Overcurrent Protection for Transformers and Automatic Transfer Switches

[Machiavelli999]

Got into a pretty heated argument recently with a colleague and hoping that you guys can help clear this up.

First, a 480 - 208/120V delta-wye transformer is being installed and is being fed on the primary side from a switchboard breaker. On the secondary side, is another breaker necessary? I said yes and in the NEC Article 240.4(F) it says it pretty explicitly

"Single-phase and multiphase transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device"

There are a couple of exceptions given (a single-phase 2 wire or a delta-delta 3 phase transformer can be considered protected on the secondary side just by having a primary breaker) but it seems pretty clear that secondary breaker is needed.

The second question is for an Automatic Transfer Switches, if I have a breaker protecting the Normal feeder and the Emergency feeder, do I need a breaker to protect the Load feeder. My colleague says no. I think yes, but I am not as sure about this one. Couldn't find anything in the code about this.

 

[iwire]

First, a 480 - 208/120V delta-wye transformer is being installed and is being fed on the primary side from a switchboard breaker. On the secondary side, is another breaker necessary? I said yes and in the NEC Article 240.4(F) it says it pretty explicitly

I agree with you, the transformer may not require secondary protection but the conductors do require protection.

The second question is for an Automatic Transfer Switches, if I have a breaker protecting the Normal feeder and the Emergency feeder, do I need a breaker to protect the Load feeder. My colleague says no. I think yes, but I am not as sure about this one. Couldn't find anything in the code about this.

Here I disagree with you. Assuming that the 'load feeders' rating is equal to or exceeds the rating of the normal and emergency feeder breakers you do not need to add another breaker.

 

[Machiavelli999]

Thanks iwire.

What is the technical explanation though?

In the NEC handbook it delves deeper into this and says that a single phase 120V overload could cause sufficient current to cause problems for the secondary conductors but not trip the breaker on the primary (480V) side. However, I don't understand how that could happen.

In a balanced or unbalanced wye system, the phase (Ian) currents are the same as their respective line currents (Ia). So, lets say I had a 75KVA (480 - 208/120V) transformer. I would have a 100A breaker on the primary side. On the secondary side, my conductors would be rated to handle 250A. If I had an overload on one of my single phase loads and it started drawing 300A. The primary side, 100A breaker would see 300 * (208/480) = 130A on one of the phases and would trip.

That is what my colleague is thinking and I can't really disprove this. Any thoughts?

 

[Smart $]

... The primary side, 100A breaker would see 300 * (208/480) = 130A on one of the phases and would trip.

...

The voltage ratio is 120/480, i.e. secondary to primary winding. Two primary lines, with no other loads, would only see 75A.

 

[DARUSA]

The second question is for an Automatic Transfer Switches, if I have a breaker protecting the Normal feeder and the Emergency feeder, do I need a breaker to protect the Load feeder. My colleague says no. I think yes, but I am not as sure about this one. Couldn't find anything in the code about this.

Asuming we are talking about the transfer switch, the load side will always be protracted for the breaker on the normal feeders or if the transfer switch is actived the EM load side breakers will protect your load side wire on the TS

Now I assume that the wire size of your load side of the transfer switch is the same size as your normal power and your EM power wire size!!!