Length
Length is the simplest relationship on the page: drop is directly proportional to it. Twice the distance, twice the drop, all the way up.
| One-way run | Drop | Percentage | At the load |
|---|---|---|---|
| 50 ft | 3.11 V | 1.30% | 236.89 V |
| 100 ft | 6.22 V | 2.59% | 233.78 V |
| 150 ft | 9.34 V | 3.89% | 230.66 V |
| 200 ft | 12.45 V | 5.19% | 227.55 V |
| 300 ft | 18.67 V | 7.78% | 221.33 V |
| 400 ft | 24.90 V | 10.37% | 215.10 V |
Computed with the site calculator. 240 V single-phase, 40 A, 8 AWG copper at 75 °C, one-way length.
Because it is strictly linear, length is also the easiest factor to plan around. If a 150 ft run gives you 3.9% and you need 3%, you need the run to be about 115 ft, or the conductor to be bigger.
Conductor size
Drop is inversely proportional to cross-sectional area. Each AWG step is about 1.26 times the area of the one below it, so each step down in gauge number cuts the drop to roughly 79% of what it was. Three steps roughly doubles the area and halves the drop.
| Size | Resistance | Drop | Percentage | vs 8 AWG |
|---|---|---|---|---|
| 12 AWG | 1.980 Ω/kft | 15.84 V | 6.60% | 254% |
| 10 AWG | 1.240 Ω/kft | 9.92 V | 4.13% | 159% |
| 8 AWG | 0.778 Ω/kft | 6.22 V | 2.59% | — |
| 6 AWG | 0.491 Ω/kft | 3.93 V | 1.64% | 63% |
| 4 AWG | 0.308 Ω/kft | 2.46 V | 1.03% | 40% |
| 2 AWG | 0.194 Ω/kft | 1.55 V | 0.65% | 25% |
Computed with the site calculator. 240 V single-phase, 40 A copper at 75 °C, 100 ft one-way.
Current
Drop is directly proportional to current, exactly as it is to length. What is different is the power lost, which goes with the square of the current. Halving the current halves the drop and quarters the heat.
This is why splitting a large load across two circuits helps twice over: each circuit carries half the current, so each drops half as much voltage and wastes a quarter as much power. It is also why raising the supply voltage is such an effective fix: the same power at twice the voltage is half the current.
Material
Conductor-grade aluminium is 61% IACS against copper’s 100%, so at the same cross-sectional area it has about 1.64 times the resistance and produces about 1.64 times the drop.
Matching a copper conductor on drop generally means going up two AWG sizes in aluminium. 2 AWG copper is 0.194 Ω per 1000 ft; the nearest aluminium equivalent is 1/0 at 0.201. The full comparison covers cost, weight and termination as well as resistance.
Temperature
Every reference table on this site is at 75 °C, which is the standard basis. Real conductors are rarely at exactly 75 °C. The correction is R₂ = R₁ × [1 + α × (T₂ − 75)], with α of 0.00323 for copper.
| Conductor temp | Resistance | Drop | vs 75 °C |
|---|---|---|---|
| 25 °C | 0.6524 Ω/kft | 5.22 V | -16.1% |
| 45 °C | 0.7026 Ω/kft | 5.62 V | -9.7% |
| 60 °C | 0.7403 Ω/kft | 5.92 V | -4.8% |
| 75 °C | 0.7780 Ω/kft | 6.22 V | — |
| 90 °C | 0.8157 Ω/kft | 6.53 V | +4.8% |
Resistance corrected per NEC Chapter 9, Table 8, Note 2, from the 0.778 Ω per 1000 ft value for 8 AWG stranded copper at 75 °C.
The practical reading: a conductor at its 90 °C insulation rating drops about 5% more than the table says, and a lightly loaded conductor in a cool space drops noticeably less. Neither is a large enough effect to change a design decision on its own, but calculating at 75 °C for a conductor you know will run hot is mildly optimistic.
Reactance and raceway
On AC there is a second contribution to the drop. Alternating current sets up a changing magnetic field around each conductor, and that field produces inductive reactance. Unlike resistance it dissipates no power, but it does add to the voltage drop, weighted by sin θ, so it contributes nothing at unity power factor and a great deal at 0.7.
Raceway matters because of this. Steel is ferromagnetic and concentrates the field, raising reactance roughly 25% over PVC and adding eddy-current losses that show up as higher AC resistance. On 250 kcmil copper the reactance goes from 0.041 Ω per 1000 ft in PVC to 0.052 in steel.
For small conductors at good power factor this is a rounding error. For a large feeder serving motors it is the difference between a design that passes and one that does not. the AC calculator quantifies it across the whole power factor range.
Which lever to pull
| Change | Effect on drop | Practicality |
|---|---|---|
| Shorten the run | Proportional: halve the length, halve the drop | Free at design stage, usually impossible later |
| Raise the supply voltage | Quadruples reach for a fixed-power load | Only if the supply and the equipment allow it |
| Increase conductor size | Inversely with area: three AWG steps roughly halves it | Always available; costs money and raceway space |
| Parallel conductors | Divides by the number of sets | Code conditions apply; generally 1/0 and larger |
| Copper instead of aluminium | Cuts drop by about 39% at the same size | More expensive per foot, easier to terminate |
| Split the load | Halving current halves the drop, quarters the loss | Needs a spare circuit and a way to divide the load |
| Correct the power factor | Reduces both current and the reactance contribution | Only relevant on inductive AC loads |
Questions
How does cable length affect voltage drop?
Directly and proportionally. Double the run and you double the drop; halve it and you halve the drop. There is no threshold and no diminishing return: every extra foot of cable adds the same increment of resistance and therefore the same increment of drop. It is the most predictable of the four factors and often the cheapest to change, since moving a panel closer costs nothing at the design stage.
How much does going up one wire size reduce voltage drop?
Each AWG step down in number is about 1.26 times the cross-sectional area, so the drop falls to roughly 79%, a 21% reduction. Three steps roughly doubles the area and halves the drop. Going from 12 AWG to 8 AWG, for instance, takes the resistance from 1.98 to 0.778 Ω per 1000 ft, a 61% reduction in drop.
Does cable temperature change voltage drop?
Yes. Conductor resistance rises with temperature, so a hot cable drops more voltage than a cool one carrying the same current. Copper resistance changes about 0.32% per °C around the 75 °C reference. A conductor at 90 °C drops about 4.8% more than the tabulated 75 °C figure; one running at 25 °C drops about 16% less.
Does the type of conduit affect voltage drop?
On AC, yes. Steel conduit is ferromagnetic, which raises inductive reactance by about 25% compared with PVC and adds eddy-current losses that show up as higher AC resistance. The effect is negligible at unity power factor and on small conductors, and significant on large conductors feeding inductive loads. On DC, conduit type makes no difference at all.
Is voltage drop worse in a longer cable or a smaller cable?
Neither in general: it depends on the numbers. Both act in proportion, so doubling the length has exactly the same effect as halving the cross-sectional area. What differs is cost and practicality: shortening a run is often free at the design stage and impossible afterwards, while increasing conductor size is always possible but costs money and may not fit the raceway.
Try any of these changes on your own circuit with the voltage drop calculator, or work backwards from a limit with the wire size calculator.