125A ckt feeding 2 100A switches

Status
Not open for further replies.

barclayd

Senior Member
Location
Colorado
I have an existing installation that is causing some local discussion.
125A breaker in a 480Volt, 800A, 3-Phase panel. It feeds two individual 100A 3-phase fused disconnect switches via (3 #1/0) copper. There is a j-box somewhere, that transitions from (3 #1/0) to two sets of (3 #1/0) - one for each switch. Each switch is fused at 60Amps.
Can you feed a 100A switch with a 125A circuit?
I've blearied my eyes trying to find a code reference.
thanks
db
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
If you properly protect the feeder per 240.4, any taps per 240.21, and use properly sized terminations, then you can feed as many 100 amp switches as you deisre with a 125 amp circuit (or any amperage).
A good example might be installing several 100 amp "drops" for a piece of portable equipment and feeding the drops with one circuit.
 
Last edited:

raider1

Senior Member
Staff member
Location
Logan, Utah
I have an existing installation that is causing some local discussion.
125A breaker in a 480Volt, 800A, 3-Phase panel. It feeds two individual 100A 3-phase fused disconnect switches via (3 #1/0) copper. There is a j-box somewhere, that transitions from (3 #1/0) to two sets of (3 #1/0) - one for each switch. Each switch is fused at 60Amps.
Can you feed a 100A switch with a 125A circuit?
I've blearied my eyes trying to find a code reference.
thanks
db

What is the calculated load on each of the disconnects?

The disconnects might be rated for 100 amps but they are being fused at 60 amps, so they are really just a combine load of 120 amps max (Probably less thats why I asked what the calculated load was)

Chris
 

barclayd

Senior Member
Location
Colorado
If you properly protect the feeder per 240.4, any taps per 240.21, and use properly sized terminations, then you can feed as many 100 amp switches as you deisre with a 125 amp circuit (or any amperage).
A good example might be installing several 100 amp "drops" for a piece of portable equipment and feeding the drops with one circuit.

I guess I'll have to find that J-Box. The 25' Rule 240.21(B)(2) might work.

What if there was only one switch? No Taps.

thanks
db
 

barclayd

Senior Member
Location
Colorado
What is the calculated load on each of the disconnects?

The disconnects might be rated for 100 amps but they are being fused at 60 amps, so they are really just a combine load of 120 amps max (Probably less thats why I asked what the calculated load was)

Chris

The total load on the 125A circuit is about 50Amps.
thanks
db
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Something about this just doesn't seem right, but I also could find no code article to forbid it. Clearly, the conductors are being properly protected. But a 125 amp breaker cannot protect a disconnect switch that is itself only rated for 100 amps. I thought there was a blanket statement somewhere that required all things electrical to be protected against overcurrent in accordance with their ratings. Was I wrong about that? :confused:

A week or so ago, I posted a comment to the effect that a breaker's sole function was to protect the conductor. But someone (who shall remain unnamed but whose initials are i.w.i.r.e.) pointed out that the manufacturers of cetain equipment are counting on the breaker to protect their stuff against overcurrent as well.

What do yas all think?
 
Last edited:

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I don't think the tap rules are going to come into play here. All the conductors have an ampacity in excess of the 125 amp rating of the breaker. So the connection point inside the j-box is not a "tap."
 

raider1

Senior Member
Staff member
Location
Logan, Utah
I guess I'll have to find that J-Box. The 25' Rule 240.21(B)(2) might work.

What if there was only one switch? No Taps.

thanks
db


What you have described is not a tap, because the wires that spliced in the j-box are full sized and fully protected by the 125 amp OCPD. This would anly be a tap if the conductors that are spliced onto the 1/0's were smaller.

Chris
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
But someone (who shall remain unnamed but whose initials are i.w.i.r.e.) pointed out that the manufacturers of certain equipment are counting on the breaker to protect their stuff against overcurrent as well.

What do yas all think?
Charlie, when I designed textile machinery line control panels, we always specified a maximum short circuit current available based on our use of class of fuse or breaker ratings. This number was part of the bid package, and if they could not provide current limiting to our "normal" requirements, we would add appropriate (usually current limiting) fusing. Our machinery was almost always either 460 or 575 nominal.

An issue here, in addition to continuous (over)load, is SCCR. We never knowingly used a disconnect that was not rated in excess of source available current. I've not looked at residential / single phase disconnects except to note that "most" residential breakers have a 10kA SCCR. I wonder if our utilities are OK with that?
 

barclayd

Senior Member
Location
Colorado
Clearly, the conductors are being properly protected. But a 125 amp breaker cannot protect a disconnect switch that is itself only rated for 100 amps.

That's what I always thought, too. I've never used the various tap rules, much. I'm not sure why a 125A circuit was used, probably because that was the size of the spare breaker.
 

raider1

Senior Member
Staff member
Location
Logan, Utah
Something about this just doesn't seem right, but I also could find no code article to forbid it. Clearly, the conductors are being properly protected. But a 125 amp breaker cannot protect a disconnect switch that is itself only rated for 100 amps. I thought there was a blanket statement somewhere that required all things electrical to be protected against overcurrent in accordance with their ratings. Was I wrong about that? :confused:

A week or so ago, I posted a comment to the effect that a breaker's sole function was to protect the conductor. But someone (who shall remain unnamed but whose initials are i.w.i.r.e.) pointed out that the manufacturers of cetain equipment are counting on the breaker to protect their stuff against overcurrent as well.

What do yas all think?

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
 

barclayd

Senior Member
Location
Colorado
If there was only one switch - no "taps" - would that be legal?
I would have always terminated a 125A circuit in a 200A switch (if I used a switch).
But, like Charlie, I can't find a reference.

This just Frazzles my Bismarks
db
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Something about this just doesn't seem right, but I also could find no code article to forbid it. Clearly, the conductors are being properly protected. But a 125 amp breaker cannot protect a disconnect switch that is itself only rated for 100 amps. I thought there was a blanket statement somewhere that required all things electrical to be protected against overcurrent in accordance with their ratings. Was I wrong about that? :confused:
...
What do yas all think?
Charlie,
If fuses disconnects and breakers have to have protection on their line side at or below their rating, how would you ever use them?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
But a 125 amp breaker cannot protect a disconnect switch that is itself only rated for 100 amps. I thought there was a blanket statement somewhere that required all things electrical to be protected against overcurrent in accordance with their ratings. Was I wrong about that?

If a larger device could not feed a smaller device, we would not be allowed to have panels with branch breakers smaller than the main breaker.

You maybe thinking of 110.10 that says everything must be rated for its fault current.

I agree with you that according to the NEC a circuit breaker is designed and tested to protect a feeder/branch circuit conductor. The fact that a manufacturer takes advantage of a branch protective device's characteristics is incidental to its original intended purpose.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
If fuses disconnects and breakers have to have protection on their line side at or below their rating, how would you ever use them?
Good point, Don. I need to make a habit of finishing my coffee, before I post anything on this forum. :roll:

Perhaps the elusive (and perhaps even the non-existent) NEC statement I half remember, the one about everything being protected at the level of its rating, does not include any type of disconnecting means or overcurrent device.

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?
 

barclayd

Senior Member
Location
Colorado
Charlie,
If fuses disconnects and breakers have to have protection on their line side at or below their rating, how would you ever use them?

You feed a 100A Non-Fused switch with 100 amps or less. (At least I would)
Are the line side lugs rated for higher than 100A? (I don't know).
A 100A Fused switch would have those same lugs. (I don't know that either)

Ahhh. I'm glad tomorrow's Friday.


db
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
A week or so ago, I posted a comment to the effect that a breaker's sole function was to protect the conductor. But someone (who shall remain unnamed but whose initials are i.w.i.r.e.) pointed out that the manufacturers of cetain equipment are counting on the breaker to protect their stuff against overcurrent as well.
I found the comment that that "someone" made. It turns out to have been more than a week ago. Here it is:
I can not agree, IMO the listing agencies and the NEC consider the OCPD to be a part of utilization equipment protection.
I misread that comment. It is not, as I first thought, saying that the OCPD is there to protect anything and everything downstream.
 

barclayd

Senior Member
Location
Colorado
Let's say I need to feed a 70 amp load.
I've got a 150 amp spare breaker in the panel.
I run 1/0 from the breaker to a fused switch containing 70 amp fuses, and from there to the load.
What size is that switch? 100A, or 200A?

If it was a 60 amp load, could I terminate in a 60A switch?

(I know this is not a real good design, but, for the sake of argument, let's say it's an old panel and breakers are hard to find so I use an existing spare.) (this part is all hypothetical anyway)

Are there non-tap-rule limits?

thanks
db
 

raider1

Senior Member
Staff member
Location
Logan, Utah
Let's say I need to feed a 70 amp load.
I've got a 150 amp spare breaker in the panel.
I run 1/0 from the breaker to a fused switch containing 70 amp fuses, and from there to the load.
What size is that switch? 100A, or 200A?

IMHO it would be a 70 amp switch.

If it was a 60 amp load, could I terminate in a 60A switch?

Yes, you could use a 60 amp fused switch.

Chris
 

barclayd

Senior Member
Location
Colorado
IMHO it would be a 70 amp switch.



Yes, you could use a 60 amp fused switch.

Chris

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
 
Status
Not open for further replies.
Top