Wire Length Calculator

This answers two questions about the length of a wire run. The first runs the voltage drop formula backwards: give a conductor size, the current, the voltage and a drop limit, and it returns the longest one-way run that stays inside it. The second turns a route into an order, adding slack at both ends, multiplying by the conductors in the run and the number of runs, and counting the spools. It does not size a conductor, because a length that passes voltage drop says nothing about whether the wire can carry the current at all.

Calculator Type
Run Measured In
Sets the run you type and every length in the answer. The drop itself is the same either way.

You know the conductor and how far it goes. This gives the drop over that run, in volts and as a percentage.

The current the circuit will actually carry, not the breaker size.
The distance from the supply to the load, not there and back. The formula doubles it for you.
Only used to say whether the run is inside the limit. The NEC has no mandatory one.
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You already know the conductor. This gives the furthest you can take it before the voltage drop passes the limit you set.

The current the circuit will actually carry, not the breaker size.
The NEC has no mandatory limit. 3 percent on a branch circuit and 5 percent overall are a recommendation in an informational note to 210.19(A).
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Lengths In
Sets the route and the slack you type, the price per length, and every length in the answer. The spool sizes are products and are listed in their own units either way.

Turns a route into an order: slack at both ends, more than one conductor in the run, more than one run, and the spool it comes on.

The path the cable actually takes, up and along and down, not the straight line across the room.
Individual conductors pulled into a raceway, counting the neutral and the ground. Enter 1 for a jacketed cable such as 12/2, which is sold by the cable foot.
Enough tail to reach into the box and make up the connection. Counted at both ends of every run. Left blank it is 3 ft, which is 0.91 m.
For a mismeasured route, a pull that has to be redone and the end of a spool that is too short to use.
A packaged length, so it is a product and not a measurement: pick what your wholesaler actually racks. The lengths themselves do not follow the unit choice above, because a 500 ft reel is a 500 ft reel wherever it is sold; the bracketed figure beside each one is the same spool in the unit you chose.
Per foot of the thing you are buying, so per cable foot for a jacketed cable and per conductor foot for single conductors.
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