Re: Overcurrent protection device for Motor compressor
This may be an interesting read for some an hopefully will open some discussion.
Interesting question. I always wondered why the 250% being too high myself.
If one backs up the truck a bit and look at the FLA of the motor, the inrush value (not LRA or starting current) and the reason for the breaker it is a bit confusing.
One must agree that the motor should be protected from OL with an properly sized OLR or internal thermal protection, and as such, the breaker does not protect the motor. However, the OLR does not protect the distribution system from a motor failure, often referred to as ground fault protection in NEC art 430-52.
The key to the issue really is looking at how the NEC 430-52(C)(3) Exception No 1 states that a mag only MCP will be applied which they are very specific. Note that it includes setting from 800%, 1100%, 1300%, and 1700% depending upon the motor as well as the "necessity." I think that it is common that most agree that motor LRA is about 700% the FLA so take a look at that 800% starting value which is just above the 700%.
Think about what the MCP is supposed to do, protect the distribution system from motor failure, to take a motor that has failed off line. To provide as close a protection as possible I have always recommended that the MCP be set as low as can be without nuisance tripping when the motor is started. Normal LRA is about 7x the FLA. Inrush current is above that which reflect the 800, 1100, 1300, and 1700% as included in the NEC. Of course that can only be with a combination starter.
What does all that have to do with applying a thermally mag breaker? Select the TM breaker that the NEC allows you to apply and multiply the breaker rating by 10 and that will give you the mag trip value(+-20% cal. allowance.) The compare that with that which is allowed should one applied an MCP.
If one were to simply use 100FLA as an example and you were allowed to apply a breaker a 175% the mag trip would be 1750a which is 1700%. Which is reasonable. Apply that 250% or a 250a breaker that would have a 2500a mag trip that's way above the maximum 1700a as allowed if an MCP were applied. Make sense??
Well, before the NEC change the rules a few years back they limited the mag setting to 1300% which raised nothing but hell with energy efficient motors which inrush ran often time significantly more than that. This is where we advised the customer to consider a TM breaker which got you out of the woods where one could apply up to 250%.
