Transformers & NEC

[Machiavelli999]

Two part question.

First, the secondary of a delta-wye transformer, does it need over current protection even if the primary has over current protection? I am almost positive that it does (despite my idiot boss' claims!!! AHH!)

Second question. Why? I know there is a phase shift between the delta and wye sides but how does that affect the magnitude of the current? And why do I need the protection on the secondary? That I couldn't explain to him. Help me out guys.

Thanks.

 

[Flyer_PE]

The definitive answer is: "It depends."

Depending on how you set up the protection for the transformer, secondary protection may or may not be required. Take a look at NEC Article 450.3. Specifically, Table 450.3(B).

 

[Machiavelli999]

The definitive answer is: "It depends."
Depending on how you set up the protection for the transformer, secondary protection may or may not be required. Take a look at NEC Article 450.3. Specifically, Table 450.3(B).

Hmm...really? I was pretty sure that the secondary of any delta-wye transformer needs protection on the secondary. But I'll take a look at the NEC when I get a chance. Thanks.

 

[Machiavelli999]

So I looked and I am pretty sure that in my case secondary protection is required. The delta-wye transformer that we are installing will have a feed on the primary coming from a switchboard with a breaker. On the secondary side, the transformer is feeding a panel that has no main breaker and only main lugs.

It's not even Article 450 that applies here IMO, because it's not the protection of the transformer that is the problem. The problem is protecting the conductors on the secondary side.

Article 240.4(F) states that "Single-phase and multiphase transformer secondary conductors shall not be considered to protected by the primary over current protective device." It goes on to give some exceptions (delta-delta 3 phase transformers don't need secondary connections) but none apply in my case.

I think.

 

[GabeM]

So I looked and I am pretty sure that in my case secondary protection is required. The delta-wye transformer that we are installing will have a feed on the primary coming from a switchboard with a breaker. On the secondary side, the transformer is feeding a panel that has no main breaker and only main lugs.
It's not even Article 450 that applies here IMO, because it's not the protection of the transformer that is the problem. The problem is protecting the conductors on the secondary side.

Article 240.4(F) states that "Single-phase and multiphase transformer secondary conductors shall not be considered to protected by the primary over current protective device." It goes on to give some exceptions (delta-delta 3 phase transformers don't need secondary connections) but none apply in my case.

I think.

Does anyone have an explanation for the thinking behind Article 240.4(F)? The explanatory information in the brown box talks about single phase and split phase transformers (why single phase transformer secondary conductors are covered by the primary conductors overcurrent protection, and split phase secondary conductors are not), but doesn't explain what the thinking is behind the three phase transformers (why only delta-delta secondaries are covered by the primary overcurrent protection, but not the others).

 

[Waterboy]

Two part question.
First, the secondary of a delta-wye transformer, does it need over current protection even if the primary has over current protection? I am almost positive that it does (despite my idiot boss' claims!!! AHH!)

Second question. Why? I know there is a phase shift between the delta and wye sides but how does that affect the magnitude of the current? And why do I need the protection on the secondary? That I couldn't explain to him. Help me out guys.

Thanks.

I answered your question on the other thread you have posted. But here it goes again. Article 240.4 talks about the overcurrent protection of the conductors on transformer secondaries. For a delta wye transformer you need to protect the secondary conductors. You gave an example in your other thread where you calculated the turns ration for the transformer, but you made a mistake. The turns ratio is determined by the phase voltage (480 and 120 in your example..you used the phase voltage and line voltage in your calculation). Wye secondary transformers can experience phase currents in excess of what the secondary conductors are capable of handling, but yet the primary overcurrent protection will not trip because the overcurrent condition is not great enough. In your example you stated that a secondary single phase load experienced an overcurrent condition whereby it draws 300 amps, then you thought the 100 amp breaker on the primary would trip. In actuality, the primary current would be 300*(120/480) = 75 amps. The secondary conductors could not handle the current, but the 100 amp breaker on the primary would not trip. If this were a delta-delta transformer, then the primary current would be 300*(208/480) = 130 amps and the primary overcurrent device would open. This is why the code stated it is acceptable on delta-delta transformers and not on delt-wye. I hope this helps.

 

[GabeM]

According to Article 240.4(F) the secondary conductors of a delta-wye transformer require a breaker for over current protection even if there is a breaker on the primary side.
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.

Any thoughts? Thanks

To find the actual transformer turns ratio you have to use the phase voltage. On the primary side you would use 480 however on the secondary side you would use 120. Thus 300*(120/480) = 75 amps. If this were a delta-delta then your calculation would be correct. Thus the wording in the NEC. I hope this helps.

I copied the other thread here so that we have everything in one place.

I disagree with the equation 300*(120/480) = 75 amps. I believe that 75 amps is the current through the delta primary windings. In a delta transformer, the line current is 1.73 times more than the phase current (current through windings). So the primary side line current is 1.73*75=130 amps.

Also, what about wye-wye transformers? They are also not exempt from having overcurrent protection on the secondary side.

 

[Waterboy]

According to Article 240.4(F) the secondary conductors of a delta-wye transformer require a breaker for over current protection even if there is a breaker on the primary side.
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.

Any thoughts? Thanks

To find the actual transformer turns ratio you have to use the phase voltage. On the primary side you would use 480 however on the secondary side you would use 120. Thus 300*(120/480) = 75 amps. If this were a delta-delta then your calculation would be correct. Thus the wording in the NEC. I hope this helps.

I copied the other thread here so that we have everything in one place.

I disagree with the equation 300*(120/480) = 75 amps. I believe that 75 amps is the current through the delta primary windings. In a delta transformer, the line current is 1.73 times more than the phase current (current through windings). So the primary side line current is 1.73*75=130 amps.

Also, what about wye-wye transformers? They are also not exempt from having overcurrent protection on the secondary side.

The scenario described above states that there is an overload on one of the single phase loads. This would cause a phase imbalance. You would not be able to simply multiply the imbalanced phase current by 1.73 to find the line current on the delta side.

 

[Wolverine]

Wye secondary transformers can experience phase currents in excess of what the secondary conductors are capable of handling, but yet the primary overcurrent protection will not trip because the overcurrent condition is not great enough.

Another way to think of this is that a high side delta will not see low side ground faults on a wye, especially hi-impedance ground faults.

Consider a short circuit deep inside a customer plant served by a D-Y transformer. Just looks like load to the high side.

 

[GabeM]

I see now how a delta-wye (delta on the incoming side, wye on the outgoing side) should require overcurrent protection on the secondary in addition to the overcurrent protection on the primary. But what about wye-delta or wye-wye? I think that with wye-delta, the primary overcurrent protection would trip before the secondary would. And wye-wye to me is the same situation as delta-delta.

 

[GabeM]

I know this isn't going to be on the exam, but I am still wondering about this. Any ideas on why wye-delta and wye-wye transformers are not excluded from article 240.4(F) along with delta-delta?