ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Top Posters(30 Days)
Potseal 11
Recent Posts
Permit Snafus...AHJs and Contractors Jump in
by sparky. 04/29/17 05:59 PM
201 volt equipment?
by bigpapa. 04/29/17 12:15 AM
Schneider LC1D09
by jraef. 04/28/17 11:05 PM
Electrode boilers question
by annemarie1. 04/27/17 01:40 PM
Why cables look like they do
by LongRunner. 04/26/17 09:36 AM
New in the Gallery:
SE cable question
Popular Topics(Views)
235,027 Are you busy
169,873 Re: Forum
163,062 Need opinion
Who's Online Now
0 registered members (), 83 guests, and 8 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
#193964 - 05/01/10 10:36 PM Sizing contactors for transformer loads  
bigpapa  Offline
Member
Joined: Mar 2006
Posts: 81
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?


Tools for Electricians:

#193967 - 05/02/10 01:46 PM Re: Sizing contactors for transformer loads [Re: bigpapa]  
Yoopersup  Offline
Member
Joined: Mar 2003
Posts: 822
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.
Yoopesup


#193969 - 05/03/10 12:54 AM Re: Sizing contactors for transformer loads [Re: Yoopersup]  
twh  Offline
Member
Joined: Mar 2004
Posts: 905
Regina, Sask.
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?



Member Spotlight
Scott35
Scott35
Anaheim, CA. USA
Posts: 2,707
Joined: October 2000
Show All Member Profiles 
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box
Powered by UBB.threads™ PHP Forum Software 7.6.0
Page Time: 0.010s Queries: 15 (0.003s) Memory: 0.7520 MB (Peak: 0.8784 MB) Zlib enabled. Server Time: 2017-04-30 16:52:11 UTC