Breaker sizing

[kenaslan]

I have a continuous load of 15A. Now I multiply this by 1.25 and I get 18.75A. Now, 15A is less than the 16A max of a 20A breaker, and 18.75 is more than the 16A max of a 20A breaker. IMO I should use a the next size up breaker 30A. Your thoughts please

 

[infinity]

No you do not need a 30 amp circuit. You'll have 15 amps continuous on a 20 amp circuit which is code compliant. You're allowed up to 16 amp continuous load on a 20 amp circuit.

 

[kenaslan]

I got this from Mike Holt Site https://www.mikeholt.com/nec-conductor-sizing-and-protection.php this seems to contridict what you just said.
Step 1 - Size the overcurrent device in accordance with 210.20(A) and 215.3. These two NEC rules require the overcurrent device (breaker or fuse) be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load.

 

[Seven-Delta-FortyOne]

You’re overthinking it. 16 amps is not the max allowed on a 20A breaker, it max continuous load you're allowed to put on a 20A circuit.

Your load is only 15A, but because it’s continuous you need a “safety factor” if the equipment is not rated for 100% continuous load, so it comes out to 18.75.

18.75 is not a standard size breaker, so you use a 20A

 

[kenaslan]

I stand corrected. See example from the Handbook

 

[infinity]

I stand corrected. See example from the Handbook

That shows a method by which you're using the 125% rule. You can also use the inverse or 80% rule. That means that the OCPD can be loaded to 80% of it's rating. So in your example the 20 amp OCPD * 80% = a maximum continuous load of 16 amps.

 

[wwhitney]

I have a continuous load of 15A. Now I multiply this by 1.25 and I get 18.75A. Now, 15A is less than the 16A max of a 20A breaker, and 18.75 is more than the 16A max of a 20A breaker.

Sounds like you've got it figured out, but I just wanted to point out that your "16A max of a 20A breaker" is a maximum for continuous loads. So you are effectively double counting by both applying the continuous load 125% multiplier and using the "16A max of a 20A breaker." You do one comparison or the other, not both: 15A continuous < 16A max continuous on a 20A breaker, that's fine; or 125% * 15A continuous < 20A breaker rating, that's fine.

Cheers, Wayne