Conduit fill calculator

How the calculation actually works

Conduit fill is one division. Add up the cross sectional area of everything going in the pipe, divide by the internal area of the pipe, and compare the result to a percentage. All the difficulty is in picking the right percentage and the right two areas.

Step one: the conduit's internal area

Trade size is a name, not a measurement. Half inch EMT has a bore of 0.622 inches, half inch rigid has 0.632, half inch schedule 40 PVC has 0.602 and half inch schedule 80 PVC has 0.526. Turned into area that spread is much wider than it looks: 0.304 square inches for the EMT against 0.217 for the schedule 80, a difference of 29 percent in the same nominal pipe. Schedule 80 has the same outside diameter as schedule 40 and a much thicker wall, and every thousandth of that wall comes out of the bore.

Area is pi over four times the diameter squared. Everything printed on this page is computed that way from a diameter, not looked up.

Step two: the conductors

Each insulated conductor has a published approximate diameter, which depends on the conductor size and on the insulation over it. Square it, multiply by pi over four, multiply by how many you have, and add the lot together.

Insulation matters more than people expect. A 500 kcmil conductor is 0.7073 square inches in THHN, 0.6984 in XHHW, and 1.0082 in rubber insulated RHH with an outer covering. That last one is 43 percent more pipe for exactly the same copper.

Count everything. Phases, the neutral, travellers, spares you are pulling for later, and the equipment grounding conductor. The ground occupies space like anything else and it counts toward the conductor total that selects your limit.

Step three: the limit

One conductor, 53 percent. Exactly two conductors, 31 percent. Three or more, 40 percent. A nipple of 24 inches or less between two enclosures, 60 percent.

A worked example

A feeder: three 3/0 THHN phase conductors and one 6 AWG THHN equipment ground, in EMT.

Conductor area

3/0 THHN is 0.584 inches across, so 0.2679 square inches each, and three of them is 0.8036. The 6 AWG ground is 0.254 inches across, so 0.0507. Total 0.8543 square inches.

The limit

Four conductors, so three or more, so 40 percent.

Try 1-1/4 inch EMT

Bore 1.380 inches, area 1.4954 square inches, 40 percent of that is 0.5982. We need 0.8543. Not close.

Try 1-1/2 inch EMT

Bore 1.610 inches, area 2.0358 square inches, 40 percent of that is 0.8143. We need 0.8543, so this is 41.96 percent full and it fails, by four hundredths of a square inch. It looks like it should fit and it does not.

Try 2 inch EMT

Bore 2.067 inches, area 3.3556 square inches, 40 percent is 1.3422. At 0.8543 the run is 25.5 percent full and it passes with plenty in hand.

The two conductor trap

Take two 6 AWG THHN conductors in half inch EMT. Each is 0.0507 square inches, so 0.1013 together, in a bore of 0.3039 square inches. That is 33.3 percent full.

Against 40 percent it passes with room to spare, and that is what most calculators and most people will tell you. But two conductors are held to 31 percent, which is 0.0942 square inches, and 0.1013 is over it. The run is illegal, and the tool that told you it was fine did so by applying the three or more rule to a case it does not cover.

The reason 31 is lower than both of its neighbours is that two circles in a circle waste space that three or more do not, and a two conductor pull has no third conductor to roll against. It is counterintuitive, it looks like a misprint, and it is the single most reliable source of wrong conduit fill answers there is.

Where this tool will mislead you

It does not derate for heat. More than three current carrying conductors in a raceway means their ampacity has to be reduced, and that is an entirely separate rule with its own table. Four to six conductors lose 20 percent of their ampacity, seven to nine lose 30 percent. A run can sit at 35 percent fill and still be undersized once that is applied. This tool answers the fill question only. The ampacity and derating calculator does the other half.

It does not know about bends. Fill says nothing about how hard the pull will be. Total bend is capped separately, and a run at 39 percent through four bends is a different job from the same run in a straight line.

It uses standardised conductor diameters. Real wire from a real reel is usually within a few thousandths of the published figure and sometimes not. The published figure is what an inspector checks, so it is the right basis for a verdict, but it is not a promise about your reel.

It does not cover everything that goes in a pipe. Compact stranded conductors, bare conductors and multiconductor cables all have their own dimensions and none of them are here. For a multiconductor cable you use the actual outside diameter of the cable, and if there is exactly one of them in the raceway there is a further rule about non circular cables that this tool does not handle.

It is not an inspection. Your local jurisdiction may amend or may be on an older code cycle, and either can change the answer.

Terminology

Trade size

The name a conduit is sold under. Not its bore and not its outside diameter. Two conduits of the same trade size in different types hold different amounts.

Bore

Internal diameter. The only conduit dimension that matters for fill.

Equipment grounding conductor

The bonding conductor that gives fault current a path back. Counts toward fill and toward the conductor count, and never counts as a current carrying conductor for derating.

Nipple

A conduit run of 24 inches or less between two enclosures. May be filled to 60 percent.

Jam ratio

Conduit bore divided by conductor diameter, for three conductors of the same size. Between about 2.8 and 3.2 they can wedge at a bend.

kcmil

Thousands of circular mils, the area unit used above 4/0. A circular mil is the area of a circle one thousandth of an inch across, so it is a conductor area, not an insulated diameter.