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Voltage Drop Calculator

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

Circuit
A
V
ft

Measure the routed path, not the straight-line distance.

Phase
Conductor
Material

NEC 210.19(A) and 215.2(A) Informational Notes.

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
SizeΩ per 1000 ftDropDrop %Max run at limit
8 AWG — selected0.77806.22 V2.59%116 ft
6 AWG0.49103.93 V1.64%183 ft
4 AWG0.30802.46 V1.03%292 ft
3 AWG0.24501.96 V0.82%367 ft
2 AWG0.19401.55 V0.65%464 ft
1 AWG0.15401.23 V0.51%584 ft

Quick answer

Most branch circuits aim for 3% or less voltage drop on the feeder run (5% total from service to load). On a 120V circuit, 3% is 3.6V; on 240V it is 7.2V. If drop exceeds that, upsize the conductor or shorten the run. Voltage drop depends on current, one-way length, wire size, and material (copper vs aluminum) — not ampacity alone. Enter your values below to see drop in volts and percent; treat results as planning numbers and confirm with a licensed electrician for the installed circuit.

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).

Feeder drop is calculated separately

The 5% combined limit applies to feeder plus branch circuit together. A sub panel 150 feet from the main may pass branch-circuit drop at the load but fail when feeder drop is included. Calculate each segment independently, then add the percentages to check the combined limit.

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.
What is acceptable voltage drop?
NEC recommends 3% maximum on branch circuits and 5% combined for feeder plus branch. On a 120V circuit, 3% is 3.6 volts (116.4V at the load). On 240V, 3% is 7.2 volts. Many local amendments enforce these as requirements.
How do you calculate voltage drop?
Single-phase: VD = 2 × K × I × L ÷ CM, where K is resistivity (12.9 for copper), I is amps, L is one-way length in feet, and CM is circular mils. A 30-amp load on 150 feet of 10 AWG copper: 2 × 12.9 × 30 × 150 ÷ 10,380 = 11.2V drop (9.3% on 120V — too high).
What voltage drop for 200 amp service 150 feet away?
A 200-amp feeder 150 feet on 2/0 copper at full load drops about 7.2V (3% of 240V) — borderline. 3/0 copper or 250 kcmil aluminum holds the drop under 3%. Most 200A services use 2/0 copper or 4/0 aluminum for runs under 100 feet.
Does voltage drop affect motors?
Yes. A motor rated for 230V that receives 210V (8.7% drop) draws more current to produce the same torque, runs hotter, and may fail to start. NEC recommends 3% drop for branch circuits partly because motor loads are sensitive to low voltage.
How much voltage drop is too much for LED lights?
LED drivers tolerate wider voltage range than incandescent, but below 110V on a 120V circuit, dimming and flickering appear. At 5% drop (114V), most LEDs operate normally. Beyond 8% drop (110V), performance degrades noticeably.
What wire size for 100 amp sub panel 100 feet away?
A 100-amp sub panel 100 feet away needs 1/0 copper or 2/0 aluminum for ampacity, but voltage drop may require 2/0 copper or 3/0 aluminum. At 80-amp actual load, 1/0 copper gives about 4.5% drop — acceptable for a feeder but worth checking.
Does aluminum wire have more voltage drop than copper?
Yes. Aluminum has 61% of copper's conductivity, so you need a larger aluminum conductor for the same ampacity and drop. For voltage drop calculations, aluminum's K factor is 21.2 versus 12.9 for copper — roughly 64% more drop per size.
How do you reduce voltage drop?
Four options: upsize the conductor (most common), shorten the run, raise the system voltage (240V instead of 120V halves the drop for the same power), or reduce the load. Upsizing from 12 AWG to 10 AWG on a 100-foot run cuts drop by roughly 37%.
What is voltage drop on 12 AWG wire 100 feet?
At 20 amps on 100 feet of 12 AWG copper: 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.9V drop — 6.6% on 120V, which exceeds the 3% recommendation. Upsize to 10 AWG for 4.9% or 8 AWG for 3.1%.

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