Fused disconnect feeding sub-panel

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titan1021

titan1021

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Northern California
I am installing a 75kva transformer to provide 200amps to a lighting panel, primary 480v, secondary 208y/120. I've got everything figured out regarding the transformer installation and requirements. However, I've run into a snag. I need to run a 125a sub-feed from the 200a panel but apparently standard BR breakers in 3-pole max out at 100a. I really don't want to get into a panelboard and molded case breakers if possible for the one larger sub-feed I need. The only solution I've come up with at this point would be to install a 125a fused disconnect to feed the sub-panel which is appox. 16ft away on an opposing wall. Just wondering if anyone has a different suggestion or see any issues with my approach.

Thanks.

Edison schematic.jpg
 
Seems legit, as long as your conductors are sized properly and you stay within your tap rule specifications.
If you could double lug the load side of the 200a disconnect you could save yourself some messy splicing in the trough & just run conduit to the panel and disconnect...

If I am correct you could use the 25' tap rule off the 200A disconnect and feed the 125A panel as long as the wire terminates in a main in the 125A panel.
 
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boptrop said:
Seems legit, as long as your conductors are sized properly and you stay within your tap rule specifications.
If you could double lug the load side of the 200a disconnect you could save yourself some messy splicing in the trough & just run conduit to the panel and disconnect...
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Great idea! I will look into that option. Splicing isn't going to be fun.

Thanks
 
A couple more options:

1. Note you can come off the secondary with more than one set of conductors: a 125A set and the 200A set
2. Use a panelboard with feedthru/subfeed lugs (or replace single lugs with double lugs. May not work on a "loadcenter" type panelboard) and use the feeder tap rules.
3. Some manufactures make a "subfeed terminal block" that goes in like a breaker. Not sure if those have the same amperage restriction as the breakers do.
 
electrofelon said:
A couple more options:

1. Note you can come off the secondary with more than one set of conductors: a 125A set and the 200A set
Click to expand...

As I remember you cannot come off the secondary with combined OCPD's that exceed the transformer rating.

for example...if the trans is rated for 200A on the secondary, then the breakers/fuses combined rating cannot exceed 200A

in what you suggest you would be able to draw 325A off a 75KVA transformer?

Also in what you suggest you could connect (up to) 6 -200A MCB panels to the 75KVA trans... Better hope the Primary OCPD catches this!
 
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As I remember you cannot come off the secondary with combined OCPD's that exceed the transformer rating.

for example...if the trans is rated for 200A on the secondary, then the breakers/fuses combined rating cannot exceed 200A

in what you suggest you would be able to draw 325A off a 75KVA transformer?

Also in what you suggest you could connect (up to) 6 -200A MCB panels to the 75KVA trans... Better hope the Primary OCPD catches this!
Click to expand...

It depends on if secondary protection is required. If it is, the combined value of the OCPD's served cannot exceed the next size up of 125% of the transformer rated current. 450.3(B) and notes.
 
......didnt feel like that was complete: So if the OP had primary protection over 125%, then the most he could have is 300 amps of OCPD's on the secondary, so clearly a 200 and 125 would not be compliant. Now if primary protection was 125% or less, then it would be fine.
 
Unless there is some reason you cant protect the primary at 125%, the dual secondary conductors would be the route I would take. Very commonplace on jobs I inspect.
 
electrofelon said:
It depends on if secondary protection is required. If it is, the combined value of the OCPD's served cannot exceed the next size up of 125% of the transformer rated current. 450.3(B) and notes.
Click to expand...
240.21(C)(1) requires secondary protection of this system.

OP could run a feeder tap to the 125 amp panel - he said it was only 16 feet away. (needs to be less then 25 feet of tap conductor length and will need a single overcurrent device at the load end.
 
electrofelon said:
I agree. I cant recall that I have ever needed secondary protection for a transformer secondary.
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Only single phase two wire or delta-delta three wire secondaries can be protected by the primary. Read 240.21(C)(1). Tap rules may allow the protection to be other then right at the transformer.
 
electrofelon said:
I was talking article 450 protection for the transformer, not conductors.
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I suppose that 240.21(C)(1) is about conductor protection, more so then about transformer protection.

But then what size to the conductors in OP need to be if you considered them protected by the primary device? You also need to provide overcurrent protection that doesn't exceed the rating of the panelboard being supplied. Not so simple with anything other then a two wire or delta - delta system.
 
kwired said:
But then what size to the conductors in OP need to be if you considered them protected by the primary device? You also need to provide overcurrent protection that doesn't exceed the rating of the panelboard being supplied. Not so simple with anything other then a two wire or delta - delta system.
Click to expand...

I am not clear on the question. I know you know your stuff and that we cannot protect the secondary conductors in a D->Y transformer with the primary device. Perhaps I am misunderstanding? When I said I didnt recall ever needing secondary protection, There was still a breaker on the secondary to meet 408.36 and 240.21, but it wasnt serving a specific purpose for the transformer itself.
 
This topic is headed to left field with opinions...

The combined OCPD rating of taps off the transformer cannot be more than 125% of 75000/(208*(Sqrt3))=260.A Table 450.3(A) note (2) 2002 NEC

If a single tap is terminated in a disconnect of less than or equal to the above you can come off the disconnect with what ever amperage you want to how ever many disconnects/panels other BS you want....

*provided all other rules are followed
 
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"......didnt feel like that was complete: So if the OP had primary protection over 125%, then the most he could have is 300 amps of OCPD's on the secondary, so clearly a 200 and 125 would not be compliant. Now if primary protection was 125% or less, then it would be fine."

Please show me the math?
 
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