Xfmr Secondary Conductors

[msteiner]

2000kVA delta-wye xfmr, 25kV primary, 480/277V secondary, located outdoors. Primary protection consists of 80A fuses. Secondary conductors consist of 8 sets of 4-500kcmil, total length = 40ft. The conductors terminate on a switchboard bus (located indoors), which has two circuit breakers on its load side: one at 1600A, one at 1200A. This is not an industrial establishment.

Are the secondary conductors adequately protected against overcurrent, per NEC?

 

[charlie b]

Nope. This exceeds the limit of how far you can go from a transformer, before hitting an overcurrent protection device. Reference 240.21(C).

 

[msteiner]

My thoughts as well charlie. What would be your recommended fix, assuming this was an existing installation?

 

[charlie b]

If you can't get the transformer or the switchboard moved, so that they are within 10 feet of each other (don't flinch, that may prove to be the cheapest option), then I would put another overcurrent device close to the transformer. A single breaker is all that is needed. But it will be big, and not cheap, and the wires to the switchboard will have to be rerouted to go via the new breaker. A fused disconnect would also serve. Take notice that if you do install an intermediary OCPD, than that location becomes the place at which the N-G bond must take place, and the existing N-G bond at the switchboard would have to be removed.

 

[cripple]

Xfmr Secondary Conductors

If the secondary conductors are being fed from an outside transformer there is no length requirement, but they are required be terminate on a single overcurrent device that will limit the load to the ampacity of the conductors.
All these requirement are found in 240.21(C)(4) Outside Secondary Conductors.

 

[charlie b]

I infer that we need some clarification about the installed configuration. I understood the transformer to be immediately outside the building, and the switchboard to be inside the building and 40 feet away. Now I wonder if the transformer is 40 feet from the building, and the switchboard is just inside the wall. So which is it? Also, you say that the switchboard has two load breakers, but you do not say if it has a single main breaker as well. Can you clarify for us?

 

[msteiner]

The xfmr is outside, the switchboard is inside, and the total length of the conductors is 40ft.

There is no main breaker in the switchboard.

 

[Mike01]

Utility owned xfmr.

Utility owned xfmr.

The seconday conductors come from the utility xfmr underground to the switchboard, if the secondary conductors are "outside" or "underground" can't they have unlimited length, also for the breakers could you apply the six disconnect rule if in a common location?

 

[charlie b]

Much depends on whether you can say the two breakers are "closest to the point of entry of the conductors."

So the total distance is 40 feet. How much of that is inside the building?

 

[msteiner]

Charlie - I'm not sure I understand why that matters. As I read 240.21(C), we have two options: Outside Secondary Conductors (subparagraph 4) or Secondary Conductors Not over 25ft Long (subparagraph 6). The conductors are definitely over 25ft long, so that leaves the first option. Part of the requirements of that paragraph require that the conductors terminate on a SINGLE circuit breaker or a SINGLE set of fuses, which isn't the case either.

 

[charlie b]

It matters in terms of what your options are to rectify the situation. For example, if the switchboard can be modified to include a main breaker, and if the location of the board is close enough to the wall to be considered "closest to the point of entry of the conductors," than that may provide your best solution. But if the transformer is right next to the outside wall, and if conductors run inside the building for 30 feet before hitting the switchboard, then adding a main breaker to the board will not solve the problem.

 

[dana1028]

It matters in terms of what your options are to rectify the situation. For example, if the switchboard can be modified to include a main breaker, and if the location of the board is close enough to the wall to be considered "closest to the point of entry of the conductors," than that may provide your best solution. But if the transformer is right next to the outside wall, and if conductors run inside the building for 30 feet before hitting the switchboard, then adding a main breaker to the board will not solve the problem.

msteiner - remember, these secondaries are considered 'outside' the building if they are run underground [beneath the building] and emerge from underground into the switch gear. [240.21(C)(5) and 230.6]

If this is the situation as Charlie indicates, then you do not have a limitation based on distance - however you do have the 'terminate at a single circuit breaker' problem.

 

[Mike01]

however you do have the 'terminate at a single circuit breaker' problem.

Why? If the transformer and the secondary conductors are routed outside / underground and penetrate the slab directly into a switchboards main lugs why do you need a single OCPD? as long as the OCPD's are "grouped" (debatable definition of grouped common enclosure or multiple disc. sw. adjacent to each other) than you could apply the six disconnect rule correct? Or am I missing something.

 

[charlie b]

There is difference between providing a means for disconnecting power to a building (6 throw rule applies) and providing overcurrent protection for a transformer's secondary conductors at a place other than their point of origin (6 throw rule does not apply).

 

[dana1028]

Why? If the transformer and the secondary conductors are routed outside / underground and penetrate the slab directly into a switchboards main lugs why do you need a single OCPD? as long as the OCPD's are "grouped" (debatable definition of grouped common enclosure or multiple disc. sw. adjacent to each other) than you could apply the six disconnect rule correct? Or am I missing something.

Why? - because this particular installation [if it is outside] must comply with 240.21(C)(4)(2) - "The conductors [must] terminate at a single circuit breaker."

RE: "the OCPD's are 'grouped'" - as Charlie said - different rules apply to services than for unprotected XO secondaries.

 

[jim dungar]

Is this a utility owned transformer, so the conductors are the service entrance, or is this a customer owned transformer?