Motors

[Djelite]

Is this correct?In non motors a ocpd is sized at 80% if its not a 100% rated ocpd and the next size up is allowed. In motors the ocpd is sized to 125% of motor calculated value but nect size up is not allowed because it was already sized at 125%

 

[Dsg319]

I’m not sure I understand fully. You can size loads at 100percent as long as they’re not a continuous load. But common practice it to load the circuit to only 80percent (for some folks).

A motor OCPD can be up to 250percent the ampacity of the conductors depending on the circumstances. Which others can explain because I’m don’t have remembered and too lazy to look up lol.

 

[Jraef]

I’m not sure I understand fully. You can size loads at 100percent as long as they’re not a continuous load. But common practice it to load the circuit to only 80percent (for some folks).

A motor OCPD can be up to 250percent the ampacity of the conductors depending on the circumstances. Which others can explain because I’m don’t have remembered and too lazy to look up lol.

250% for a thermal-mag breaker on a motor circuit, 300% for a non time-delay fuse, 175% for a dual element time-delay fuse.

As to the original premise, you (OP) are looking at it kind of backward. For other than some specific loads like motors and transformers, you size CONDUCTORS for 125% of the continuous load (defined as 3 hours or more), then you size the OCPD to protect the conductors. The so-called “80% rule” is simply the inverse of 125%, meaning that when you have properly sized the conductors for 125% of your load, and then the breaker is sized for those conductors, the breaker is indirectly sized where any continuous load will only be 80% (or less) of the breaker rating.

Then BECAUSE of that, the breakers use in most panels, where they are mounted right next to each other with no air flow between them, are not de-rated for temperature because they are not being expected to handle more than 80% of their rating continuously. So again, the fact that they are USED at 80% of their rating is indirect. All breakers are tested at 100% of their rating, but in “free air” under lab controlled conditions.

You CAN load a breaker to 100% continuously IF it says it has a 100% rating, which usually means it has to be stand-alone in its own enclosure so that sufficient air can flow around it (there are exceptions when it comes to direct bus connections). There are other restrictions and requirements that make it difficult to implement, so most people just stick to using them at 80%.