How to Size Wire for Continuous and Non-Continuous Loads

Sizing Wire Isn’t Guesswork — It’s a Process

One of the most common calculation topics on any electrician’s exam is conductor sizing. It shows up again and again, and yet it trips up test-takers because they skip steps or read the wrong part of the table. In Lesson 9.2 of his ampacity series, the Electrical Code Coach breaks the process down into a repeatable routine that works every single time.

The good news? Once you understand the two core rules and follow the same steps for every problem, sizing wire for continuous and non-continuous loads becomes almost automatic. Let’s walk through it.

The Two Rules You Must Recap First

Before touching a single problem, the Code Coach reminds us of the fundamentals that every sizing question is built on:

“If the question states that it terminates to 60, 75 or 90 degree C terminals, we’re going to make our selection from that respective column under normal circumstances.”

And the second rule, which mirrors what you already learned about overcurrent protection:

“We’re going to do 125% demand factor for all loads that are expected to run for three or more hours.”

That’s the entire foundation. Terminal rating tells you which column to read in the ampacity table, and the continuous-load status tells you whether to multiply by 1.25. Keep both in mind and you’re most of the way there.

There’s one more principle repeated throughout the lesson that you should tattoo on your brain: all wire must meet or exceed the known load. You never round down to a conductor that carries less than your calculated value.

Example 1: A Straightforward Non-Continuous Load

Question: What size copper conductor would you select for a 40 amp load using THW, terminating on 75 degree C terminals?

The process starts the same way it always does — find the total connected load:

• The known load is 40 amps.
• Check for demand factors. The question doesn’t mention specific equipment or that the load is continuous, so there are no demand factors to apply.
• Our target stays at 40 amps, and our wire must meet or exceed it.

Now we head to the primary ampacity table (NEC Table 310.16). The conductor is copper, so we work from the left-hand side. We’re in the 75 degree C column because that’s our terminal rating. Slide down until we find a conductor that meets or exceeds 40 amps, and we land on an 8 AWG copper.

Example 2: Adding the Continuous-Load Multiplier

Question: What size copper conductor would you select for a 60 amp load running for more than 5 hours, using THHN, terminating on 75 degree C terminals?

Here’s where the 125% rule earns its keep. “Running for more than 5 hours” is the tell — that’s well beyond the three-hour threshold, so this is a continuous load.

• Starting known load: 60 amps.
• Apply the continuous-load math: 60 × 1.25 = 75 amps.
• Our new known load is 75 amps, and the wire must meet or exceed that.

Note that although the wire is THHN (a 90 degree C insulation), we still select from the 75 degree C column because the terminals are rated 75 degrees C. Reading down the copper side, a conductor that meets or exceeds 75 amps is a 4 AWG copper.

Example 3: Continuous Load, Smaller Amperage

Question: What size copper conductor would you select for a 40 amp load that’s considered a continuous load, using THHN, terminating on 75 degree C terminals?

This one reinforces the same pattern with a different starting point:

• Starting known load: 40 amps.
• The question already tells us it’s continuous, so multiply: 40 × 1.25 = 50 amps.
• Wire must meet or exceed 50 amps.

Copper, left-hand side, 75 degree C column. The conductor that satisfies 50 amps is an 8 AWG copper. Notice that even after the 125% bump, a 50 amp requirement still lands on an 8 AWG — a good reminder to always run the math and check the table rather than assuming a bigger conductor is needed.

Example 4: Switching to Aluminium

Question: What size aluminium conductor would you select for a 90 amp load running for more than 5 hours, using THHN, terminating on 75 degree C terminals?

The steps are identical, but this problem hides a common trap:

• Starting known load: 90 amps.
• Running more than 5 hours means continuous, so multiply: 90 × 1.25 = 112.5 amps.
• Wire must meet or exceed 112.5 amps.

Now for the trap. The conductor is aluminium, so we must read from the right-hand side of Table 310.16, not the left. As the Code Coach warns:

“This time we have to be careful, ‘cause they are wanting aluminium conductors, so we’re actually going to start on the right-hand side of this table.”

Staying in the 75 degree C column on the aluminium side, the conductor that meets or exceeds 112.5 amps is a 1/0 AWG aluminium. If you’d absent-mindedly read the copper column, you’d have picked the wrong wire — which is exactly why this step catches so many people.

The Repeatable Routine

Every single problem in this lesson followed the same sequence. Commit it to memory and you can handle any continuous/non-continuous sizing question:

1. Find the total connected load — start with the stated amperage.
2. Check for demand factors — is it continuous (three hours or more)? If yes, multiply by 1.25 to get your new known load.
3. Identify the conductor material — copper (left side of Table 310.16) or aluminium (right side).
4. Choose the correct temperature column — match the terminal rating (60, 75 or 90 degree C).
5. Slide down and select — pick the first conductor that meets or exceeds your known load.

The beauty of this method is its consistency. Whether the load is 40 amps or 90 amps, copper or aluminium, continuous or not, the routine never changes — only the numbers do.

Common Mistakes to Avoid

• Forgetting the 125% multiplier on a continuous load. Watch for phrases like “continuous load” or “running for more than three/five hours.”
• Reading the wrong side of the table — copper is left, aluminium is right.
• Choosing the column by insulation type instead of terminal rating. THHN is a 90 degree C wire, but if it terminates on 75 degree C terminals, you use the 75 degree C column.
• Rounding down. Your conductor must always meet or exceed the known load, never fall short of it.

How NEC Mastery Fits Into This

Conductor sizing is a skill you build through repetition — the more problems you work, the faster the routine becomes second nature. That’s exactly what NEC Mastery is designed for.

• 8,000+ exam-style questions give you endless ampacity and load-sizing problems to drill, so applying the 125% rule and reading Table 310.16 becomes automatic.
• Detailed explanations referencing specific NEC articles walk you through each step — which column to use, which side of the table, and why — reinforcing the process the Code Coach teaches here.
• Timed mock exams weighted to your exam type let you practise calculation questions under real pressure, building the speed and confidence to finish with time to spare.

Learn the routine once, then prove to yourself through practice that you can run it every time. As the Code Coach says: don’t give up, keep grinding — you can do it.