Raceway Fill with Mixed Conductor Sizes – NEC Chapter 9 Explained

Why Mixed Conductor Fills Trip People Up

If you’ve already learnt how to calculate raceway fill when all the conductors are the same size, you know it’s fairly straightforward — you can go straight to the informative annexes and look up how many conductors fit. But the moment you have a mixture of conductor types and sizes, that shortcut disappears. You have to work through each conductor individually using the Chapter 9 tables, and that’s where mistakes creep in on exam day.

Paul Abernathy — a veteran NEC instructor of nearly 40 years who serves on Code Making Panel 5 and Code Panel 17 — walks through exactly how to handle this in a detailed breakdown. Let’s follow the same process step by step.

The Setup: What’s Going in the Raceway?

The problem involves determining the correct trade size of rigid metal conduit (RMC) for the following mix of conductors:

• Four 10 AWG THW
• Two 12 AWG RHW without outer covering
• Two 14 AWG RHH with outer covering
• Six 14 AWG THW
• Three 12 AWG bare stranded equipment grounding conductors

The very first thing to recognise is that with this many conductors, you’re well over two — which means Chapter 9 Table 1 tells you the maximum fill is 40%. That number is locked in before you do any maths.

“We didn’t get past the first one when we knew we had four tens. So, we’re already over two. So, 40% is our maximum.”

Step 1: Write Everything Down

Before jumping into the tables, write down every conductor type, size, and quantity on your scratch paper. This sounds basic, but it saves you from flipping back and forth between the question and the code book under time pressure.

“Get used to taking good notes… you want to write these things down so you’re not having to go back and forth, back and forth, back and forth.”

This is practical exam strategy. Every time you flip pages unnecessarily, you’re burning seconds you can’t get back.

Step 2: Look Up Each Conductor’s Area

Now comes the core of the calculation. For insulated conductors, you go to Chapter 9 Table 5 (Dimensions of Insulated Conductors and Fixture Wires). For bare conductors, you go to Chapter 9 Table 8 (Conductor Properties).

The critical column is Approximate Area — not diameter. Here’s where each value comes from:

10 AWG THW — Find THW in Table 5, go to the 10 AWG row:

• Area = 0.0243 sq. in.
• 4 conductors: 0.0243 x 4 = 0.0972

12 AWG RHW without outer covering — This is the one with the asterisk (*) in Table 5. The asterisk indicates no outer covering:

• Area = 0.0260 sq. in.
• 2 conductors: 0.0260 x 2 = 0.0520

14 AWG RHH with outer covering — No asterisk, meaning it has the covering (larger area):

• Area = 0.0293 sq. in.
• 2 conductors: 0.0293 x 2 = 0.0586

14 AWG THW — Back to THW in Table 5, 14 AWG row:

• Area = 0.0139 sq. in.
• 6 conductors: 0.0139 x 6 = 0.0834

12 AWG bare stranded EGC — This one uses Table 8 instead of Table 5, since there’s no insulation. Select the 7-strand row (stranded, not solid):

• Area = 0.0060 sq. in.
• 3 conductors: 0.0060 x 3 = 0.0180

The Asterisk Trap: Covered vs. Uncovered

This is the detail that catches people on exams. In Chapter 9 Table 5, RHH, RHW, and RHW-2 entries appear in two forms:

• With an asterisk (*) = without outer covering (smaller area)
• Without an asterisk = with outer covering (larger area)

If you grab the wrong value, you’ll calculate a different total area and potentially select the wrong raceway size. On a multiple-choice exam, that slight difference is often the gap between the right answer and a wrong one.

“On an exam, if they’re giving you those, there may be a fact that that slight difference in that covering is a difference between you getting the question right or wrong.”

Step 3: Add Up the Total Area

Once you have all five values, add them together:

Conductor	Area per Conductor	Qty	Subtotal
10 AWG THW	0.0243	4	0.0972
12 AWG RHW (no covering)	0.0260	2	0.0520
14 AWG RHH (with covering)	0.0293	2	0.0586
14 AWG THW	0.0139	6	0.0834
12 AWG bare stranded EGC	0.0060	3	0.0180
		Total:	0.3092

Step 4: Find the Right Raceway Size

Now take that total — 0.3092 square inches — and go to the Chapter 9 Table 4 entry for rigid metal conduit (Article 344). Look at the Over 2 Wires (40%) column:

• 3/4” trade size: 40% fill = 0.220 sq. in. — too small (0.3092 exceeds 0.220)
• 1” trade size: 40% fill = 0.355 sq. in. — this works (0.3092 fits within 0.355)

The answer is trade size 1.

Could you use 1-1/4”? Technically yes — but on the exam, you always select the best possible answer, which is the smallest conduit that doesn’t exceed the fill percentage.

“Even though you could argue, well, both of them are right, it’s not right when it comes to an electrical exam because it always wants the best possible answer.”

Quick Reference: The Process

For any mixed-conductor raceway fill problem, the method is always the same:

1. Determine the fill percentage — Chapter 9 Table 1 (40% for over two conductors)
2. Look up each conductor’s area — Table 5 for insulated, Table 8 for bare (watch for solid vs. stranded)
3. Multiply each area by its quantity and add up the total
4. Find the raceway size — Go to the correct Table 4 entry for your raceway type and find the smallest size whose 40% column meets or exceeds your total

Once you’ve practised this process a dozen times, you can work through these problems in under two minutes. The first time through, take it slow and make sure you understand each table lookup.

How NEC Mastery Fits Into This Strategy

Raceway fill calculations with mixed conductor sizes are a staple on electrician licensing exams — and the only way to get fast at them is repetition. Paul Abernathy can do these in two minutes because he’s done thousands of them. You need the same volume of practice.

• 8,000+ exam-style questions include dozens of raceway fill problems with varying conductor types, covering scenarios, and raceway materials — so you build speed through sheer repetition
• Detailed explanations referencing specific NEC articles walk you through exactly which Chapter 9 tables to use and why, reinforcing the Table 5 vs. Table 8 distinction and the asterisk trap every time you review an answer
• Timed mock exams weighted to your exam type let you practise these calculations under real time pressure — because knowing the process is only half the battle; doing it quickly enough to finish with time to spare is what actually passes the exam
• No shortcuts, just structured practice — just like Paul says, there’s no way around working through each conductor individually. NEC Mastery gives you the volume of problems you need to make that process second nature