We have a 300kva transformer being hooked up to a chiller drawing 250kva. The problem is that the bldg has an 800 amp breaker and when the transformer is hooked up, it is going to draw 833 amps for a fraction of a second. The breaker is not a GFCI, so it seems as though going 4.125% over its rating for less than a second would not trip the breaker.

I just want to know if anyone with experience on this can offer some information.

 

dang your talking some serious draw there. the transformer is just for the chiller or the entire building?? a momentart spike should not trip the breaker off. breakers are designed to instant trip on a direct short or in the case of an overload they will hold for alitttle while to allow start up. give you an example. in a residence an A/C condencer will spike well over 100amps i have seen them as high as almost 200 and on a VERY hot day if you short cycle the compressure it will hold that high amperage for several seconds. even though the unit is on a 30 or 40 amp breaker it won't trip unless it contiunes to hold there. you should be fine. what is the running load of the chiller. might wanta make sure your not going to be maxing out your entire service.

 

It's over spec, and even though most "components" are rated at 66% of what they can stand - the system source should be overcapable for the draw. That one isn't...

If it's a slow blow, it may last a while, but will inevitably degrade and there will be major conflicts and outages over time - rely on that. (The system will die a miserable death)

Never run more than a system is rated for.

If it's fast blow (*grinz*) the problem will become immediately obvious...
Try not to be standing anywhere near ground zero when the top brass come down and want to assign responsibility!

Never overload a load center.


~Wolf

"The current source is unsuited for it's end duties"

*BANGS GAVEL*

"Next case..."
(He states, boredly)

HINT: Suggest upgrading the breaker. Try to get a statement that you did recommend it, and the user/supervisor was aware of the problem. In fact - I would insist on it being documented

 

Bigredtruck, one thing I am unsure of from your post, does the 800 ampere breaker feed the transformer then the transformer feeds the chiller? Or is the 800 amp breaker on the secondary side of the transformer? If the 800 ampere breaker serves the primary transformer, you need to take magnetic inrush of the transformer into account as well.

833 amperes on an 800 ampere breaker is fine, unless there is something unusual about the application you haven't told us about. Remember, 800 amperes is the continuous current rating of the breaker, motor starting is not a continuous amp draw. If it were a busy elevator that starts and stops 30 times an hour, that would need to be taken into account. But most buildings I have designed electrical systems for, with a large chiller it starts a few times a day at most, they aren't designed to cycle constantly. If I had enough information, I might even say a 600 ampere breaker would work.


Circuit breakers typically are designed to hold 135% of rated amperes overload for 2 hours, fuses something like 150%. You are at 104% or so, the breaker could probably operate that way continuously without tripping unless it was 120 degrees in the panel or switchboard enclosure that holds the breaker. Even then it would probably take 4 hours or more.

Do you have access to the name plate information for the chiller? It should indicate full load amps (FLA or RLA), minimum circuit ampacity (MCA or WSA) and maximum circuit breaker size (MOCP, OCPD, or similar). If you comply with all of those, it should be fine.