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#193964 - 05/01/10 07:36 PM Sizing contactors for transformer loads
bigpapa Offline

Registered: 03/18/06
Posts: 75
Loc: canada
How do you size a contactor that is in the primary circuit of a transformer?
I am looking for a specific CEC rule or guidance.
In this case 75KVA 600V 3PH primary with upstream 70A circuit breaker.

I know you can use definite purpose contactors but i think you have to upsize them for inductive loads. If that is the case, does the code address this specifically?

2014 / 2011 NEC & Related Books and Study Guides
#193967 - 05/02/10 10:46 AM Re: Sizing contactors for transformer loads [Re: bigpapa]
Yoopersup Offline

Registered: 03/04/03
Posts: 826
Loc: Michigan
2008 NEC Article 450 450.3 (B).
See Table 450.3(B).
The way I usually do it (if no charts handy).
480 primary/120-208 secondary.
480x1.73 = 830.4 round out to 830
208 x 1.73 = 359.84 round out to 360
75 x 1000 = 75000 Divided by 830 = 90.36 amp pri.
7500 divided by 360 = 208.33
Pri can be up to 250% if seconday protected no more then 125%
So 70 amp breaker Pri. Fine
Secondary 125% x 208.33 = 260.4 amps
See (450.3 (B) Table note 1.
Then go to 240.6.
Thats the way I;ve done it.

#193969 - 05/02/10 09:54 PM Re: Sizing contactors for transformer loads [Re: Yoopersup]
twh Offline

Registered: 03/11/04
Posts: 895
Loc: Regina, Sask.
I don't see a rule about controlling transformers, so 14-400 should cover it.
14-400 Control devices shall have ratings suitable for the connected load of the circuits that they control and, with the exception of isolating switches, shall be capable of safely establishing and interrupting such loads
I wouldn't think the transformer is the load. Rather, it has a load. If you connect only capacitors to the transformer, doesn't the primary have a leading power factor like a capacitor and not a lagging power factor like a motor?


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