Conductor Ampacity and Wire Sizing for the C-10 Exam
Wire-sizing questions are some of the most predictable points on the C-10 exam. The trouble isn't that they're hard — it's that people memorize one column of one table and then get blindsided when the question layers in a termination rating, a hot attic, or a bundle of conductors in a raceway. Get the sequence right and these turn into free points.
Here's the mental model. A conductor has a theoretical ampacity straight from the table, but the number you're actually allowed to use is whatever survives after you check the terminals and apply any derating. Table first, reality second.
Start with Table 310.16
Table 310.16 is the workhorse. It gives allowable ampacities for insulated conductors rated up to 2000V, run in a raceway, cable, or directly in earth, based on an ambient of 30C (86F). Every column you care about lives here: the 60C, 75C, and 90C insulation ratings, for both copper and aluminum.
A handful of values are worth burning into memory because they show up constantly. At the 75C copper column: #12 is 25A, #10 is 35A, #8 is 50A, #6 is 65A, #4 is 85A, #1 is 130A, 1/0 is 150A, 2/0 is 175A, 3/0 is 200A. Aluminum runs roughly two sizes behind copper for the same ampacity — that's the intuition to carry, not another table to recite.
Knowing the table value is only step one. The exam wants to know whether you understand which column you're actually allowed to read.
The termination-temperature rule: 110.14(C)
This rule trips up more candidates than any other ampacity concept. Section 110.14(C) says the temperature rating of the conductor has to be coordinated with the lowest-rated component in the circuit — the wire, the lugs, the breaker, the terminal. You size to the weakest link.
In practice, most equipment terminals are rated only 60C or 75C even when the wire itself is a 90C insulation type like THHN. So even though THHN copper #6 shows 75A in the 90C column, you can't use 75A if it lands on a 75C-rated lug. For standard equipment the rule shakes out like this:
| Circuit rating | Terminal rating assumed | Column to use |
|---|---|---|
| 100A or less, or #14–#1 wire | 60C (unless marked otherwise) | 60C column |
| Over 100A, or larger than #1 wire | 75C | 75C column |
The catch that catches everyone: equipment can be marked and listed for 75C terminals, and most modern panels and breakers are. When the equipment is rated 75C, you get the 75C column even on small circuits. Read the question — it will tell you the terminal rating, or expect you to assume 60C for the small stuff.
So why does the 90C column even exist if you almost never get to use it directly? Because it's the starting point for derating. You begin with the 90C ampacity, apply your correction and adjustment factors to that higher number, then compare the derated result against the termination-limited ampacity. The final answer is the lower of the two. That two-track check is exactly the kind of thing the exam builds a question around.
Correction and adjustment factors
Two things force you to knock the ampacity down, and they multiply together.
Ambient temperature correction — Table 310.15(B)(1). The base table assumes 30C. Run conductors through a 50C attic and copper carries less current, so you multiply by a correction factor below 1. Hotter ambient, smaller factor.
Conductor bundling adjustment — Table 310.15(C)(1). When you have more than three current-carrying conductors in a raceway or cable, heat can't escape, so you derate: 4–6 conductors drop to 80%, 7–9 to 70%, 10–20 to 50%. Note the phrase "current-carrying" — a grounded neutral that only carries unbalanced current on a normal circuit usually doesn't count, and the equipment grounding conductor never counts.
When both apply, start at the 90C ampacity and multiply by both factors. Say THHN copper #6 is 75A at 90C. In a 46C ambient (roughly a 0.87 correction) with seven current-carrying conductors (0.70 adjustment): 75 × 0.87 × 0.70 = about 46A. Then check that against the termination limit — #6 copper at 75C is 65A — and use the lower number, 46A. This is a classic multi-step problem, and it rewards doing the steps in order instead of guessing.
To see how this feeds the bigger picture, ampacity is the front half of nearly every dwelling load calculation and it sets up choosing the right overcurrent device. It also runs into conduit fill limits, since bundling more conductors both fills the pipe and triggers the adjustment factor.
The 83% dwelling service rule: 310.12
Here's the one that saves real copper and shows up as its own question type. Section 310.12 allows single-phase dwelling services and main power feeders — the feeder carrying the total current supplied by that service — to be sized at 83% of the service rating.
That's why a 200A residential service doesn't need a conductor rated for a full 200A. 200 × 0.83 = 166A, and 2/0 copper at the 75C column carries 175A — plenty. Aluminum works out to 4/0 for the same 200A service. For a 100A service the rule points you at #4 copper or #2 aluminum. The Code even includes a table in 310.12 that lists these common service sizes so you don't have to run the math every time — but understanding why the 83% multiplier exists is what lets you answer a question worded a little differently than you expect.
One boundary to keep straight: 310.12 applies to the service and main feeder of a dwelling, not to every feeder in the building and not to commercial services. Branch circuits and sub-feeders go back to standard sizing. The exam loves to slip a commercial or sub-feeder scenario in to see if you'll wrongly apply the 83% break.
Putting the sequence together
Every wire-sizing problem follows the same order:
- Find the load in amps.
- Look up the conductor in Table 310.16 — usually starting at the 90C column if you'll be derating.
- Apply ambient correction (Table 310.15(B)(1)) and bundling adjustment (Table 310.15(C)(1)).
- Check the termination rating per 110.14(C) — 60C or 75C.
- The allowable ampacity is the lower of the derated value and the termination-limited value.
- For a dwelling service or main feeder, remember the 83% option in 310.12.
Do that in order and you won't get fooled by the layered questions, which is most of them. If you're still shaky on where these problems fit in the exam overall, the C-10 electrical exam study guide maps out how much weight the trade calculations carry.
These concepts are explained here to help you study; the printed 2025 California Electrical Code always governs on exam day and on the job, so work from the current tables. Sit down with a stack of timed ampacity problems and drill the six-step sequence until it's automatic. Get comfortable reading Table 310.16 under a clock, and these questions become the easy ones.