240.4 Overcurrent

[shockin]

I have a 150 amp breaker feeding a 60 amp fused disconnect. The engineer would like to run #6 and claims the fused disconnect will provide for the overcurrent as required in 240.4. This doesn't seem correct to me. Any one else.

P.S. - Assume that no tap rules apply.

Thanks for the help

 

[iwire]

If you cannot make it fit a 240.21 tap rule it is an likely a violation.

What will it ultimately supply?

 

[charlie b]

You have to protect every conductor against overcurrent at the point at which the conductor gets its power. The tap rules are exceptions to that rule. But since you stipulated that they do not apply, then the #6 conductor has to be protected at the upstream point, not at the downstream point. The engineer is wrong here. A 150 amp breaker cannot protect a #6, and you cannot take credit for the 60 amp fuse at the end of the #6 run.

 

[shockin]

I don't think I can make any of the tap rules fit. It's over 40' away.

The 480v 60a disconnect feeds a controll cabinet which operates a couple 4160v VFD's which control a couple 2500HP motors.

 

[Dennis Alwon]

Why does he want a 150 amp breaker on this?

 

[shockin]

If I could ask a follow-up question.

If I run 1/0 from the 150amp breaker to the 60 amp disconnect is this a problem because the disconnect is only rated at 60 amps?

Obviously the load can't exceed 60 amps because of the fuses

 

[shockin]

Why does he want a 150 amp breaker on this?

The 150 breaker is existing - the load changed from 150 to 60. Long story but the short version is - the 60 amp disconnect was already purchased by the owner.

 

[charlie b]

I think the disconnect would also have to be rated 150 amps, but you could use 60 amp fuses to serve the load.

 

[Dennis Alwon]

Jeez- just buy a 60 amp breaker and be done with it. It ain't worth being in violation.

 

[Dennis Alwon]

I think the disconnect would also have to be rated 150 amps, but you could use 60 amp fuses to serve the load.

Not sure of that but I am sure you will have problems with the lugs and perhaps room for the bends.

 

[One-eyed Jack]

If I could ask a follow-up question.

If I run 1/0 from the 150amp breaker to the 60 amp disconnect is this a problem because the disconnect is only rated at 60 amps?

Obviously the load can't exceed 60 amps because of the fuses

The 1/0 would not be a problem. Any chance you could move the disconnect within 25 ft.?

 

[charlie b]

. . . just buy a 60 amp breaker and be done with it.

Agreed. And if parts of this are already in place, I might infer that there is already a set of conductors rated 150 amps or higher in place. You can leave that as-is, install a 60 amp breaker, and not have to pay for new conductors. If in addition there is an existing 150 amp rated fused disconnect in place, all you need to do is replace the fuses with 60 amp fuses.

So I am a bit confused. What is still in place, and has some of it already been demolished in preparation for the new configuration?

 

[iwire]

I think the disconnect would also have to be rated 150 amps, but you could use 60 amp fuses to serve the load.

The switch need only be rated for the load not the supply.

 

[charlie b]

I think the disconnect would also have to be rated 150 amps. . . .

Not sure of that. . . .

The switch need only be rated for the load not the supply.

I did say, ?I think.? Current will flow through the switch part, before it flows through the fuses. So it makes sense to me that the enclosure itself, or the ?fused disconnect? as an entity, has to be rated for the total current being supplied to it (i.e., the rating of the circuit that feeds it). But I could not find a reference that confirms or refutes this viewpoint. Anybody have a code citation that clarifies this issue?

 

[shockin]

So I am a bit confused. What is still in place, and has some of it already been demolished in preparation for the new configuration?
[/QUOTE]

The motors / controllers / test cabinet were all assembeled, confirgured and tested off site. The "plan" was when it showed up to simply set it in place and make our connections. The plans I have called for a 150 amp circuit which I have run, but as someone else mentioned the terminations to a 60 a disc. are a big problem. It's an AB disc (I didn't even know they made them) and there is zero room inside of it. No chance of getting an 1.5" conduit into it, let alone terminate the conductors on the lugs that are too small.

Changing the breaker out is an option, but would require a shut down of the main service as well as some time constraints and costs in geting the breaker.

I am proposing to install a 200 amp disc and fuse it down to 60 amps.

 

[winnie]

It is quite common that the 'upstream' to a switch has far greater current available than the switch is rated for; think service rated disconnects, tap rules, even breakers on bus stabs.

Back to the OP's situation: you have a 150A breaker, 150A circuit conductors, and a 60A disconnect. I think that the following approaches are viable:

1) Place a junction box, transition to smaller wires (allowed by tap rules) and enter the 60A disconnect.

2) Place a larger disconnect, fused to 60A.

3) Upsell and place some sort of sub-panel, with 60A supplying the load being installed, and capacity for other load now provided 'for free'.

-Jon

 

[augie47]

I agree with Jon (Winnie). Select whichever of the 3 options he has shown thats best suits your sitaution.

 

[Cow]

2) Place a larger disconnect, fused to 60A.

3) Upsell and place some sort of sub-panel, with 60A supplying the load being installed, and capacity for other load now provided 'for free'.

-Jon

I like #2 for your situation.

I like #3 if there is the possibility for future expansion.