Secondary Disconnect Location

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Rock86

Senior Member
Location
new york
Occupation
Electrical Engineer / Electrician
Situation:
480-208/120V 500kVA step down, located outdoors
primary OCP: 600A
Secondaries tap to two distribution panels inside a building. Panels are mounted such that the conductors will stub into the panels after penetrating through the building.

Regardless if it is one or two secondary taps, should secondary protection be supplied outside at the transformer, or is the MCB at the panel sufficient? My understanding that because it outside, tap rule secondary tap rule would not apply and the OCP should be at the nearest point of entry.
 
Rock86 said:
Situation:
480-208/120V 500kVA step down, located outdoors
primary OCP: 600A
Secondaries tap to two distribution panels inside a building. Panels are mounted such that the conductors will stub into the panels after penetrating through the building.

Regardless if it is one or two secondary taps, should secondary protection be supplied outside at the transformer, or is the MCB at the panel sufficient? My understanding that because it outside, tap rule secondary tap rule would not apply and the OCP should be at the nearest point of entry.
Click to expand...
My understanding without pulling out the code is that in order to comply with the outside tap rule the conductors must be completely outside. No "nearest the point of entry" exception.
 
Strathead said:
My understanding without pulling out the code is that in order to comply with the outside tap rule the conductors must be completely outside. No "nearest the point of entry" exception.
Click to expand...
Okay, I can accept that. Therefore would you require an OCP at the transformer or on the outside of the building?
 
I'm afraid I have to disagree with Post #2..
From the NEC 240.21(C) "Outside"
(4) The disconnecting means for the conductors is installed at a readily accessible location complying with one of the following:
a. Outside of a building or structure
b. Inside, nearest the point of entrance of the conductors
c. Where installed in accordance with 230.6, nearest the point of entrance of the conductor
 
augie47 said:
I'm afraid I have to disagree with Post #2..
From the NEC 240.21(C) "Outside"
(4) The disconnecting means for the conductors is installed at a readily accessible location complying with one of the following:
a. Outside of a building or structure
b. Inside, nearest the point of entrance of the conductors
c. Where installed in accordance with 230.6, nearest the point of entrance of the conductor
Click to expand...
Therefore, would you prefer the disconnect at the transformer, or at the panel?
 
for servicing the panels I would prefer it inside since it will be next to them but you have to do what makes sense for the customer in this particular circumstance. I don't think the answer will be the same in every situation.
 
Krusscher said:
for servicing the panels I would prefer it inside since it will be next to them but you have to do what makes sense for the customer in this particular circumstance. I don't think the answer will be the same in every situation.
Click to expand...
The discussion in our office is whether or not the underground conductors should be protected. There is still the intent of a MCB at the panel regardless if there is an OCP device at the transformer.
 
Transformer secondary conductors installed outside are considered to be protected as long as you meet the requirements of 240.21(C)(4).
The MCB at the panel can meet one of the requirements of that Section.
The nature of the beast means transformer secondary conductors will run some length prior to an OCP device. Rules for outside conductors are more lenient as they are similar to service conductors and treated in a similar manner.
 
augie47 said:
I'm afraid I have to disagree with Post #2..
From the NEC 240.21(C) "Outside"
(4) The disconnecting means for the conductors is installed at a readily accessible location complying with one of the following:
a. Outside of a building or structure
b. Inside, nearest the point of entrance of the conductors
c. Where installed in accordance with 230.6, nearest the point of entrance of the conductor
Click to expand...
If I had taken 3 seconds to think about it, I would have realized this.
 
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