correct type of breaker

[mull982]

I have feeder coming off of an 480V MCC out to a temporary power rack consisting of some heaters and two motors. The load I calculated was about 65A at 480V and a #2 so is being used as the feeder circuit. The contractor wants to use an HMCP breaker to protect the feeders out to this rack. From a previous fourmn I understood that an HMCP was only to be used with motors and would only provide short circuit protection (Even if the HMCP he wants to use is 100A). I told the contractor that we must use a thermal magnetic breaker in order to protect our feeder circuit from an overload condition since the magnetic breaker only provides short circuit protection and not overload protection. Am I correct with telling him to use a thermal magnetic breaker.

The breaker I had in mind for using is a Siemens HED4 breaker if anyone is familiar with it.

 

[Jraef]

You are correct, he is wrong.

He would be in violation of the NEC if he used an HMCP as a feeder, or for any other purpose other than as a replacement going into an existing starter that already had one in it.

Good choice on the breaker by the way, but you may also want to consider the new HEG series breaker that Siemens is selling. Smaller, rated higher (65kAIC vs 42kAIC), field installable accessories.

 

[mull982]

Good choice on the breaker by the way, but you may also want to consider the new HEG series breaker that Siemens is selling. Smaller, rated higher (65kAIC vs 42kAIC), field installable accessories.

I'm not really familiar with the HED4 breaker, it was just one of the spares that we had avaliable. I was curious what makes it such a good selection for this application.

I was also told by the manufacturer that these breakers are made to carry 80% of their rated value. For instance if I had a 100A breaker it is only rated to carry 80% of that value. Is this correct? If so, is this taken into account by the fact that we add 125% for motors and continuous loads, or is derrating the breaker by 20% an additional step that must be taken?

 

[bsh]

You only need to derate once. Most breakers can "continuously" carry 80% of their current rating. Having a breaker rated for 125% of the load does the same thing. Example: load of 80 amps, 80 X 1.25 is 100 so use a 100 amp breaker. The 100 amp breaker will then "continuously" carry the 80 amp load.