MCB & Xfrmr Size

[Alwayslearningelec]

If I’m looking at it correctly a 15kva step down(480 to 208v) xfrm puts out about 41 amps on the secondary side.

If I’m feeding a 120/208v panel on the secondary side would this 41 amps come into play when determining the panel MCB size or it would just be the loads the panel are feeding to determine the main breaker size?

 

[Johnhall30]

When determining the C/B size on the secondary of a transformer, you can multiply the FLA by 125%, and then also use the next size up rule.

41 x 1.25 = 51 Amps (Next size up) 60 Amp breaker.

60 Amp breaker is code compliant. However you should only load the transformer up to the 15KVA rating.

 

[jim dungar]

If I’m looking at it correctly a 15kva step down(480 to 208v) xfrm puts out about 41 amps on the secondary side.

If I’m feeding a 120/208v panel on the secondary side would this 41 amps come into play when determining the panel MCB size or it would just be the loads the panel are feeding to determine the main breaker size?

I work from the loads back towards the source. Technically your panel could have a main breaker much larger than the transformer output.
The transformer secondary output affects the primary side protection value.

To mitigate the possibility of future confusion many designers use 125% of the transformer output when allowing for future growth.

 

[Alwayslearningelec]

THank you very much guys.
On a seprate note.
I have an existing 480v 100A disconnect feeding (1) unknown load. They are asking me to tap this disconnect to provide:
1. Power to another 100A disconnect switch
2. Power to a 15kva xfrm.

IMO to many unknows to say these taps can be done. Can you even tap a disconnect switch twice?

 

[augie47]

Not nearly enough info to answer...
"Tap" load side or line side of your disconnect ??
(If your disconnect is fed by a tap you can't 'tap a tap' )
The 15 kva transformer will obviously need proper protection
The method of "tapping" will come into consideration. Most 100 amp switches don't have lugs for more than 1 conductor..
etc etc etc

 

[Alwayslearningelec]

Not nearly enough info to answer...
"Tap" load side or line side of your disconnect ??
(If your disconnect is fed by a tap you can't 'tap a tap' )
The 15 kva transformer will obviously need proper protection
The method of "tapping" will come into consideration. Most 100 amp switches don't have lugs for more than 1 conductor..
etc etc etc

Thanks Augie. The xfmr with be within 25' feed of the panel it's feeding on the secondary side. Would I still need a disconnect switch?
You can't get 2 barrel lugs to add 2 wires onto one lug?

 

[augie47]

Thanks Augie. The xfmr with be within 25' feed of the panel it's feeding on the secondary side. Would I still need a disconnect switch?

There are a some transformer secondary rules which allow a 25 ft secondary.. the answer would be in the particulars of your install... most importantly the panel would need a main breaker if you have no disconnect (408.36)

You can't get 2 barrel lugs to add 2 wires onto one lug?

There are numerous methods you can use to connect more than one conductor..again the specifics come into play. Most often a field decision on how to address it.

 

[kwired]

If I’m looking at it correctly a 15kva step down(480 to 208v) xfrm puts out about 41 amps on the secondary side.

If I’m feeding a 120/208v panel on the secondary side would this 41 amps come into play when determining the panel MCB size or it would just be the loads the panel are feeding to determine the main breaker size?

If you only had 35 amps of load you certainly could have a 35 amp breaker and 35 amp conductor (x 1.25 for continuous load) or this:

When determining the C/B size on the secondary of a transformer, you can multiply the FLA by 125%, and then also use the next size up rule.

41 x 1.25 = 51 Amps (Next size up) 60 Amp breaker.

60 Amp breaker is code compliant. However you should only load the transformer up to the 15KVA rating.

would be the maximums allowed regardless the actual load. If you do go with the 60 amp breaker you also must also have at least 51 ampacity conductor.