How to determine the transformer size

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dahualin

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We have two service switchboards with 800A main circuit breaker and 1200A main circuit breaker respectively. The corresponding service transformers don't have nameplate on them. The transformers are on primary service loop on big campus owned by our client rather than local utility company. Based on your experience, can you tell the transformer size (rating)? The services are 480V 3-phase 4-wire secondary. Thanks.:-?
 
Well, 800 amps times 480 volts times the square root of three (1.732) is 665,000. So I would guess either a 500 kVA or 750 kVA. By similar math, I would guess the 1200 amp service transformer is a 1000 kVA.
 
From a NEC standpoint - Table 450.3(B) - You could easily feed the 800A Switchboard with a 500kVA transformer, and the 1200A Switchboard with a 750kVA transformer.
HOWEVER
If the transformers were sized using Utility standards, the 800A could be fed with a 300, or even a 225. The 1200A could be fed with a 500.
Are they Dry-Type, or Oil-Filled Padmount type? Where did the nameplate go? It also has the % impedance value, and other important information.
db
 
dahualin said:
We have two service switchboards with 800A main circuit breaker and 1200A main circuit breaker respectively. The corresponding service transformers don't have nameplate on them. The transformers are on primary service loop on big campus owned by our client rather than local utility company. Based on your experience, can you tell the transformer size (rating)? The services are 480V 3-phase 4-wire secondary. Thanks.:-?
Click to expand...
The 1200A CB is the right size for a 750KVA. 1000KVA secondary protection is usually a 1600A frame set down to 1500A. Secondary OCPD is usually set at 125% FLA

cf
 
IMO.... with out a name plate... there is no math that will do you any good... who's to say they were undersized... over sized .. or just right? I would search the internet, find a match and call them for help in sizing them.. send manufacturer the dimensions and a picture??? I don't know what else to do in order to be sure of there capacity.
 
charlie b said:
Only if it is a 1200 amp frame with a trip point of 1128 or lower.
Click to expand...
You are likely looking at 240.4, Protection of conductors.

For this application I'm usuing 450.3B, note 1. So 1200A is satispactory for the xfm secondary protection. The conductor ampacity has to be 1200A or more. 4-350kcmil is a nice fit

The tone of the OP led me to believe the 1200A and 800A CBs were molded case - which, of course might not be.

As for a 1200A secondary main on a 1000kva xfm - that would be the first one I ever saw that was designed for a 1000kva xfm and not just thrown together with what they had on hand. I'm sure that someone somewhere has done that, but I wouldn't know why.

As for using a 1600A set down to 1500, 4-500kcmil is a nice fit. A 1600A secondary main doesn't fit quite so nicely.

I don't think I'm goofed up on this - but I could be:)

In any case, the best post so far is from bobsherwood. Go measure it and match the pictures - everything else is guess work

cf
 
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