Motor disconnect size

A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
Does it have to match the panel OCPD size? Or rather it sized based on what? I ask because I often see wire size rated for less than the motor OCPD as shown below. #12 THHN at 75 degree is rated for 25A but the motor OCPD is 30A. Isn't that a problem? Obviously not.

1774358322207.png
 
I don't think the code requires it to specifically match the "panel OCPD", by which I'm pretty sure you mean the motor feeder OCPD.

The rating of a motor disconnecting means is determined by 430.110.

What I think you are calling a motor OCPD is actually the motor disconnecting means. Also, motors do not have overcurrent protection devices, they have overload protection devices. All OCPD's in motor circuits are for protecting conductors, not motors.

Also, this looks like it's a rooftop AC unit, not a motor, in which case this is all governed by article 440, not 430, but you would know better than I, so just ignore that if I'm wrong.

You might need to clarify your question and the situation if you want a better answer.
Or wait for someone smarter than me to come along.

Generally speaking the rating of a disconnecting means is not tied to the rating of any upstream OCPD, it is tied to the rating of the load. An oversized disconnecting means is code compliant and common since disconnects are only made in a few specific ratings, but loads come in all shapes and sizes.
 
Last edited:
Disconnects are typically HP rated. When you're sizing them, they most closely correspond to the wire size. But bear in mind, the number of sizes available are limited to 30A, 60A, 100A, 200A, 400A, etc. There are no in-between sizes like the breaker and fuse sizes listed in 240.6(A).
 
You must log in or register to reply here.
Top