Installing two panelboards on secondary of a transformer 450.3(A) (B) Note 2

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iwire2

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I have two panelboards on the secondary of a 480-120/208V Wye 75kVA transformer. From the secondary of the transformer, each panelboard is fed with its own set of conductors (250 MCM THHN Copper, phase and neutral) in separate conduits. The transformer has one primary feed which is protected at not more than 125% of rated primary current (Primary is 125 amp breaker with #1 THHN Copper conductors). Each 3 phase 120/208 volt panelboard has a 250 amp MCB.

Is this code compliant, as you can have multiple (up to six sets of secondary breakers) on secondary of the transformer, but it states where secondary protection is required (not sure that it is, as it has primary protection) they must be grouped, (which they are), and the total of all the device ratings shall not exceed the the allowable value of a single overcurrent device (which they do not, as 250 amp x 2 panels exceeds the 250 amp total allowed).

Basically, I am thinking that if secondary protection is not required then 450.3 (A) or (B), Note 2 would not apply, and this installation is code compliant?:?
 
iwire2 said:
Basically, I am thinking that if secondary protection is not required then 450.3 (A) or (B), Note 2 would not apply, and this installation is code compliant?:?
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It sounds like your transformer has its required overcurrent protection on the primary side, and secondary overcurrent protection for the "transformer" would not be required.
 
david said:
It sounds like your transformer has its required overcurrent protection on the primary side, and secondary overcurrent protection for the "transformer" would not be required.
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You cannot take credit for primary side protection unless you are talking about one of two specific transformer types. The OP does not have one of those two. So primary-only protection is not allowed. Reference NEC 240.4(F).

 
iwire2 said:
Basically, I am thinking that if secondary protection is not required then 450.3 (A) or (B), Note 2 would not apply, and this installation is code compliant?
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Sorry, but Note 2 does apply. You need primary and secondary protection. So the total of all OCPDs on the secondary side cannot exceed 125% of the transformer's secondary side current rating of 208 amps. 125% of that value is 260 amps. Therefore, you can't have two panels, each with a 250 amp MCB, powered by this transformer.

 
I think we have a situation where both posters are correct. As far as transformer protection, the 125% primary protection will allow you to skip secondary protection, but Art 240 for conductor protection would require you to have protection per 240.21. If you satisfy both scenarios, you are compliant.
 
iwire2 said:
From the secondary of the transformer, each panelboard is fed with its own set of conductors (250 MCM THHN Copper, phase and neutral) in separate conduits.
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iwire2 said:
Each 3 phase 120/208 volt panelboard has a 250 amp MCB.
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charlie b said:
You cannot take credit for primary side protection unless you are talking about one of two specific transformer types. The OP does not have one of those two. So primary-only protection is not allowed. Reference NEC 240.4(F).
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The OP asked about overcurrent protection for the transformer and also asked if 250 MCM THHN Copper secondary conductors protected with 250 MCB was compliant

Not sure how 240.4 (F) would not be met

Do not think the OP mentioned the tap length
 
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Engineer concern with an Unbalanced Load Scenario

Engineer concern with an Unbalanced Load Scenario

The engineer has the following unbalanced load scenario, to the above two panelboard installation.

Here is the scenario based on your original setup (two 225A MCB, 3-ph, 4W panelboards):

If the load on panels 1 & 2 are as follows:

Panel 1:
Phase A – 140A
Phase B – 0A
Phase C – 0A
Neutral – 140A

Panel 2:
Phase A – 140A
Phase B – 0A
Phase C – 0A
Neutral – 140A

Then the resulting loads are as follows:
Transformer Secondary:
Phase A – 280A
Phase B – 0A
Phase C – 0A
Neutral – 280A

Transformer Primary:
Phase A – 121A
Phase B – 121A
Phase C – 0A

Since the 225A panelboard main circuit breakers are loaded at 140A, they will never trip. Since the 125A
transformer primary circuit breaker is loaded at 121A, it will only trip after a very long time (> 3 hours).
Since a 480V-208Y/120V, 75kVA transformer is only rated for 208A on the secondary, phase A and the
neutral of the transformer is in an overload condition and will fail before any circuit breaker trips. The
transformer is not properly protected from this type of overload condition, so the installation is not in
compliance with the NEC.

Having said that my thinking is that 210.11(B) requires that "This load shall be evenly
proportioned among multioutlet branch circuits within the panelboard(s)", and the scenario
above would create a violation of the NEC.
 
iwire2 said:
The engineer has the following unbalanced load scenario, to the above two panelboard installation.

Here is the scenario based on your original setup (two 225A MCB, 3-ph, 4W panelboards):

If the load on panels 1 & 2 are as follows:

Panel 1:
Phase A – 140A
Phase B – 0A
Phase C – 0A
Neutral – 140A

Panel 2:
Phase A – 140A
Phase B – 0A
Phase C – 0A
Neutral – 140A

Then the resulting loads are as follows:
Transformer Secondary:
Phase A – 280A
Phase B – 0A
Phase C – 0A
Neutral – 280A

Transformer Primary:
Phase A – 121A
Phase B – 121A
Phase C – 0A

Since the 225A panelboard main circuit breakers are loaded at 140A, they will never trip. Since the 125A
transformer primary circuit breaker is loaded at 121A, it will only trip after a very long time (> 3 hours).
Since a 480V-208Y/120V, 75kVA transformer is only rated for 208A on the secondary, phase A and the
neutral of the transformer is in an overload condition and will fail before any circuit breaker trips. The
transformer is not properly protected from this type of overload condition, so the installation is not in
compliance with the NEC.

Having said that my thinking is that 210.11(B) requires that "This load shall be evenly
proportioned among multioutlet branch circuits within the panelboard(s)", and the scenario
above would create a violation of the NEC.
Click to expand...

The engineer needs to submit a Public Input to change the 2023 code if that is his position. The code very clearly permits the primary of the transformer to be protected at 125 amps and the secondary of the transformer to not have any protection. In addition the loading specified in that scenario would be almost impossible to achieve in the real world.
 
iwire2 said:
Having said that my thinking is that 210.11(B) requires that "This load shall be evenly
proportioned among multioutlet branch circuits within the panelboard(s)", and the scenario
above would create a violation of the NEC.
Click to expand...

The engineer gets the primary current wrong as well. The primary in his scenario would see 70A on A and B.

Your proposed installation is Code compliant.
 
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