling: 3 VA/ft² plus small-appliance and laundry circuits, demanded at 100% of the first 10 kVA and 40% of the remainder, plus fixed appliances — then rounds up to a standard service size. All three are estimates; verify against the current NEC, and apply derating for temperature and conduit fill on the job.
Voltage drop = 2 × K × I × L ÷ circular mils, where K is 12.9 for copper (21.2 for aluminum), I is the load in amps, and L is the one-way length in feet. Multiply by √3 instead of 2 for three-phase. The calculator does it and shows the percentage.
The NEC recommends (in informational notes) keeping branch circuits at or below 3% and the combined feeder plus branch at or below 5%. Above that, size the conductor up or shorten the run.
For a 100-amp load at 75°C, that's typically #3 copper or #1 aluminum per NEC 310.16 — but service and derating rules can change it. Enter your load in the wire-size tab for copper or aluminum.
Run the standard method: 3 VA per square foot plus small-appliance and laundry circuits, demand the first 10 kVA at 100% and the rest at 40%, add fixed appliances, then divide by voltage. The calculator rounds up to 100/125/150/200/400 A.
No. These are quick field estimates for planning and sanity checks. Final designs must follow the current NEC and local code, account for temperature and conduit-fill derating, and — where required — be performed or reviewed by a licensed electrician.
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