15kva XFMR Calculation

[kencoel]

I wanted to figure the size of a primary breaker for a 15kva transformer:
480v single phase primary to 120/240v single phase secondary,
15000 divided by 480 = 31.25 amps. Is this correct? The load draw would be
around 55 amps, would a 30amp primary breaker be sufficiant for this transformer?

 

[Geraldselectric]

Look at table 450.3A

15Kva/480 = 31.25

Assuming your Z in 6% or less, I believe you would muliply 31.25 x 600% for a circuit breaker (187.5a) or 300% for a fuse (93.75a) selecting next size up OCPD is allowed. In this case, a 200a CB would be allowed.

 

[kencoel]

Look at table 450.3A

15Kva/480 = 31.25

Assuming your Z in 6% or less, I believe you would muliply 31.25 x 600% for a circuit breaker (187.5a) or 300% for a fuse (93.75a) selecting next size up OCPD is allowed. In this case, a 200a CB would be allowed.

As for clarification?: A 200amp breaker should be the primary size for a 15kva xfmr? The secondary load is less than 55amps.

 

[Geraldselectric]

Look at 430.3b, You have a 480v primary.

So, 31.25 x 250% (if both primary and secondary need protected) = 78.125a would require 70a OCPD and the secondary 15000/240 = 62.4a x 125% = 78.125a = 70a OCPD or (next size up) 80A is allowed.

 

[kingpb]

I wanted to figure the size of a primary breaker for a 15kva transformer:
480v single phase primary to 120/240v single phase secondary,
15000 divided by 480 = 31.25 amps. Is this correct? The load draw would be
around 55 amps, would a 30amp primary breaker be sufficient for this transformer?

If I understand correctly what you are asking, I don't believe a 30A breaker would be sufficient.

Your load is "around" 55A so that equates to 13.2KVA @ 240V, and 27.5A at 480V. You can only load the breaker to 80% which means you need a 34.4A breaker, rounding up then to a 40A breaker. This method would size a breaker for the load, but leave no room for margin, or additional load on the transformer later.

If your talking about a 55A single motor load, then the transformer will be too small unless a reduced voltage starter or VFD is involved.

The method just described is for sizing the breakers based on the load, not for sizing the protection for a transformer.