Sizing Breakers and Fuses: Table 240.6(A) and the Next Size Up Rule

Breakers and Fuses Are All OCPDs

Every electrician sizes breakers, but on the exam you need to know exactly which rules you’re applying and why. In Lesson 8.1 of his video series, the Electrical Code Coach works through Article 240 and the basics of sizing overcurrent protective devices.

You’ll see the term OCPD all over exam prep material and in the code itself. It stands for overcurrent protective device, but the Code Coach puts it more simply:

“Anytime you see OCPD it just means breakers and fuses — whether it’s the individual breaker at a branch circuit, 20 amp, or it’s a 100 amp feeder breaker, or our main 200 amp disconnect.”

Once you think of OCPDs as “breakers and fuses”, Article 240 is much easier to work through.

Table 240.6(A): The Table That Unifies the Industry

The Code Coach calls Table 240.6(A) one of his favourite tables in the NEC. It doesn’t look like much, but you’ll come back to it constantly.

As with any table, start by reading the heading so you know you’re in the right place. This one covers standard ampere ratings for fuses and inverse time circuit breakers.

Here’s how the table is laid out:

• In the 2023 NEC and later, it starts at 10 amps
• In the 2020 NEC and earlier, it starts at 15 amps
• From there it goes up in steps of 5: 15, 20, 25, 30, 35, 40, 45, 50
• After 50 it goes up in steps of 10, and the gaps get bigger at higher amperages

Why does such a simple table matter so much? Because everyone in the trade works from the same list:

“The manufacturers make these size breakers, the stores stock these size breakers, installers purchase these size breakers, and when we are doing our load calculations in the NEC, these are the sizes that we’re actually going to use.”

Get your codebook out and find this table now. You’ll use it in nearly every calculation question on the exam.

These Are the General Rules

The Code Coach calls these general OCPD rules for a reason. They apply across the whole NEC unless another section tells you otherwise. Other parts of the code can modify or override them, and motor overload protection is a good example. Treat these rules as your default, and watch for the exceptions as you get further into the code.

He covers three main points.

Rule 1: The Next Size Up Rule

Start with what the Code Coach calls a known load. Say you’ve worked out a load of 59 amps. Is there a 59 amp breaker? No.

So you go to Table 240.6(A) and find the next standard size above your load:

1. Run down the table: 15, 20, 25, 30… 45, 50
2. There’s no 55, so after 50 the next size is 60
3. A 59 amp load goes on a 60 amp OCPD

This can feel uncomfortable when you’re new, because the breaker is larger than the load. The Code Coach says that’s fine. Later in the program you’ll learn the conductor sizing rules that make sure everything stays protected.

Rule 2: Continuous Loads at 125%

The second point is continuous loads. You may also see the term continuous duty. A continuous load is one where the maximum current is expected to continue for three hours or more.

On the exam, a question might look like this: “A load is known to run for 5 hours. What size breaker is required?” It might say 5 hours or 10 hours. The number doesn’t matter. If it’s three hours or more, it’s a continuous load.

Exam writers may also use the word “indefinitely” to throw you off. It means the same thing, so apply the 125% multiplier. As the Code Coach puts it, the multiplier is “just another demand factor.”

Worked Example: 59 Amp Continuous Load

Take the same 59 amp load, but now the question says it’s continuous:

1. Apply the multiplier: 59 A × 1.25 = 73.75 A
2. Replace the old load: The 59 amps is gone. 73.75 A is your new known load, and you size the breaker (and later the wire) from it
3. Go to Table 240.6(A): 73.75 is above 70, so the next standard size is 80
4. Answer: an 80 amp OCPD

“That old load is gone, the 59 is gone. The 73.75 is our new known load and that’s what we would go size our breaker and subsequently our wire by.”

A common exam mistake is sizing from the original load after calculating the 125% figure. Once you’ve applied the multiplier, the original number is no longer your design load.

Rule 3: The OCPD Sets the Service Size

The third point catches a lot of people out: the size of the main OCPD sets the size of the service.

Say a whole house load calculation comes to 180 amps. You need to size the main breaker, so you go to Table 240.6(A), take the next standard size up, and choose a 200 amp main.

So what size is the service? Suppose the panel’s bus is rated at 225 amps. Is it a 225 amp service?

No. The service size comes from the main breaker or main fuse, not the equipment’s bus rating. This is a 200 amp service.

Each step depends on the one before it:

• The load calculation sets the breaker size
• The breaker size sets the service size

The Code Coach says this will matter more and more as the program covers service and feeder calculations.

Quick Reference Summary

Concept	Rule	Example
Standard sizes	Use Table 240.6(A)	2023 NEC starts at 10 A; 2020 starts at 15 A
Next size up	No exact match? Go to the next standard size	59 A load → 60 A OCPD
Continuous loads	3+ hours (or “indefinitely”) → × 1.25	59 A × 1.25 = 73.75 A → 80 A OCPD
Service size	Set by the main OCPD, not the bus rating	180 A calc → 200 A main → 200 A service

The Bottom Line

Sizing breakers and fuses comes down to a few habits:

1. Know Table 240.6(A) and be able to find the next standard size quickly
2. Look for continuous load wording such as “three hours”, “five hours” or “indefinitely”, and multiply by 125% first
3. Replace the old load with the new figure. Don’t size from the original number
4. Remember the order: the load sets the breaker, and the breaker sets the service

These are the general rules. Other sections, such as those for motors, can change them, so read each question carefully and check whether a more specific article applies.

How NEC Mastery Fits Into This

Exam calculation questions often test these exact rules: picking the next standard size, spotting the continuous load wording, and knowing what actually sets the service size. The rules are simple, but you need practice to apply them quickly and correctly under time pressure.

• 8,000+ exam-style questions include plenty of OCPD sizing and load calculation problems, so working through Table 240.6(A) becomes routine
• Detailed explanations that cite specific NEC articles show you where each answer comes from, whether it’s Article 240, the continuous load rules, or service sizing
• Timed mock exams weighted to your exam type help you practise spotting words like “continuous” or “indefinitely” and applying the 125% multiplier without second-guessing
• Use it alongside your codebook. Each question you work through sends you back to Table 240.6(A), so on exam day you’ll find it quickly