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Copper, or aluminum — the metal printed on the jacket (CU or AL).
Aluminum has about 1.64 times the resistance of copper for the same area (K 21.2 against 12.9), so the same size drops about 64% more voltage. It is met mostly on services and feeders; the small branch circuits of a house are usually copper. The NEC's smallest aluminum conductor is 12 AWG, so the page gives no drop for 14 AWG aluminum.
The size printed on the jacket: an AWG number up to 4/0, then kcmil.
Smaller AWG numbers mean thicker wire — 12 AWG is thicker than 14 AWG — and past 1 AWG the sizes run 1/0 to 4/0. Above that, conductors are named by their area in thousands of circular mils: 250 kcmil is a quarter of a million circular mils, a little larger than 4/0.
Single phase — including a 240 V circuit and DC — or a balanced three-phase circuit.
A single-phase circuit loses voltage on the way out and on the way back, so the one-way length counts twice. A balanced three-phase circuit uses 1.732 in place of the 2, and the drop it gives is between lines, to be compared with the line-to-line voltage.
The distance from the panel to the load, one direction only.
Don't double it for the return path — the formula already accounts for both directions. Measure the route the conductor takes, including the rises and drops, not the straight line between the ends.
The expected current draw of the load in amps.
Use the device's rated amps, or watts divided by voltage. For a three-phase load, the current in one line.
The nominal circuit voltage — line to line for three phase.
120 V for standard outlets, 240 V for large appliances, 208 or 480 V for three-phase equipment. The voltage does not change the volts lost, only the percentage they are of the supply.
Voltage drop
1.936 V
- Voltage drop
- 1.61 %
- Voltage at the load
- 118.06 V
- K constant, Ω·cmil per ft
- 12.9
- Conductor area, circular mils
- 6,530
They open the calculator with your figures already in it
Wire Gauge Voltage Drop Calculator (Copper or Aluminum, AWG to 1,000 kcmil): 1.94 V — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard single-phase voltage drop formula: Vdrop = (2 x K x one-way length in ft x amps) / circular mils, K=12.9 for copper
- NEC recommends keeping voltage drop to 3% or less for branch circuits (and 5% total including feeders) for reasonable efficiency, though it is a recommendation, not a hard code violation
- IAEI Magazine, "Voltage Drop Formulas" (March/April 2017): K is a conductor's resistance per foot times its area in circular mils — the resistance of a conductor one circular mil in area and one foot long — at 75 °C (167 °F), from NEC Chapter 9 Table 8: 1,000 kcmil uncoated copper at 0.0129 Ω per 1,000 ft (0.0423 Ω/km) gives 12.9, 1,000 kcmil aluminum at 0.0212 Ω per 1,000 ft (0.0696 Ω/km) gives 21.2, and every size in the table works out close to these; the single-phase multiplier 2 becomes 1.732 for three phase
- EC&M, "Stumped by the Code? NEC Requirements for Conductor Material Types and Sizing Requirements" (Mike Holt, 9 February 2023, on the 2023 NEC): Section 310.3(A) sets the smallest conductors at 14 AWG for copper and 12 AWG for aluminum or copper-clad aluminum, except where the code permits otherwise
Inputs used
- Conductor metal
- Copper (uncoated)
- Conductor size (AWG or kcmil)
- 12 AWG
- Circuit
- Single phase (or DC), out and back
- One-Way Wire Run Length
- 49 ft
- Current Draw
- 10
- Circuit Voltage
- 120 V
- Voltage you are working at (V)
- 600
Intermediate steps
- Voltage drop
- 1.61 %
- Voltage at the load
- 118.06 V
- K constant, Ω·cmil per ft
- 12.9
- Conductor area, circular mils
- 6,530
What this calculation does not cover
- VOLTAGE DROP IS NOT AMPACITY, and the two are different questions with different answers. A conductor can stay inside the 3% suggestion and still be too small to carry the current without overheating, and it can be thermally adequate and still drop too much over a long run. Both checks have to be made, and only one of them is made here.
- The K constants are DC resistance at 75 °C (167 °F) for uncoated copper and for aluminum, the basis of NEC Chapter 9 Table 8. The table's size-by-size resistances differ from the single constant by a percent or so either way, and the table is not reproduced here. A conductor running cooler drops a little less and one at a 90 °C (194 °F) rating a little more; tinned (coated) copper has its own, slightly higher resistance.
- Treats the circuit as resistive, which is close for lighting, heating and most branch circuits. On a large AC feeder, and above all one in steel conduit feeding an inductive load, the conductor's reactance adds to the drop and the power factor matters; Table 9 of the same chapter carries the AC figures and is not reproduced here.
- Three phase assumes a balanced load and gives the drop between lines. A single-phase load taken from one line to neutral of a three-phase supply is a single-phase circuit: choose single phase and the line-to-neutral voltage.
- Aluminum conductors need terminations and devices listed for them; the code does not let dissimilar metals be joined except in a device listed for the purpose. Nothing here checks a termination, a lug or a splice.
- The 3% and 5% figures are suggestions in the code's informational notes rather than requirements, though a local amendment, an equipment maker's instructions or a specification can make a tighter figure binding.
Add the equipment this sizes
This result is a specification — 1.936 V — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.1.0
Regulatory standards & verification citations4
- Standard single-phase voltage drop formula: Vdrop = (2 x K x one-way length in ft x amps) / circular mils, K=12.9 for copper
- NEC recommends keeping voltage drop to 3% or less for branch circuits (and 5% total including feeders) for reasonable efficiency, though it is a recommendation, not a hard code violation
- IAEI Magazine, "Voltage Drop Formulas" (March/April 2017): K is a conductor's resistance per foot times its area in circular mils — the resistance of a conductor one circular mil in area and one foot long — at 75 °C (167 °F), from NEC Chapter 9 Table 8: 1,000 kcmil uncoated copper at 0.0129 Ω per 1,000 ft (0.0423 Ω/km) gives 12.9, 1,000 kcmil aluminum at 0.0212 Ω per 1,000 ft (0.0696 Ω/km) gives 21.2, and every size in the table works out close to these; the single-phase multiplier 2 becomes 1.732 for three phase
- EC&M, "Stumped by the Code? NEC Requirements for Conductor Material Types and Sizing Requirements" (Mike Holt, 9 February 2023, on the 2023 NEC): Section 310.3(A) sets the smallest conductors at 14 AWG for copper and 12 AWG for aluminum or copper-clad aluminum, except where the code permits otherwise
Which documents these citations point at
- National Electrical Code (NFPA 70) — Chapter 9 (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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Protection this work requires
- Stop and check first. Textured ceiling coatings, board behind panels, floor tiles and bitumen adhesive in older buildings — built before asbestos was banned or phased out where you are — can contain asbestos, and drilling or sanding them releases fibres. Do not disturb it — have it sampled first. This is not a job for better PPE.
Tools and safety for this job
To first-fix electrical. Generic types, no brands, no prices.
- Assume every cable is live until proved dead at the point you will touch, with a two-pole tester you have just proved on a known source.
First-fix electrical: This site does not publish a tool list for electrical installation work. In every market it serves, fixed wiring is either reserved to a registered electrician or notifiable to a building authority, and the failure mode is a fire or an electrocution months later rather than a visibly bad job on the day. The calculator gives you the quantities to discuss and to buy against. The installation is a job for a qualified electrician, and the certificate they issue is the point of them.