ECN Forum Forums Code Related Discussion Canadian Electrical Code Topics Sizing contactors for transformer loads

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