Transformer Secondary conductors.

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Mattelectricnj

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Been having a debate with a coworker.

We are currently drawing up a plan to have a 75kva 3PH 480v Delta to 3PH 208/120Y transformer installed to feed a 3PH 208/120Y 42 circuit lighting and appliance panel.


We will have a 3PH 480v fused disconnect installed next to the transformer for the primary feed. The Secondary panel will be installed next to the transformer allowing for the secondary conductors to be no more than 10' in length.

Our debate is, I say that the secondary panel requires a main breaker to be installed to protect the secondary conductors. He says the primary disconnect will be sufficient to protect both the primary and secondary. I keep referencing NEC OCP "Tap rule" articles and he keeps referencing Transformer articles.

What are your thoughts and what NEC articles may we reference to finally get the right answer?


Thanks
 
The secondary conductors for that type of transformer require OCP and the panelboard requires OCP {408.36}. One device such as a main in the panel can accomplish both tasks.
 
Inn addition to the mentioned "tap rules" and "transformer articles", add Art 408.36 to your references. There you will find the requirements for panels to have an overcurrent protective device (with some exceptions)

sorry, rob, posted at the same time.
 
Two wire single phase secondary or delta -delta secondaries serving a straight three phase load are about the only places you can protect the secondary with the primary overcurrent device. Most any other "multiwire secondary" is capable of having enough current imbalance without tripping the primary side device. Gets more complicated with three phase but similar and easier to understand example would be a 25kVA single phase transformer with 120/240 secondary. It is only rated for 12.5 kVA for each 120 volt leg, but if you were to load one leg to 20 kVA you are significantly overloading one half of the secondary even if the other half is not supplying any load, yet the primary device is not going to open if it is only seeing a 20 kVA load. Properly sized secondary protection still opens in this case.
 
If the transformer is arranged in such a way, that fault current is guaranteed to line up winding-to-winding, then the primary OCPD can protect the secondary conductors and no secondary OCPD is needed. Provided that the secondary wires have enough ampacity to be protected by the primary OCPD multiplied by the primary-to-secondary voltage ratio (this should be an intuitive calculation if you think about what a transformer does to a given current on one of the sides). Examples are 2-wire single phase on both sides, and 3-wire delta three phase on both sides.

If transformer topology can allow a fault on one secondary winding to be distributed onto multiple windings of the primary, and go unnoticed by the primary OCPD, then you need an overcurrent device on both sides of the transformer. Examples being any transformer with a WYE system on either side, or a high leg delta.
 
There are multiple NEC sections that all must be addressed, 240, 408, and 450.
As iwire pointed out, one device (e.g. a panelboard main breaker) may provide all of the protection required.
 
Carultch said:
If the transformer is arranged in such a way, that fault current is guaranteed to line up winding-to-winding, then the primary OCPD can protect the secondary conductors and no secondary OCPD is needed. Provided that the secondary wires have enough ampacity to be protected by the primary OCPD multiplied by the primary-to-secondary voltage ratio (this should be an intuitive calculation if you think about what a transformer does to a given current on one of the sides). Examples are 2-wire single phase on both sides, and 3-wire delta three phase on both sides.

If transformer topology can allow a fault on one secondary winding to be distributed onto multiple windings of the primary, and go unnoticed by the primary OCPD, then you need an overcurrent device on both sides of the transformer. Examples being any transformer with a WYE system on either side, or a high leg delta.
Click to expand...

I agree with the logic, but IMHO the install would not comply with the specific wording in 408.36 as far as the panelboard itself is concerned.
 
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