HyreElectrical
Find a Local Electrician

HyreElectrical tools

Voltage drop, and why 3% is not a code requirement

The 3% branch and 5% total figures everyone quotes live in informational notes. Informational notes are explanatory and are not enforceable. That distinction changes what a contractor can tell you a run 'has to' do.

Not a requirement the 3% figure is an informational note NEC 210.19(A) and 215.2(A) carry the 3% branch-circuit and 5% total voltage-drop figures in informational notes. Informational notes are explanatory material rather than enforceable requirements, so a run exceeding 3% is generally not a violation on that basis. They remain a sound design target — they are simply not what an inspector fails you on.

Enter the conductor size and metal, the load, the one-way run length and the system voltage. The tool returns the volts lost, the percentage, the voltage arriving at the load, and the size that would bring it under 3%.

Check the voltage before you price a bigger conductor. The same wire carrying the same load loses the same volts regardless of system voltage — but as a percentage that halves when you double it. Our base case is 6.34 V either way: 5.28% on 120 V and 2.64% on 240 V. Where a load can be served at 240 V, the problem often disappears for nothing. Live electrical work is not a do-it-yourself subject here. Nothing on this site instructs a reader to open, probe or modify energised equipment.

Drop over the run

One-way length. The tool doubles it for the return path. Nothing is emailed.

— Drop
— Against the recommendation
— To reach 3%
— Volts lost
— Voltage at the load
— On the 3% figure

What this assumed —

Simplified DC approximation. Ignores reactance, power factor and temperature rise. Not a design and not instructions to perform electrical work.

The 3% figure is a recommendation, not a rule

Almost every voltage-drop calculator presents 3% as a code requirement. It is not one, and the difference matters when someone tells you a run fails inspection.

The 3% branch-circuit and 5% total figures appear in informational notes to NEC 210.19(A) and 215.2(A). Informational notes are explanatory material. They are not enforceable requirements, and a branch circuit exceeding 3% is generally not a code violation on that basis alone.

That does not make them unimportant. They are a widely used design target because excessive drop is a real performance problem — motors run hot and start poorly, heating elements underperform, lights dim on load, and electronics behave unpredictably. The targets are good engineering. They are simply not what an inspector fails you on.

Why the distinction is worth having. If a contractor tells you a run "has to" be upsized to meet code, that specific claim is usually wrong — while the underlying recommendation to upsize may well be right. Knowing which you are being told changes the conversation from compliance to engineering judgement, and lets you weigh the cost against the benefit rather than treating it as non-negotiable.

Voltage is the lever nobody thinks of. The same conductor carrying the same load loses the same number of volts regardless of system voltage — but as a percentage it halves when you double the voltage. Our base case drops 6.34 V either way: that is 5.28% on 120 V and 2.64% on 240 V. Where a load can be served at 240 V, the drop problem often disappears without a larger conductor.

Nothing on this site is a stamped load calculation, a design, or a permit document. A load calculation that an inspector accepts is produced by someone who has seen the building and signs for it. Live electrical work is not a do-it-yourself subject here. Nothing on this site instructs a reader to open, probe or modify energised equipment.

What actually moves the number

Every row computed by the calculator on this page, from a 12 AWG copper run carrying 16 A over 100 ft at 120 V.

ChangeVolts lostAs a percentageVerdict
Base case12 AWG copper, 16 A, 100 ft, 120 V6.34 V5.28%Over 5%
Halve the run to 50 ft3.17 V2.64%Within 3%
Extend the run to 150 ft9.50 V7.92%Over 5%
Serve it at 240 V instead6.34 V2.64%Within 3%
Upsize to 10 AWG3.97 V3.31%Over 3%, within 5%
Upsize to 8 AWG2.49 V2.07%Within 3%
Aluminium instead of copper, same 12 AWG10.18 V8.48%Over 5%

When drop is worth spending money on

Upsizing a long run is expensive. These are the cases where it earns the cost, and the case where it usually does not.

  • Motor loads at the end of a long run

    Motors are the least tolerant load. Low voltage at start means high current, heat, and shortened life. A well pump or a compressor at the end of a long run is the classic case for upsizing.

  • Anything with a heating element

    Heat output falls with the square of voltage, so a modest percentage drop costs a disproportionate amount of performance. A drop that a light fixture would shrug off makes a heater measurably worse.

  • EV charging and other long-duration high-current loads

    Hours at near-full draw turn a small percentage into real heat in the conductor and real time added to a charge. Worth getting right at installation, when the run is open.

  • Detached structures — garages, shops, barns

    These are where the long runs actually are, and where a feeder rather than a branch circuit is usually the right answer. This is also where the 5% total figure starts to matter, because feeder and branch drop accumulate.

  • When it usually is not worth it

    A short run to an ordinary receptacle or lighting circuit inside the house. If the calculator returns comfortably under 3% on a 40-foot run, there is nothing here to buy.

  • Consider the voltage before the conductor

    Where the load can be served at 240 V, that halves the percentage drop for free. Ask whether it can before pricing a larger conductor.

Questions this calculator answers

Is 3% voltage drop a code requirement?
No. The 3% branch-circuit and 5% total figures appear in informational notes to NEC 210.19(A) and 215.2(A). Informational notes are explanatory material and are not enforceable requirements, so a branch circuit exceeding 3% is generally not a code violation on that basis alone. It may still be poor design — the distinction is between "this fails inspection" and "this will work badly", and only one of those is true.
Then why does it matter?
Because excessive drop is a real performance problem even when it is not a violation. Motors start poorly and run hot, heating elements underperform disproportionately, and long-duration high-current loads waste energy as heat in the conductor. The recommendation is good engineering. Knowing it is a recommendation lets you weigh the cost of fixing it rather than treating it as non-negotiable.
How accurate is this calculation?
It is a simplified DC-resistance approximation using NEC Chapter 9 Table 8 values. It ignores conductor reactance, power factor, temperature rise under load and conduit material. For ordinary residential branch circuits that is close enough to make decisions with; for large feeders, high-reactance situations or anything with a poor power factor, the real figure diverges and an electrician should run it properly.
Should I enter one-way or round-trip length?
One-way — the distance from the panel to the load. The tool doubles it internally for a single-phase circuit, because the current travels out and back. Entering round-trip length will double your answer.
Why does 240 V help so much?
Because the same conductor carrying the same load loses the same number of volts, but that loss is measured against twice the voltage. Our base case loses 6.34 V either way: 5.28% of 120 V and 2.64% of 240 V. Where a load can be served at 240 V — many can — that halves the percentage drop without a larger conductor and without any additional cost.
Why is aluminium so much worse at the same size?
Lower conductivity means higher resistance for the same physical size. Our 12 AWG base case goes from 5.28% in copper to 8.48% in aluminium over the same run at the same load. Aluminium feeders are common and perfectly legitimate; they are simply sized larger for the same job.
Does a large voltage drop damage anything?
It can, indirectly. Motors are the main concern — low voltage at start means high current, which means heat and shortened life. Heating elements simply underperform. Electronics with switching supplies are largely tolerant. The damage case is real but it is specific to motor loads rather than general.
My electrician says the run has to be upsized to meet code. Is that right?
The "to meet code" part is usually not, if voltage drop is the reason given — the figures are recommendations. The underlying advice to upsize may well be sound, particularly for a motor load or a long run to a detached structure. Ask which it is. That converts the conversation from compliance, where you have no say, to engineering judgement, where you can weigh the cost.
Can I use this to plan my own wiring?
No. Live electrical work is not a do-it-yourself subject here. Nothing on this site instructs a reader to open, probe or modify energised equipment. Nothing on this site is a stamped load calculation, a design, or a permit document. A load calculation that an inspector accepts is produced by someone who has seen the building and signs for it. It screens a run so you can understand a proposal or ask a better question.

Sources and methodology

Figures dated 5 September 2026. Last reviewed .

  • NFPA 70, National Electrical Code — 210.19(A) and 215.2(A) informational notes, and Chapter 9 Table 8 (National Fire Protection Association, retrieved 2026-09-05. Source for the 3% branch and 5% total voltage-drop figures being informational notes rather than requirements, and for the DC resistance values used in the calculation. NFPA provides free read-only access to the code text.)
  • NEC adoption maps — which edition is enforced where (National Fire Protection Association, retrieved 2026-09-05. Adoption and amendment of the National Electrical Code is local. A state or city may be on an older edition, may amend it, and may add rules of its own. Confirm the adopted edition and the amendments with the authority having jurisdiction before relying on any code figure here.)

Related

Find a Local Electrician Back to the homepage