Sparky's Journal

📅 April 4, 2026

15 Amp on 20 Amp Breaker? Not so Fast! (Continuous Loads)

So I threw this up as a quick challenge:

• 900W heater

• 600W microwave

• 300W lighting

Total = 1800W

Simple math:

1800 ÷ 120 = 15 amps

And yeah… first instinct is:

“20A breaker? You’re chilling.”

That’s exactly what I wanted people to think.

Because This Is Where It Gets Twisted 

A lot of the comments were basically:

• “15A, you’re good”

• “Duh”

• “Obviously it works”

And I get it—that’s surface-level thinking.

But this is one of those situations where the math is right, but the answer can still be wrong depending on the situation.

The Real Question I Was Asking

Not:

“Can a 20A breaker handle 15 amps?”

Obviously yes.

The real question was:

How long is that load running?

Because that changes everything.

The “80% Rule” Everyone Talks About

Couple people asked for code reference—which I actually respect.

The NEC doesn’t say “80% rule” straight up.

What it DOES say is:

• Continuous loads (3+ hours) must be sized at 125%

So when I said 80%, I’m just flipping it:

Same rule, just looked at backwards.

That realization honestly blew my mind when I first learned it.

Now Let’s Look at It My Way

If that heater is just running for a bit?

• Cool. 15A on a 20A breaker = ✅ no issue

But if that heater is sitting there cooking for 3+ hours?

Now I’m thinking:

“Alright… this just became a continuous load.”

So mentally I go:

• 20A breaker → 80% = 16A usable

Now compare:

• Actual load = 15A

• Max continuous = 16A

👉 It technically still works…

But now we’re riding that line.

And That’s the Part I Care About

I’m not just asking:

“Does it work?”

I’m asking:

“Is this how I should actually build it?”

Because yeah—it passes.

But:

• You’ve got a heater (high, steady load)

• You’ve got other loads on the circuit

• Real life isn’t perfect calculations

Now you’re in that gray area where things start getting iffy over time.

The Comments Killed Me Though 

• “I just load it until it trips”
  → The legendary field testing method

• “Throw a 25A breaker in”
  → Yeah… and now your wire is the fuse 

• “Duh”
  → It is… until it isn’t

What I Want People to Take From This

This wasn’t about tricking anyone.

It was about showing:

Electrical isn’t just numbers—it’s context.

Same exact load:

• One situation = perfectly fine

• Another situation = pushing limits

How I Think About It in the Field

I keep it simple:

• Step 1: Do the math

• Step 2: Ask “is it continuous?”

• Step 3: Give myself margin

Because I’d rather be at:

12–14 amps on a 20A circuit

Than sitting right at the edge like:

“Yeah… it should be fine”

Final Thought

Anyone can do:

Watts ÷ Volts = Amps

But not everyone is thinking:

“What’s this load doing over time?”

And that’s the difference between just passing a test…

and actually knowing what you’re looking at.

A Note About Microwaves on Dedicated Circuits

Some commenters pointed out that microwaves are often installed on their own circuit. That’s a great observation and reflects common practice in many kitchens today.

The purpose of the example in this article wasn’t to suggest that a heater and microwave should normally share a circuit. Instead, it was simply a load calculation example used to demonstrate how to determine amperage and understand the continuous load rule.

In real-world installations, microwaves are frequently placed on a dedicated branch circuit for several reasons:

Microwaves can draw a significant amount of current while operating.

Sharing a circuit with other appliances can lead to nuisance breaker trips.

Many manufacturer installation instructions require a dedicated circuit, which must be followed per NEC 110.3(B).

Additionally, kitchen countertop receptacles are typically supplied by small-appliance branch circuits, which have specific requirements under NEC 210.52(B).

While the calculation example focuses on the electrical math and code concepts, good circuit planning in the field often means placing larger appliances like microwaves on their own circuit to improve reliability and avoid overloads.