AmpSizerBreaker, conduit and box calculators
The conductor is only part of it. These work out the device in front of it, the pipe around it, the box at the end of it, and whether the circuit you already have can take one more thing.
Box fill calculator
Conductors, devices and clamps in. Cubic inches and a box that fits out.
Breaker size calculator
Watts or amps in, the standard device rating and the wire under it out.
Cable tray fill calculator
Conductor diameters added up against the tray width you have.
Circuit load calculator
Watts on the circuit in. How much of the breaker is left out.
Conduit fill calculator
Conductors in, fill percentage and the smallest trade size that takes them out.
Sub panel feeder size calculator
Panel amps and distance in. Feeder, neutral, ground and conduit out.
Conduit fill is the second most searched question in this whole subject after voltage drop, and it is the one where a wrong answer shows up physically: conductors that will not pull, insulation scraped on the way in, and a fault years later in a pipe nobody wants to open.
Box fill is the same problem at the other end of the run, and it is the single most common thing an inspector writes up. Both are arithmetic on areas and volumes published in the code, and both are computed here from those tables rather than from a chart somebody retyped.
Frequently asked questions
What fill percentage am I allowed in a conduit?
53% for a single conductor, 31% for exactly two, and 40% for three or more — Chapter 9, Table 1. A nipple no longer than 24 inches between enclosures gets 60%. The counter-intuitive part is that two conductors get the tightest limit of the three, because of how they can jam against each other going in.
Does the ground wire count toward conduit fill?
Yes. Fill is about physical space, so every conductor in the pipe counts — grounds, neutrals, travellers, all of it. Derating is the opposite: there, the equipment grounding conductor never counts, and the neutral of a balanced multiwire circuit does not either.
Do I count the pigtail in box fill?
A conductor that comes into the box and leaves again counts once. A conductor that comes in and ends there counts once. A pigtail or jumper that both begins and ends inside the box counts as nothing at all, which is 314.16(B)(1) — and it is the exception people either forget or apply to everything.
Is 80% of the breaker a code rule?
Not as a blanket rule. What the code says is that a circuit supplying a continuous load — anything on for three hours or more — is sized at 125% of that load, which works out to the same thing from the other direction. For a bank of intermittent loads there is no 80% ceiling, only the breaker rating.
How we calculate this
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