Circuit breaker overload

[XingFu]

Hello:

The National Electric Code requires the branch circuit to use no more than 80% of the circuit breaker (and wiring) rating for continuous loads. A continuous load is considered to be 3 hours (section 100 of NEC).

My question: is it against the code if the circuit breaker runs at 125% rated current or so for a short period of time (e.g. several minutes)?

Xing Fu

 

[iwire]

Not a easy question,

It is against the NEC to design a circuit where the load exceeds the rating of the circuit.

However that happens all the time during in rush of transformers, start up of motors etc.

 

[kwired]

You may want to look at the current/time curves for the breaker you wish to use.

You may also want to consider sizing your circuit for the additional 25% if that is normal operating current.

There is other information necessary to really be able to answer your question. What type of load is it?, Why the 25% extra load for short time.

 

[Jim W in Tampa]

Under normal conditions you should not reach even 100 %. Yes can and will happen with the result of tripping a breaker if for more than very short time. I have seen offices that if you wire according to prints and all the lights are on that there will be a problem. Some may be a storage closet or restroom so technically they are ok. If typical receptacles there simply is no way to know what they will reach. Will tell you that if breakers trip often you will have an unhappy customer. This where the low ball EC might not been such a bargain. NEC just tells us not to design a circuit that will be running for 3 or more hours at 100 %. In commercial uses that usually means a 9 hour day. Do not assume that print your working from was done right. Do your own math and ad up the ballast amps. Your name is on that job.

 

[brian john]

Under normal conditions you should not reach even 100 %.

It should be noted SOME circuit breakers are rated for 100%

Yes can and will happen with the result of tripping a breaker if for more than very short time.

At 125% a molded case CB could go for days without tripping. I do not have specific curves on my Mac at home, but here is some reading that alludes to this.

http://www.brainfiller.com/articles/TimeCurrentCurves.php