Voltage Drop on a 100 ft Wire Run: Fix It by Upsizing
NEC recommends max 3% drop on branch circuits (5% total with feeders). A 100 ft 120V run pulling 16A on 12 AWG copper drops about 5.1V โ 4.3%, over the limit. Upsizing to 10 AWG drops it to ~3.2V (2.7%) and the circuit is compliant. Here is the exact arithmetic plus the distance rules of thumb.
Step-by-Step Takeoff
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Measure one-way length (100 ft here โ round trip is 2ร in the formula).
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Voltage drop (single-phase): VD = 2 ร K ร I ร L รท CM, with K = 12.9 for copper, I = amps, L = one-way feet, CM = circular mils.
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12 AWG (6530 CM) at 16A over 100 ft: 2 ร 12.9 ร 16 ร 100 รท 6530 = 6.3V round trip รท 2 legs โ 5.1V effective, or 4.3% of 120V โ too much.
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10 AWG (10380 CM): same math gives โ 3.2V, or 2.7% โ inside the 3% branch allowance.
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Rule of thumb: past ~60 ft at 120V/16A, price 10 AWG instead of 12 AWG; past ~100 ft, consider 8 AWG or a 240V circuit.
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Motors and compressors care most โ size to the starting current, not just running amps.
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Enter your exact length, load, and voltage in the Electrical Calculator โ it flags drop over 3% automatically.
Guide FAQ
Is 5% voltage drop acceptable?
Only 3% is recommended on the branch itself; 5% is the total budget including the feeder. Fix the branch first.
Does 240V halve the drop problem?
Better: at 240V the same watts draw half the amps, and drop percentage quarters versus 120V for the same wire.
What is K in voltage drop?
12.9 for copper, 21.2 for aluminum โ resistivity constants in the 2KIL/CM formula.
Does voltage drop waste money?
Yes โ lost volts become heat in the wire, and motors run hotter and shorter-lived.
When to run a subpanel instead?
Past ~150 ft or for multiple circuits, a subpanel with one fat feeder beats many long branch runs.
Related Guides
What Wire Size for a 20A Branch Circuit?
12 AWG copper on a 20A breaker: ampacity, the 80% continuous-load rule, and when to upsize.
Is My 200A Panel Overloaded? Do the Math
Existing 80A load plus new circuits against a 200A service, demand factors, and upgrade triggers.
How Many Wires Fit in 3/4" PVC Conduit?
The 40% fill rule worked: THHN areas by gauge, derating limits, and conduit upsizing calls.