About the Handiwork Voltage Drop Calculator
The Voltage Drop Calculator estimates how much voltage is lost over a wire run given its gauge, material, length, load current, and system type (DC, single-phase AC, or three-phase AC), then checks the result against the NEC’s informational voltage drop guidance. It also works backward, recommending the thinnest standard AWG size that keeps a given run under the commonly recommended 3% branch-circuit limit.
How to use the Handiwork Voltage Drop Calculator
- Choose the system type, system voltage, and load current the circuit will carry.
- Choose the wire gauge, conductor material, and the one-way length of the run.
- Read the voltage drop in volts and percent, the voltage that reaches the load, and whether it is within the recommended limit — plus the minimum gauge that would keep it there.
The voltage drop formula
Voltage drop is current times the total resistance of the circuit: for DC and single-phase AC, Vdrop = 2 × I × R × L, where the factor of 2 accounts for the round trip through both the hot and return conductors. For balanced three-phase AC, Vdrop = √3 × I × R × L — the √3 factor comes from the 120° phase displacement between conductors, and a balanced three-phase circuit does not need the round-trip factor of 2. R is the one-way resistance per unit length of the conductor at the chosen gauge and material, and L is the one-way run length.
What the 3% and 5% figures mean
NEC Informational Note No. 4 to section 210.19(A) recommends that branch-circuit conductors be sized so voltage drop does not exceed 3% at the farthest outlet, and that the combined drop of feeder plus branch-circuit conductors not exceed 5%, for reasonable operating efficiency. These are informational notes, not enforceable code requirements — the NEC itself states informational notes are explanatory only. That said, some local jurisdictions adopt the 3%/5% figures as a mandatory amendment, and utilities, commercial energy codes, and general engineering practice (IEEE Std 141) often treat them as a practical design target regardless.
Why the minimum recommended gauge matters
A wire gauge that is safely rated for a load's current (its ampacity) is not necessarily thick enough to deliver that current over distance without a large voltage drop — the two are separate checks. A motor or LED strip running on a marginal voltage can underperform, overheat, or trip protective devices even on a circuit that is otherwise within its ampacity rating. This calculator's recommended-gauge panel searches upward in wire thickness until the 3% target is met, independently of the ampacity of the chosen gauge — always confirm the final gauge also meets ampacity requirements using the AWG wire size calculator.
Typical situations where this matters most
Long DC runs (solar panel-to-charge-controller wiring, 12/24 V battery systems, LED strip runs) are especially sensitive to voltage drop because the system voltage is low, so the same voltage loss is a much larger percentage. Long single-phase branch circuits to detached garages, workshops, or outbuildings, and three-phase feeders to remote equipment, are the other common cases where the standard 12–14 AWG in-wall wire is not thick enough for the distance involved.
Continue this workflow
Use the adjacent tool when the next step calls for a different input, output, or method.
- AWG Wire Size CalculatorCheck that the gauge recommended here also has enough ampacity for your load — voltage drop and ampacity are two separate checks on the same wire.
- Ohm's Law CalculatorSee the underlying voltage, current, resistance, and power relationship this calculator builds on, useful for spot-checking a single segment of the circuit.
Assumptions and limitations
- Uses DC resistance at 20°C for the chosen gauge and material; it does not model AC skin effect or conductor reactance, which become significant for larger conductors and longer three-phase runs.
- The 3% and 5% thresholds are the NEC’s informational recommendation for reasonable efficiency, not a mandatory code requirement, unless your local jurisdiction has adopted it as one.
- Assumes a balanced load for three-phase calculations and a simple two-conductor circuit for DC and single-phase; it does not model shared neutrals, unbalanced loads, or power-factor effects.
- The recommended minimum gauge only targets the voltage drop limit — it does not check ampacity, conduit fill, or any other code requirement for the same run.
Sources and standards
These authoritative references were used to verify the method and guidance on this page.
Frequently asked questions
Is 3% or 5% the voltage drop limit I should use?
The NEC recommends 3% for a branch circuit alone, and 5% for the combined drop of a feeder plus the branch circuit connected to it. If your circuit runs directly from the panel, use 3%; if it runs through a sub-panel or feeder first, the branch circuit’s share of the total should leave room under the 5% combined figure.
Is exceeding 3% voltage drop actually against code?
Generally no — it's an NEC informational note, which the code itself defines as explanatory rather than enforceable. Some cities and jurisdictions have adopted it as a mandatory local amendment, so check with your local authority having jurisdiction if you need to be certain for a permitted installation.
Why does three-phase use √3 instead of 2?
A single-phase or DC circuit needs a factor of 2 because current travels out on one conductor and back on another over the same distance. A balanced three-phase circuit has no return conductor carrying the full current — the √3 factor instead comes from the 120° phase relationship between the three line conductors.
Why is voltage drop a bigger problem on 12V and 24V systems?
Voltage drop in volts depends only on current, resistance, and length — not on system voltage. But the percentage drop is the volts lost divided by the system voltage, so the same 1-volt loss is 0.4% on a 240V circuit and over 8% on a 12V circuit. That is why solar, marine, and RV wiring at low DC voltages needs noticeably thicker wire for the same distance and current.
Does a thicker wire than the recommended minimum ever hurt anything?
No — going thicker than necessary only reduces voltage drop and resistive heating further. The tradeoffs are cost, weight, and physically fitting a larger conductor into a connector, breaker terminal, or conduit, not electrical performance.