125A ckt feeding 2 100A switches

[wasasparky]

IMHO this is a tap.
The 100A lugs/bus are tapped to a 125A feeder.
Conductors are wire, lugs, bus...

 

[raider1]

Can you provide a Code reference?
It is stated periodically within these fora that "If the Code does not prohibit it, then it is allowed." I'm not sure I would always agree with that, but is that all we have here?
thanks
db

Let me try to explain my logic in this case.

What we have is a 125 amp feeder between a 125 amp breaker and a fused switch. The fused switch has 70 amp fuses installed. From the fused switch we have a 70 amp branch circuit to the load. The 70 amp fuses protect the disconnect and the branch circuit (and possibly the load) from overcurrent. The 125 amp breaker only protects the feeder conductors from overcurrent.

If you have a 100 amp rated non-fused disconnect then the 125 amp breaker would be a branch circuit overcurrent protection and it would exceed the rating of the 100 disconnect.

Chris

 

[wasasparky]

charlie,

i am not aware of anything in the nec that requires the feeder and it's overcurrent protective device for a fused disconnect to be sized for the full rating of the disconnect.

Chris

240.4?????

 

[wasasparky]

May I infer that the phrase "100 amp rated fused disconnect" means that that is the highest rated fuse that the component is rated to contain, and not the level to which it must be protected by an upstream device?

No........

 

[barclayd]

Let me try to explain my logic in this case.

What we have is a 125 amp feeder between a 125 amp breaker and a fused switch. The fused switch has 70 amp fuses installed. From the fused switch we have a 70 amp branch circuit to the load. The 70 amp fuses protect the disconnect and the branch circuit (and possibly the load) from overcurrent. The 125 amp breaker only protects the feeder conductors from overcurrent.

If you have a 100 amp rated non-fused disconnect then the 125 amp breaker would be a branch circuit overcurrent protection and it would exceed the rating of the 100 disconnect.

Chris

I'm not sure I agree that the 70A fuses protect the line side of the switch. Everything up-stream of the top of the fuses is under the influence of the 125A breaker.

This is fun
db

 

[raider1]

I'm not sure I agree that the 70A fuses protect the line side of the switch. Everything up-stream of the top of the fuses is under the influence of the 125A breaker.

This is fun
db

The 70 amp fuses will provide overload protection for the line side of the disconnect, and the 125 amp breaker will provide short circuit and ground fault protection.

Chris

 

[LarryFine]

The 70 amp fuses will provide overload protection for the line side of the disconnect, and the 125 amp breaker will provide short circuit and ground fault protection.

This is fine if the disconnect's label states MCA and MOC numbers (which I seriously doubt.)

Otherwise, only tap rules would allow the disco's feeder to be smaller than its supplying OCPD.

 

[raider1]

This is fine if the disconnect's label states MCA and MOC numbers (which I seriously doubt.)

Otherwise, only tap rules would allow the disco's feeder to be smaller than its supplying OCPD.

The feeder in the OPer's question is sized for the 125 amp breaker, the question is can this feeder terminate in a 100 amp rated fused disconnect that has 70 amp fuses installed.

Chris

 

[LarryFine]

The feeder in the OPer's question is sized for the 125 amp breaker, the question is can this feeder terminate in a 100 amp rated fused disconnect that has 70 amp fuses installed.

Got it. I'd say no, the disco must be rated for the OCPD supplying its feeder.

 

[benaround]

In article 240.2 'Definitions'

The definition of 'Tap Conductor' seems to be aimed at this question.

 

[raider1]

Got it. I'd say no, the disco must be rated for the OCPD supplying its feeder.

What if I apply the tap rules in 240.21(B)(1) to the line side of the disconnect?

Chris

 

[LarryFine]

The 70 amp fuses will provide overload protection for the line side of the disconnect, and the 125 amp breaker will provide short circuit and ground fault protection.

Otherwise, only tap rules would allow the disco's feeder to be smaller than its supplying OCPD.

The feeder in the OPer's question is sized for the 125 amp breaker, the question is can this feeder terminate in a 100 amp rated fused disconnect that has 70 amp fuses installed.

Got it. I'd say no, the disco must be rated for the OCPD supplying its feeder.

What if I apply the tap rules in 240.21(B)(1) to the line side of the disconnect?

That's what I said in my first response, and you had an issue with it! Make up your mind, man!



(I'm not really fussing, it was just more fun this way. :grin

 

[barclayd]

Alright - back at it - got coffee & 2 breakfast burritos from Albertos.
I don't know how to do one of those bar graph surveys, but the two questions I would ask are:
1. 125A breaker, 1/0 conductors, one 100A Fused switch - Legal?? YES NO
2. 125A breaker, 1/0 conductors, two 100A Fused switches - Legal?? YES NO

I don't know if I would ever design either one like that, especially #1. It appears that the jury is still out on #2.

Frankly, this part of the existing installation is the least of my worries. One of the two disconnects has 4#4 SO cord up into and across a suspended ceiling, through a wall and down the other side.
Isn't it Lazlo's signature that says "Just because it works, doesn't mean it's right." ??

TGIF
db

 

[iwire]

1. 125A breaker, 1/0 conductors, one 100A Fused switch - Legal?? YES NO

YES

2. 125A breaker, 1/0 conductors, two 100A Fused switches - Legal?? YES NO

YES

Isn't it Lazlo's signature that says "Just because it works, doesn't mean it's right." ??

Yes, but just because it seems odd to you does not make it wrong. :smile:

All that matters is the actual load, not the size of the fused disconnects.

 

[raider1]

That's what I said in my first response, and you had an issue with it! Make up your mind, man!



(I'm not really fussing, it was just more fun this way. :grin

Your right, when I said that there was no tap I was talking about the 1/0 feeder conductors from the 125 amp breaker. I wasn't thninking about the lugs on the disconnect as a tap, my bad.

Chris

 

[raider1]

I agree with Bob.

Chris

 

[wasasparky]

All that matters is the actual load, not the size of the fused disconnects.

But this IS a tap, correct?
(This install would pass the <10'...where one of the requirements is calculated load)

 

[barclayd]

iwire - Is there a code refererce for #1 (single switch)? Are there limits? ie 250A breaker feeding a 30A switch?

The 4#4 SO cord 'works', but maybe not so right.
Also, not very 'odd' around here.

thanks
db

 

[wasasparky]

iwire - Is there a code refererce for #1 (single switch)? Are there limits? ie 250A breaker feeding a 30A switch?

240.21(B)(1)
1 switch, 2 switch, does it matter?

As long as tap requirements are met, I see no limits, other than practicality...