Run length
The voltage drop formula, rearranged. Give it the circuit and the conductor and it returns the furthest the run can go before the drop crosses your limit: one-way for planning the route, round-trip for ordering the cable.
Your maximum run appears here
Enter a supply voltage and a load current, then pick a conductor size.
Longest one-way run within the limit
96.8ft
That is 193.5 ft of conductor in total, out and back.
This is the point at which the drop exactly reaches your limit. Design to something short of it: load current, conductor temperature and supply voltage all move in service, and a run built right at the limit will cross it.
Voltage drop budget
V_allowed = V_source × (limit ÷ 100)V_allowed = 240 V × (3 ÷ 100)Result: 7.20 V
Rearrange for length
L = V_allowed ÷ (2 × I × Z)L = 7.20 ÷ (2 × 30 A × 1.24000 Ω/1000 ft)Result: 96.8 ft one-way
A two-wire circuit carries the full load current out and back, so the current travels twice the one-way run length.
Maximum one-way run
L in feet96.8 ftResult: 96.8 ft
That is 193.5 ft of conductor in total, out and back.
Formula
Because L appears once and linearly, the rearrangement is exact: there is no iteration and no approximation. The calculator then feeds the answer back through the forward calculation as a check, which is why the drop shown at maximum length lands exactly on your limit.
Reference
Copper, single-phase, 3% limit, one-way distance in feet. Resistance at 75 °C.
| Size | 120 V, 15 A | 120 V, 20 A | 240 V, 30 A | 240 V, 50 A |
|---|---|---|---|---|
| 14 AWG | 38 ft | 29 ft | 38 ft | 23 ft |
| 12 AWG | 61 ft | 45 ft | 61 ft | 36 ft |
| 10 AWG | 97 ft | 73 ft | 97 ft | 58 ft |
| 8 AWG | 154 ft | 116 ft | 154 ft | 93 ft |
| 6 AWG | 244 ft | 183 ft | 244 ft | 147 ft |
Computed with the site calculator from NEC Chapter 9, Table 8 resistance values at 75 °C, resistance only at unity power factor.
Read across a row to see what doubling the voltage buys you, and down a column to see what a size increase buys you. These figures are voltage-drop limits only. Each size also has an ampacity limit that may bite first. The wire size calculator shows both, and the online voltage drop calculator gives the drop for a length you have already settled on.
Questions
The headline figure is the one-way distance from the source to the load, which is what you measure when planning a route. The line underneath gives the same run as total conductor length, twice the one-way figure, which is what you need when ordering cable. Both describe the same run.
No. The maximum is the point at which the drop exactly equals your limit, with nothing left over. In service the load current varies, the conductors run hotter than the 75 °C the data assumes, terminations add resistance the calculation does not model, and the supply voltage sags at times of high demand. Leaving 15 to 20% of margin is ordinary practice, and it costs nothing at the design stage.
Because it changes two things at once. The voltage drop budget in volts doubles, since the limit is a percentage of a larger supply. And if the load is a fixed power rather than a fixed current, doubling the voltage halves the current, which halves the drop per foot. Together that quadruples the reachable distance. It is why a long run is often better served by stepping the voltage up than by stepping the conductor up.
They multiply it. Two identical conductors per phase halve the impedance of the path, so the same drop budget stretches twice as far. Three triple it. The code has its own conditions on paralleling (generally 1/0 and larger, with every conductor in the set matched in length, material, size and termination), which this calculator does not check.
On AC, yes. Below unity power factor the conductor reactance starts contributing to the drop, so the effective impedance rises and the reachable distance falls. The effect grows with conductor size and is worse in steel conduit. This calculator brings reactance in automatically whenever the size has published Table 9 data and the power factor is below 1.
More tools