instantaneous setting for load-center-fed 460vac motor 4

[electricpete]

Instantaneous overcurrent setting for “high-efficiency” 460vac motor fed from load center breaker

For determining instantaneous overcurrent settings, I am used to applying the NEC limits (multiples of FLA) for motors fed from molded case circuit breakers in MCC’s. (For example 800 - 1300%FLA for standard motors and 1100-1700% for energy efficient motors... possibly next higher setting depending on your interpretation).
My MAIN QUESTION: Do these NEC guidelines/limits apply to motors fed from load center breakers?

The specific example motor I’m interested in:

Motor Supply breaker: Westinghouse DS-10 (load center breaker) with Amptector trip units. The tolerance band for the instantaneous setting is +/- 20%.

Motor nameplate data: 350hp, 460vac, 350A (FLA), 1200rpm, KVA code G (equates to LRC = 2109-2372 @100% voltage)
Motor efficiency is not marked on nameplate (motor purchased in early 1990’s).

Motor data sheet shows locked rotor current of 2200A (matches above range).
Motor data sheet shows efficiency of 95.7% at full load, full voltage.
When compared with recent revisions of NEMA MG-1 standard, I believe this motor would be considered an “energy efficient” motor. (Per NEMA MG-1-2005 table 12.11, 300hp 1200rpm motor with nominal energy efficiency 95.4 and min energy efficiency 94.5 can be classified as energy efficient, although the efficiency shall be marked on the nameplate under this standard and this efficiency was not... efficiency data retrieved from data sheet).

If I considered this a standard motor, the instantaneous trip would be 800% - 1300% * 341 =
2728 – 4433A.

If I considered this an energy efficient motor (as I’m inclined to), the instantaneous trip would be 1100% - 1700% * 341A = 3751 – 5797A

If I picked twice the locked rotor current, the setting would be 2*2200 = 4400.

Twice locked rotor currrent seems fairly conservative to me. We have for many years used 1.7*LRC on larger motors. We’ve been through the math before that says the true peak could be up to double using the steady state equivalent circuit and not taking any credit for expected decay of dc component before first peak.... the factor of 2 seems to account for that (without getting too in detail about assumptions of which parameter of the non-sinusoidal current the trip unit actually responds to, which can get very murky).

Personally I’m inclined to set it as high as possible, but that is an uphill battle if I don’t have good justification.

What do you think?
Is 4400A high enough?
Should the NEC guidelines be used for this motor?
Should it be considered energy efficienct?
And in that case, we should consider up to 5800 setting?


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(2B)+(2B)' ?

 

[DRWeig]

Thanks for the clarification, Pete -- I understand the layout now. I'll still give the same answer. I'm more concerned about spurious trips than the low-as-possible scenario. In my opinion, still plenty safe.

Good on ya,

Goober Dave

 

[Bramb]

Hi Pete,
is there any automatic transfer system in your load center? (I mean a system which automatically open the incoming on undervoltage and consequently, checking adequate interlocks, close the tie).

If not, to lose coordination between HV and LV side of the transformer can be, in my opinion, acceptable.

If yes, no way: you have to study coordination over the whole chain (feeder, tie, incoming, xmfr HV side).

Ciao
Erminio

The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.