Sizing Motor OCPD's

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kaltbier

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I was in Africa a couple of months ago. Parts were very hard to acquire locally, necessitating shipment from USA, UK or RSA (South Africa) -- we preferred to source from the USA. A complicating factor was shipping times. Shipping across the Atlantic, plus clearing customs in an African nation (indifference plus graft) created long waiting periods. This means we preferred to use material on hand, whatever its condition.

We installed a 100-hp submersible pump and its manufacturer-supplied 480V starter panel. The main breaker for that starter panel was 175A, rated for 600V. In fact all devices in that panel were rated 600V except the control power transformer, which was a 460/120V transformer.

At the last minute, they wanted to change the source power from 460V to 600V. We had to order a new 575V motor, so we also ordered a new 575V starter panel, plus a new 600/120V control transformer for the old 480V starter panel. The pump motor and panel were installed.

We now needed an OCPD for the feeder to that panel, to feed 600V power. According to Table 430.250, at 575V, the 100-hp motor FLC is 99A. (I won't mention conductor sizing except to say it more than adequate for continuous currents.) According to inverse-time column of Table 430.52, we needed an OCPD (breaker) with multiplier 2.5. 99A * 2.5 = 247.5A. According to 240.6(A), the nearest, larger OCPD is 250A. A 250A feeder CB above a 175A branch circuit CB is a reasonable increase for coordination purposes.

We located (searched the entire platform) a 250A inverse-time circuit breaker and installed it on the 600V bus of the generator. It tripped. But the circuit breaker in the pump motor starter panel was 175A, and it was not tripping. Using a current-injection test set, we found the the 175A Starter CB tripped at 175-180A on all three phases. For the 250A inverse-time circuit breaker, the lowest phase tripped at around 320A, the others being higher. Obviously, the starting current was tripping the 250A feeder CB (but not the 175A starter CB). Being in Africa, and since this submersible pump was a fire pump, we needed to have it running NOW. I robbed the 175A, 600V CB from the old 480V panel and saved ourselves. I will point out that these pump motor panel circuit breakers had what appeared to be an overcurrent adjustment, adjustable to almost 1800A !!

What kind of circuit breaker was this?

I have in my hand several motor & starter selector slide rules (cardboard) from Allen-Bradley, Cutler-Hammer & Square D. There is a GE version somewhere, but I can't find it. These all show OCPD ratings that do not seem to agree with any portion of Table 430.52.

My dilemma: I would prefer to use the NEC tables to determine circuit breaker sizes, but the electrical industry seems to have an unpublished set of tables of which I am both unaware (save these cardboard selector sliderules).

Why does there appear to be no agreement?

Thx
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
Not an answer to your "why" question, but a note of concern. You mentioned the pumps were used as fire pumps. If they fall under the venue of Art 695, you need to take a look at 695.4(D). It appears your breakers may be far undersized for that operation.
 

jim dungar

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Location
Wisconsin
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PE (Retired) - Power Systems
The adjustment you see on the breaker is for the short time (magnetic) portion of the breaker's trip curve. A setting of 1800A on a 175A breaker is just past 10X. Almost all 480V and 600V breakers larger than 150A have this adjustment. It is possible that your 175A breaker was set to High (10X = 1750A) and the 250A breaker set to Low (5X = 1250A) causing your lack of coordination.

The NEC tables do not tell you what size breaker to use, rather they say the maximum that can be used.
 

jim dungar

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Location
Wisconsin
Occupation
PE (Retired) - Power Systems
The NEC does not make a suggestion as to sizing breakers for motors, it only provides maximum possible settings.

When choosing circuit breakers for motor loads you must always compare the magnetic (instantaneous) pickup point of the breaker with the starting current of the motor. Simply looking at a breaker's continuous current rating is typically not sufficient.

I don't see a conflict with the NEC maximums and manufacturer's suggestions. For a 100HP 575V motor, my Square D Motor Data Calculator says to use a 150A breaker with an adjustable trip range of 5 to 10 times. So with a 99A FLC this breaker can handle a motor with 15X inrush current. A case could be made that the maximum instantaneous setting for an adjustable inverse time breaker should not exceed the value in table 430.52 for instantaneous trip breakers.
 
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