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Voltage Drop Calculator

Check whether a conductor run stays inside the 3% branch-circuit voltage drop recommendation.

Voltage drop

2.59%6.2 V

Inside your 3% limit at 100 ft.

Voltage at the loadFrom 240 V at the source: 233.8 V
Drop in voltsCopper 8 AWG at 0.7780 Ω/1000 ft: 6.22 V
Maximum run at 3%One-way, at this size and load: 116 ft
Power lost in the conductorHeat, paid for and not used: 249 W

Within the 3% recommendation

At 2.59% you have 16 ft of run length in reserve before this size exceeds the limit. Remember that these percentages come from Informational Notes — advisory in the NEC, but frequently mandatory under a project specification or local amendment.

What upsizing buys you
Size	Ω per 1000 ft	Drop	Drop %	Max run at limit
8 AWG — selected	0.7780	6.22 V	2.59%	116 ft
6 AWG	0.4910	3.93 V	1.64%	183 ft
4 AWG	0.3080	2.46 V	1.03%	292 ft
3 AWG	0.2450	1.96 V	0.82%	367 ft
2 AWG	0.1940	1.55 V	0.65%	464 ft
1 AWG	0.1540	1.23 V	0.51%	584 ft

What this assumes

Uses NEC Chapter 9, Table 8 DC resistance for uncoated copper and aluminum at 75°C.
Single-phase drop uses 2 × length; three-phase uses 1.732 × length.
Alternating current effects such as reactance are not modeled; for large conductors in steel raceway, actual drop can be higher.

Important

NEC 210.19(A) Informational Note recommends a maximum 3% drop on a branch circuit and 5% on the combined feeder and branch circuit. These are recommendations, not enforceable requirements, except where a specific article states otherwise.

Understanding the result

Why voltage drop matters

Every conductor has resistance, so voltage at the load is always lower than voltage at the source. Excessive drop makes motors run hot and draw more current, dims lighting, and causes electronics to reset. The longer the run and the higher the current, the more conductor you need to hold the drop down.

The 3% and 5% figures

The NEC recommends holding a branch circuit to 3% and the feeder plus branch circuit combined to 5%. Because these appear in an Informational Note, they are not enforceable in most installations — but designers, specifications and many local amendments treat them as mandatory.

What to do when the drop is too high

Increase the conductor size, shorten the run, raise the supply voltage where the equipment allows it, or split the load across multiple circuits. Upsizing conductors for voltage drop is the usual answer, and the equipment grounding conductor must be upsized proportionally under NEC 250.122(B).

Common questions

Is 3% voltage drop a code requirement?

Generally no. It appears in Informational Notes to NEC 210.19(A) and 215.2(A), which are advisory. Specific articles — such as those covering sensitive electronic equipment or fire pumps — do impose hard limits, and local amendments can make the 3% figure enforceable.

Do I need to upsize the ground wire too?

Yes. NEC 250.122(B) requires the equipment grounding conductor to be increased proportionally to the increase in ungrounded conductor size whenever conductors are upsized.

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