Sizing Circuit Breaker Protection For Contactor Coil

P

Petergg98

Member
Location
New Hampshire
Occupation
Electrical
Hello,

I have a 480Vac/3Ph contactor with 120Vac coil. The specifications for the coil are 1100VA (9.16A) closing power and 12VA (0.1A) Holding Power. I believe I'd size my breaker for the holding current and choose the correct trip curve to ride out the quick closing current of the Coil. Example: a 2A miniature CB with a "C" trip curve (x5-10) so the breaker should trip if the closing power hits 10A. I'd love some advice/ recommendations.
 
I’ve always sized the OCPD to protect the control power transformer (which was selected based on the load). It’s purpose is not to protect the coil.
 
retirede said:
I’ve always sized the OCPD to protect the control power transformer (which was selected based on the load). It’s purpose is not to protect the coil.
Click to expand...
I have main branch protection for the secondary of the control transformer sized to table 450.3(B), but with having several loads on the secondary I'm looking to add protection for each circuit downstream as well.
 
Petergg98 said:
I have main branch protection for the secondary of the control transformer sized to table 450.3(B), but with having several loads on the secondary I'm looking to add protection for each circuit downstream as well.
Click to expand...
Generally, everything fed by the cpt is considered one circuit and is protected by the OCPD on the secondary. You really don’t want various loads in the control circuit to lose power separately.
 
These closing & coil sealed VA ratings appear to be incorrect. Its been many years since I looked up closing & sealed VA in an Allen Bradley catalog do googled it. Telling me that an A&B size 2 starter only used 29VA closing power. Even though most cabinets with say 6 or more motor starters seldom have every starter pulling in the same time we would install a 480 to 120 volt enclosed transformer on the side of panels ( to reduce panel heat ) sized 250% more then all of the starter closing VA plus power for panel lights, solenoids , relays etc. Side note When I questioned our fantastic Danfoss drive tech why some of the older Danfoss VLT drives around 40 to 60 HP with two & three contactor bypasses had a timer set for maybe 1.5 seconds. They undersized the control transformer and guessing could not handle the inrush closing current of two GE made in Italy contactors closing at the same time. Newer models did away with the on delay timer. I know that when bidding on jobs you have to watch every penny but we had enclosed 480/240 to 240/120 volt single phase phase transformers in bench stock from 100 VA to 10 KVA.
 
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