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National Electrical Code Explanations: Conduit Fill

by Mark Lamendola

What's the rule on conduit fill? The answer is there are many rules!

First, let's expand things a bit. Conduit is a specific kind of raceway. So, we're really talking about raceway fill--whether that raceway is conduit, EMT, NMT or some other kind of raceway. The NEC index cross-references "conduit fill" as conductor fill.

The basic NEC reference is 300.17. The NEC does not provide a specific fill number, here. It merely says the number and size of conductors can't be more than will permit heat dissipation and the ready withdrawal of conductors without damaging them.

Notice, I said the basic NEC reference. Just below 300.17, you'll find an FPN. This one happens to be highly detailed. The fill requirements are specified by first by raceway type in 342.22 - 388.22. Then, they are specified by application as follows:

  • Underfloor, 390.5
  • Fixture wire, 402.7
  • Theaters, 520.6
  • Signs, 600.31(C)
  • Audio signal processing,  640.23 and 640.24
  • Class I, II, III circuits, Article 725
  • Fire alarm circuits, Article 760
  • Fiberoptics, Article 770.

Now, you can simplify all of this by understanding something the FPN doesn't tell you. Most of these various references tell you to use Table 1 of Chapter 9. It's probably left out of the FPN to avoid duplication of information (the result of which is invariably conflict and confusion), and to allow each standards committee to decide whether to refer to Table 1 or not. Article 770, for example, says that Table 1 does not apply [770.12(A)].

Going to Table 1, Chapter 9, we don't find a whole heck of a lot. One wire can fill only 53% of a raceway, and two wires can fill only 31%. And if you have more than 2 conductors in a raceway, the maximum fill is 40%.

That 40% is subject to further downward adjustment, though. The Authority Having Jurisdiction (AHJ) can require even less fill--so, use common sense. Read the FPN in Table 1 thoroughly. Don't assume that 50% fill is OK, because it's close. It's not close--it's over the limit. But don't assume 40% is always OK, either. Circumstances may dictate otherwise, and the NEC provides some examples on that point.

Some people get confused on which conductors count for raceway fill. They all count. The confusion results from misapplying 310.15(B)(6) to raceway fill. But 310.15(B)(6) is is about ampacity calculations, not raceway fill calculations. You exclude grounding or bonding conductor(s) when determining the number of current carrying conductors for purposes of selecting an ampacity table. But you don't exclude them from determining the raceway fill. Keep Articles 300 and 310 separate!

Do the math

Conduit fill requires calculation. In the past, this involved determining the circular mils for various conductors. Fortunately, the NEC now provides tables to reduce the amount of calculation in the field.

To determine how much wire you can run in a given raceway:

  1. Find the raceway type and size you're running, in Table 4 of Chapter 9. The number you want is Total Area. This is expressed in square inches. Multiply by 0.4, and you'll know your total permissible wire fill for that raceway.
  2. Find the wire type and size you're running, in Table 5 of Chapter 9. The number you want is approximate area. This is expressed in square inches.
  3. Add the results of Step 2 for each wire you want to run as you go, and stop when you reach your total permissible wire fill for that raceway. Most likely, you'll stop before you reach it, as you are not allowed to exceed it.

To determine what size raceway you need for a given wire run:

  1. Find the wire type and size you're running, in Table 5 of Chapter 9. The number you want is Approximate Area. This is expressed in square inches.
  2. Perform Step 1 for each wire you want to run, and add the results as you go. The final total is your total wire area. Divide this number by 0.40, and you'll know your minimum raceway area.
  3. Find the raceway type you're running, in Table 4 of Chapter 9. Look down the column until you find an area number that is greater than the minimum raceway area you calculated in Step 2. You must use a raceway at least this size.