Reason for sizing OCPD 125% for continuous load?

[jim dungar]

In all the years I have been doing this I don't recall having actually seen or installed a 100% rated breaker or disconnect in the wild.

And how many times have you started your circuit design at the breaker/source and not at the load end of the conductor?

 

[TX+ MASTER#4544]

My understanding is circuit breakers are tested in open air. When you have a bunch of them crammed side by side in a panel board, under full loading they would generate excessive heat and trip prematurely. Hence the "derating". So with the breaker upsized, the conductors now need to be upsized.

2023 NEC TX

Section 310.15(B)(2) or T.310.15(B)(1)(2) for ambient temperature of 40 degrees C. This maybe the table you're thinking of because of the 40 degrees C you may see stamped on circuit breakers.

T. 310.15(B) (1)(2) looks similar except it's based on ambient temperature for conductors at 40 degrees C. See the foot note following T. 310.15(B)(1)(2) used with other tables as required. Open air?

Yeah, when you load up a panelboard with CB's you'll get some temperature heat rise.

Branch Circuits...210.19(A)(1) Exception to (1) "...If the assembly, including the overcurrent devices protecting....."
That is to say, that if the MFG built the panelboard as pointed out as described above, then, thus he is saying it is rated for 100% duty and' we" won't have to apply 125% for certain conductors, (continuous loads).

See also ,215 Feeders 215.2 (A)(1) Exception No.1. Says same as 210.19

And, Article 230 Services 230.42(A)(1) Exception No.2 ..... same as the above.
I would think to have one built at the factory it would be time consuming and most likely ....expensive.

That makes the "loop" of our electrical system, BC's FDRs, and Services.

So, you can pay extra for that luxury for your load center, panelboard, or switchboard or do what we all do is simply include the 125% for continuous loads and 80% for OCPDs. Hence, reciprocal trade off.

For 'us' we size continuous loads at 125% when calculating BCs, feeders etc.

Now, if you will excuse me, I have to go do some clothes washing and clothes drying and wash some dirty dishes!
Such is the life of a batchelor, not my choice!

Dennis, any suggestions?

Thanks for reading
Comments accepted
TX+MASTER#4544

 

[tortuga]

Now, if you will excuse me, I have to go do some clothes washing and clothes drying and wash some dirty dishes!
Such is the life of a batchelor, not my choice!

Huh I never did any of that until I got married