Xfrmr Feeding Disconnects

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Alwayslearningelec

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If I have a 3P xfrmr( 120/208v secondary) feeding (3) disconnects. Each disconnect would fed a single phase 120/208v panel.
What would the following requirements be for the disconnects be?

1. # of poles. Would it be a 2 or 3 pole and why? Asking because the xfrmr is 3 phase but the panels being fed are single phase.
2. Number of wire? Would it be a 3W or 4W
3. Would it matter if it was 240v or 600v?
 
horsegoer said:
Can you explain a little more.
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The neutral/ground bond is either made in the transformer (4-w feed) or in the disconnect (3-w feed).

This is not unlike a service, where the source neutral and the grounding system must be bonded.
 
So the receptacle fed from the disconnect has a SSBJ on the strap (the green or bare wire)? I call that an EGC, so why would the other end of it in the disconnect be called SSBJ?

I don't like when the same wire gets different terminology at the different ends. :unsure:

I assume you mean just the green from the source feeding the transformer to the case of the transformer is the SSBJ, and downstream of that it is still the EGC, and the neutral of the transformer's secondary is tied to this also.
 
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wwhitney said:
Doesn't that depend on whether there is one MBJ in the transformer, or three MBJs, one in each disconnect?

Cheers, Wayne
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If they're in the disconnects how does that change anything when they're still on the supply side? If they're not SSBJ's then what are they?
 
horsegoer said:
Well I
l be using SER cable so I hope the cable comes with correct ground.
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It might, but you have to check because the supply side bonding jumper will be larger than the EGC. Some cables have conductor that will have the grounding conductor sized so that it meets the requirements for a supply side bonding jumper, but other cables have the conductor sized based on the maximum size OCPD that can be used to protect the ungrounded conductors in the cable and Table 250.122.
 
wwhitney said:
Doesn't that depend on whether there is one MBJ in the transformer, or three MBJs, one in each disconnect?

Cheers, Wayne
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In both cases there needs to be a supply side bonding jumper between the panels and the transformer, based on the size of the ungrounded conductors in the raceways or cables that connects the transformer to the panels.
 
A safer bet you be to use a wiring method where you can properly size the SSBJ (Flex or conduit). If you do use SER, get a 4 wire SER and use one of the phase conductors as your SSBJ so you don;'t have to worry about size.
Don't forget as transformer secondary conductors to your disconnects you can't use the 240.4(B) "next size up rule".
 
infinity said:
If they're in the disconnects how does that change anything when they're still on the supply side? If they're not SSBJ's then what are they?
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Ah, I misinterpreted the terminology, it's supply side of the secondary OCPD, not supply side of the SBJ (which I incorrectly called an MBJ). Thanks.

BTW, it seems to me that the allowance on SDSs to have the SBJ upstream of the secondary OCPD(s) (at the transformer) would be a nice thing to have for services with multiple service disconnects supplied from, say, a common trough. As that would allow a single MBJ and avoid the otherwise high likelihood of objectionable current that arises from having multiple MBJs. [The objectionable current would arise if the different services' EGCs ever get interconnected anywhere by any bonded metal.] Basically amending (2017) 250.28(D)(2) to include an allowance like in 250.28(D)(3).

Cheers, Wayne
 
wwhitney said:
Ah, I misinterpreted the terminology, it's supply side of the secondary OCPD, not supply side of the SBJ (which I incorrectly called an MBJ). Thanks.
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When you mentioned it I was racking my brain trying to think what else it could be with no OCPD ahead of it.
 
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