Sparky's Journal

📅 April 7, 2026

⚡ NEC 220.20 — Continuous Loads Explained

Why Your Breaker Might Be Undersized

If you saw my recent TikTok about whether a 20A breaker can handle a load, this is where things actually get interesting.

Because this isn’t just math.

This is where the NEC changes how you’re allowed to size things.

📖 What NEC 220.20 Actually Says

NEC 220.20 — Loads Subject to Continuous Loading

Continuous loads must be calculated at 125% of the load.

Simple on paper.

But this is one of those rules people memorize without understanding what it’s doing in real installations.

🔥 What Is a Continuous Load?

A continuous load is anything expected to run for 3 hours or more.

Common examples:

• Commercial lighting

• Signs

• EV charging equipment

• Certain HVAC loads

• Anything that stays on long-term

Not every load is continuous — but when it is, it changes everything.

⚡ The 125% Rule (The Real Shift)

If a load is continuous, you can’t size the breaker at 100%.

You must size it at:

125% of the actual load

So:

Imin = 1.25 × Iload

Example:

16A continuous load:

16A × 1.25 = 20A required capacity

So now:

• A 20A breaker is at the limit

• There’s no headroom

• Real-world conditions start to matter

⚠️ Where People Get It Wrong

Most electricians think:

“If it’s under 20A, a 20A breaker is fine.”

That’s not how the NEC evaluates continuous loads.

The code is thinking long-term:

• Heat buildup

• Conductor insulation stress

• Breaker thermal performance over time

It’s not about instant trip.

It’s about sustained operation.

🧭 Where This Shows Up in Real Installs

This rule directly affects:

• Breaker sizing

• Conductor sizing

• Load calculations

And it’s why in practice you’ll see:

• Circuits not loaded to full breaker rating continuously

• The “80% rule” mentioned in the field

That 80% rule comes directly from:

100% ÷ 125% = 80%

🧠 Why This Matters in the Field

Understanding the rule isn’t enough — you have to apply it.

Every time you size a circuit, you should be thinking:

• Is this load running 3+ hours?

• Do I need the 125% factor?

• Is this breaker actually appropriate long-term?

⚡ Final Thought

NEC 220.20 is one of those sections that separates memorization from understanding.

Anyone can repeat “125% for continuous loads.”

But once you actually apply it in the field, you start seeing why certain installs don’t sit right — even when they “technically work.”

And once you see that…

you don’t unsee it.