NEC 240.21 (c)

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Daniel R

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NEC 240.21 (c), will the panel MCB be sufficient if exceeded the 25', or do you need a physical Disconect Switch before the panelboard? i don't think both.
please advise
 
If the distance from the transformer to the panel exceeds 25 feet, then you have to protect the secondary conductors right at the transformer. And it is not a "disconnect switch" that is needed, it is overcurrent protection for the wires. A fused disconnect switch set right next to the transformer would do the job, and you would not need to also have a main breaker at the panel.

Welcome to the forum.
 
charlie b said:
If the distance from the transformer to the panel exceeds 25 feet, then you have to protect the secondary conductors right at the transformer. And it is not a "disconnect switch" that is needed, it is overcurrent protection for the wires. A fused disconnect switch set right next to the transformer would do the job, and you would not need to also have a main breaker at the panel.

Welcome to the forum.
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Charles, what if It is already install, and the panel has a main circuit breaker, do I still need to add the fused disconnected in the secondary of the transformer?

thanks
 
What is the distance from the transformer to the panel? If it is more than 25 feet, you may have an existing code violation on your hands. One way to resolve the violation, if it is a violation, would be to add an overcurrent protection device (breaker or fused disconnect) near the transformer. There may be other options.
 
I agree with Charlie and would add that the 25' measurement is the length of the conductors not the physical distance between the transformer and the OCPD.
 
Totally understand, I think it was an oversigth on our part. We will need to add the protection inside the room where the transformer is located.

Thank You

"you live, you learn..."
 
Would all this not depend on our transformer configuration ?
If its delta-delta or two wire primary-secondary, depending on the ratio, might we not possibly be Code compliant ?
 
Augie
It is not often one sees a transformer installation such as you have stated.

There is an additional condition that has been added to the 2008 NEC, Condition (4).

If the room can be considered a "vault" as per 450, Part III - one may be able to use the primary protection of the transformer if it meets the condition.
 
Pierre C Belarge said:
Augie
It is not often one sees a transformer installation such as you have stated.

There is an additional condition that has been added to the 2008 NEC, Condition (4).

If the room can be considered a "vault" as per 450, Part III - one may be able to use the primary protection of the transformer if it meets the condition.
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Hmmm... the only condition (4) I see added in the 2008 NEC is under 240.21(C)(2) for tap conductors not over 10'...???
 
Daniel R said:
I need to find definition of "considered a vault"...maybe we got something.
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I found this here (I had to scroll down quite a bit to see it)

Vault construction
The floors, walls, ceilings, and roofs of vaults must have adequate structural strength with a minimum fire resistance of 3 hours, such as 6-inch-thick reinforced concrete [450.42]. Provide each vault doorway with a tight-fitting door with a minimum fire-resistance rating of 3 hours [450.43(A)]. This minimum fire resistance (for the vault and the door) drops to 1 hour, where an automatic sprinkler system protects the vault.

Note: The NEC vault requirements only apply when a vault is required for transformer containing oil-insulation or rated over 35,000V.

Vault doors must [450.43(C)]:

Swing out.

Be equipped with panic bars or pressure plates so the door can open from inside under simple pressure.

Be provided with locks that are accessible only to qualified persons.
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all being said, I consider there is no way around it. We need to add the safety disconnect by the secondary of the transformer.

Thank You All:grin:
 
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