OCPD for Primary on Isolation transformer/power center

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Mddocstanley

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Equipment involved is an Isolation transformer/power center, rated at 10kva at 208/230v 1ph.
There is a main cb rated at 60amps ahead of the transformer incorporated into the equipment.
I am just trying to determine the maximum size OCPD for the feeder to this equipment so that the upstream OCPD does not trip before the equipment main
OCPD.
OK Per 450.3(b)
The 60 amp OCPD installed ahead of the transformer it seems to me is sized to protect the secondary since it is rated at 125%.
The primary can be rated at 250% or @ 100amp. I can install a 100 amp OCPD ahead of the feeder to this.
Am I looking at this correctly?
Thanks for any input.
 
Unfortunately breaker size does not necessarily control which would trip first (or almost simultaneously). There are many cases where a "Main" trips along with a branch breaker.
You will need to determine the characteristics of your breaker such as the tip curve and that of any "upstream" breaker to assure the equipment breaker trip's first.
(In an overload situation, the 60 would probably go first, but in a short-circuit situation,. it could be ANY upstream device unless selective coordination is applied)
 
The 250% primary protection is allowed per 450.3(b) IF you have 125% secondary protection.
Note that Art 450 only addresses transformer protection, not conductor protection
 
Hey Augie47,
On this I guess what I am trying to find out now is does this 60amp main incorporated into the equipment qualify as a secondary protection, and if not then I am going to have to install a 60amp feeder with a 60amp OCPD?
 
It appears you have a two-wire prim nary and secondary.
Take a look at 240.21(C)(1). The install probably qualifies for the "secondary considered to be protected by the primary" clause that applies to delta-delta or two-wire-two-wire

Satisfy 450 for the transformer an 240 for the conductors,
 
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