600 HP Motor

[Overlord1981]

I have a 600HP motor that is controlled by a soft start motor. Nameplate FLA is 673A. I want to reuse an existing 1200A inverse time circuit breaker. I know the NEC says you can go up to 250% but that puts me well over the 1200A breaker I want to use. Is it reasonable to think that 178% FLA will be ok because its a soft start?

 

[dpc]

It should be fine. Even if the current exceeds 1200 A, the breaker has an inverse time curve that will take it quite a while to trip if the current does not go much over 1200 A.

The amount of starting current will depend on what you are trying to start, to some extent.

 

[Overlord1981]

I am going to see what the manufacturer of the soft start says. Should at least give me a better idea. I tend to agree with you dpc. I am just a bit nervous because I would hate to say sure then have it trip. If it trips I have no other place to feed that motor so the owner basically just bought a very expensive motor for nothing.

 

[dpc]

Look at the time-current curve for a 1200 A breaker. That may help you decide.

 

[jraef]

If the soft starter has motor thermal overload protection (and most do at that size), all you really need the circuit breaker for is short circuit protection, which can be as high as 1300% of motor FLC. The thermal element in the breaker is made redundant and unnecessary by the TOL in the starter. I know the code doesn't specifically say that, but most inspectors know it and accept that concept. Think of it this way, what if the soft starter had been built at the factory with a Mag-Only breaker? Still perfectly legal and it has NO thermal elements at all. You of course cannot use a Mag-Only breaker in the field, but the concept is the same, you can size it based on the magnetic trip elements.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[LionelHutz]

We sell thousands of soft-starters a year with the breaker sized at 150% of FLA and I can't recall a single time a breaker was tripping which required a larger breaker. You will be abusing the soft-starter and motor quite badly before the breaker becomes an issue.