Conduit fill, worked the way the exam wants it

Three lookups and one division. Most people who miss this question miss it on the percentage, not the arithmetic.

What the question is really asking

A conduit fill question gives you a number of conductors, a wire size, an insulation type and a raceway type. It wants to know whether they fit, or what size raceway they need.

Underneath, it is one comparison: total conductor area against allowed raceway area. Everything else is looking up the two sides of that comparison.

The percentage comes first, and it is where people lose the point

Before you touch a conductor area, decide what percentage of the raceway you are allowed to fill. Chapter 9, Table 1 sets it, and it depends only on how many conductors you have.

There are three cases: one conductor, two conductors, and more than two. They are three different percentages, and the one for two conductors is the lowest of the three.

That ordering is not intuitive. People assume more conductors means a lower allowance, so they reach for the two-conductor number when they have four. Read the row you are actually in.

Open your code book to Chapter 9, Table 1 and read the three values now. They have been stable for a long time and they are worth knowing cold.

Then the conductor area

Chapter 9, Table 5 lists the approximate area of insulated conductors, in square inches, by insulation type and size.

Type matters as much as size. THHN and XHHW at the same wire size are not the same area, because the insulation thickness differs. A question that says XHHW and gets answered with a THHN area is wrong by a margin large enough to change the raceway size.

Find your insulation type in the left column first, then your size. Multiply that area by the number of conductors of that type. If the question mixes types, do each type separately and add the totals.

Then the raceway

Chapter 9, Table 4 gives raceway areas, and it is organized by article. Electrical metallic tubing lives under Article 358. Rigid metal conduit under Article 344. Each raceway type gets its own block.

The table has already done the percentage arithmetic for you. It prints a column for each of the three fill cases alongside the total area. So once you know which case you are in, you read that column directly rather than multiplying.

This is the shortcut most candidates miss under time pressure. They compute forty percent of the total area by hand when the column beside it already holds the answer.

The comparison

1. Count the conductors and pick the fill case from Chapter 9, Table 1.
2. Look up each conductor area in Chapter 9, Table 5 and total them.
3. Go to Chapter 9, Table 4, find your raceway type by article, and read down the column for your fill case.
4. Choose the first trade size whose column value equals or exceeds your total.

If the question instead gives you the raceway and asks how many conductors fit, run it backwards: read the allowed area from Table 4, divide by the single conductor area from Table 5, and take the whole number. Never round up. A partial conductor is not a conductor.

Two traps

The first is Annex C. It contains ready-made answers for raceways filled entirely with one size and type of conductor. It is faster than the long method and it is legal. But it only applies when every conductor is identical, and exam questions frequently mix sizes to close that door. Check whether the conductors are truly all the same before reaching for it.

The second is the equipment grounding conductor. It occupies space and it counts toward fill. It is a conductor for this purpose even though it carries no current in normal operation. Leaving it out is a quiet way to get a smaller raceway than the question wants.

Why the answers are all lookups

Every value in this method lives in your code book. That is deliberate on the exam's part. Conduit fill is not testing whether you memorized areas. It is testing whether you can find three tables under time pressure and keep them straight.

The practice worth doing is not memorizing the numbers. It is running the sequence until the page turns are automatic.

What this page cites

• NEC Chapter 9, Table 1 Percent of cross section of conduit and tubing for conductors. Read it in your own code book.
• NEC Chapter 9, Table 4 Dimensions and percent area of conduit and tubing, organized by article.
• NEC Chapter 9, Table 5 Dimensions of insulated conductors and fixture wires.
• NEC Annex C Conductor and fixture wire counts for raceways filled with a single size and type.