Overcurrent protection of transformer

[Jodonnell]

I am working on an installation where we are installing a 167 KVA transformer to feed an existing service. We have provided the 125% protection for the transformer on the primary side. Are we required to protect the secondary conductors when they leave the transformer?

If the load is know and we are not going to be using the capacity of the transformer can we feed it (primary) with less then 125%

 

[pete m.]

Check out 240.21(C) for the transformer secondary conductors.

As far as the primary OCPD you can size it to the calculated load (as long as you don't exceed the maximum).

Pete

 

[Smart $]

I am working on an installation where we are installing a 167 KVA transformer to feed an existing service. We have provided the 125% protection for the transformer on the primary side. Are we required to protect the secondary conductors when they leave the transformer?

Assuming this xfmr is outside and the secondary meets the four conditions of 240.21(C)(4), no.

If the load is know and we are not going to be using the capacity of the transformer can we feed it (primary) with less then 125%

Yes... but you cannot regulate the inrush of energizing current. The lower you go with your primary ocpd rating the more likely it will trip/blow upon energizing.

I'm currently wondering about the details, since you say 125%... which is the max rating for a primary protected secondary. Most secondaries that would (re)feed an existing/former service do not qualify for primary-only protection. The typical max ocpd rating for 600V and less is 250%. What voltage and phase are the primary and secondary?

 

[Jodonnell]

The primary is 480 volts single phase. The secondary is 120/240 single phase. All of the loads are lighting and general receptacles. All of the heating and cooling loads are off the new 480 volt three phase service.

 

[augie47]

With that arrangement, you can not use the primary OCP to protect the secondary conductors. The location of the secondary OCP protection will depend on which rule of 240.21(C) applies (10 ft, outside, 25 ft, etc).
If you are terminating in a panel, 408.36 will normally require a main.