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Sizing contactors for transformer loads #193964 05/01/10 09:36 PM
Joined: Mar 2006
Posts: 81
B
bigpapa Offline OP
Member
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?

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Re: Sizing contactors for transformer loads [Re: bigpapa] #193967 05/02/10 12:46 PM
Joined: Mar 2003
Posts: 849
Y
Yoopersup Offline
Member
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.
Yoopesup

Re: Sizing contactors for transformer loads [Re: Yoopersup] #193969 05/02/10 11:54 PM
Joined: Mar 2004
Posts: 947
T
twh Offline
Member
I don't see a rule about controlling transformers, so 14-400 should cover it.
Quote
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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