240.21(c)(4)

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charlie b

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There will be a transformer in one building, and a long underground run towards another building. Conduits will penetrate the floor of the second building, then rise up to the electric room on the second floor. The conduit riser will be encased with 2 inches of concrete, until within 15 feet (WA State limit) of a fused disconnect. The fused disconnect will serve as both the required building disconnecting means and the overcurrent protection for the transformer secondary conductors.

I should think that you would want the incoming conductors to land on the disconnect switch terminals, and have the fuses on the load side of the switch. That way, you can safely replace the fuses by first opening the switch. But I read the tap rule in 240.21(C)(4) as saying that the conductors have to land on the fuses. What say you?

Related question: 240.21(C)(4)(4) requires the disconnecting means to be ?readily accessible.? I think this has been debated here before, but if the fused disconnect is inside a locked electrical room, is it still ?readily accessible??
 

jim dungar

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I should think that you would want the incoming conductors to land on the disconnect switch terminals, and have the fuses on the load side of the switch. That way, you can safely replace the fuses by first opening the switch. But I read the tap rule in 240.21(C)(4) as saying that the conductors have to land on the fuses. What say you?
Fuses are not UL Listed with direct connections to cables. So meeting the 'literal' wording of the code is not possible. This leaves you with having the conductors land on fuse holders or fused disconnects, both of which meet the 'intent' of the code.
 

david luchini

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I should think that you would want the incoming conductors to land on the disconnect switch terminals, and have the fuses on the load side of the switch. That way, you can safely replace the fuses by first opening the switch. But I read the tap rule in 240.21(C)(4) as saying that the conductors have to land on the fuses. What say you?

Charlie, the requirement of 240.21(C)(4) cannot be applied to the situation that you described, as it is for "Outside Secondary Conductors." 240.21(C)(4) would apply "where the conductors are located outdoors of a building or structure, except at the point of load termination...."

You described a situation where the conductors are outside except at the point of Line and Load termination. At least that's the way I would read it.
 

charlie b

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This leaves you with having the conductors land on fuse holders or fused disconnects, both of which meet the 'intent' of the code.
Jim, I have no problem with landing on a fused disconnect. But which of the two components within that enclosure should get the line side termination? In other words, as I put the symbol on the drawing, with a box enclosing a fuse and a switch, and with the incoming power connected at the bottom of the box, do I show the fuse symbol above the switch symbol, or do I show it the switch symbol above the fuse symbol?

 

charlie b

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You described a situation where the conductors are outside except at the point of Line and Load termination.
Sorry, David, but I am not following your description. The conductors are outside the building, as they pass underground from the first building and get to the lowest level of the second building. They are still "outside" the second building as they pass up to the second floor, since they are encased in 2 inches of concrete. They are not "inside" until they get close to the building disconnecting means.
 

steve66

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Sorry, David, but I am not following your description. The conductors are outside the building, as they pass underground from the first building and get to the lowest level of the second building. They are still "outside" the second building as they pass up to the second floor, since they are encased in 2 inches of concrete. They are not "inside" until they get close to the building disconnecting means.

I would agree with David on this one. I think that since the line side of the conductors terminate on the transformer inside the building, that would would not comply with this paragraph. In other words, I think this paragraph is reserved for transformers that are located outside (since that is the only way the line side terminations would be outside).

I would suggest putting a fused disconnect at the transfomer, and maybe an unfused disconnect at the other building.

Steve
 

charlie b

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OK, David and Steve, help me out here. I am not quite getting it. Are you two saying that because the transformer is inside the first building, the conductors from the transformer to the point they leave the first building (heading underground for the second building) are not "located outside," the article does not apply?
 

david luchini

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Sorry, David, but I am not following your description. The conductors are outside the building, as they pass underground from the first building and get to the lowest level of the second building. They are still "outside" the second building as they pass up to the second floor, since they are encased in 2 inches of concrete. They are not "inside" until they get close to the building disconnecting means.

But where they connect to the transformer secondary in the first building, are they not "inside" the first building?
 

david luchini

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OK, David and Steve, help me out here. I am not quite getting it. Are you two saying that because the transformer is inside the first building, the conductors from the transformer to the point they leave the first building (heading underground for the second building) are not "located outside," the article does not apply?

If the transformer is inside the first building, then I would say where the secondary conductors terminate is "inside" the first building, so yes, that is what I'm saying.

Unless the transformer is within a vault meeting the construction requirements of Art 450, Part III, then maybe you could argue that per 230.6(3) that they are outside the building.
 
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charlie b

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My apologies. I just checked with the engineer who did the design (I am doing a design review), and I discovered that I had misunderstood the physical layout. The transformer in question is, quite literally, outdoors. There is no "first building" supplying a second building. Mea culpa, mea culpa, mea maxima culpa.

That brings me back to my question of which way do I show the fused disconnect on the drawing (i.e., fuse on top, or switch on top, given that the power comes in from the bottom).
 

david luchini

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My apologies. I just checked with the engineer who did the design (I am doing a design review), and I discovered that I had misunderstood the physical layout. The transformer in question is, quite literally, outdoors. There is no "first building" supplying a second building. Mea culpa, mea culpa, mea maxima culpa.

That brings me back to my question of which way do I show the fused disconnect on the drawing (i.e., fuse on top, or switch on top, given that the power comes in from the bottom).

Taking the code language literally, I'd say the fuses would be on the bottom. Then of course you'd need a LOTO at the transformer main to safely change the fuses as you suggest. (Of course using an enclosed c/b would kill two birds with one stone.)

And regarding the locked electrical room, I think that meets the requirements of readily accessible as long as there is ready access to the key by those who need the access to the room.
 

jim dungar

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Jim, I have no problem with landing on a fused disconnect. But which of the two components within that enclosure should get the line side termination? In other words, as I put the symbol on the drawing, with a box enclosing a fuse and a switch, and with the incoming power connected at the bottom of the box, do I show the fuse symbol above the switch symbol, or do I show it the switch symbol above the fuse symbol?
240.40 says the disconnect must be on the 'supply' side of the fuses, except as permitted by 230.82.
 

david luchini

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240.40 says the disconnect must be on the 'supply' side of the fuses, except as permitted by 230.82.

240.40 says that for fuses that are accessible to other than qualified personnel, or for circuits over 150V to ground, the disconnect must be on the supply side. So if the secondary is 208/120 and the fuses/disconnect are in a locked closet, the fuses on the "supply" side would be OK.

240.21(C)(4) requires the conductors to terminate on a single set of fuses, either in an integral disconnect, or located adjacent to a disconnect. I don't think 240.21(C)(4) would allow termination on the disconnect first.

So taking 240.40 together with 240.21(C)(4), I'd say you're limited to a c/b disconnect where your secondary was greater than 150V to ground or where the disconnect was accessible to other than qualified personnel.
 

LarryFine

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Charlie, in my opinion, 240.21(C)(4) does not exclude the switch of a fused switch, it's merely saying a single OCP device, as opposed to more than one (breaker or fuse set), in order to limit current.

I don't believe the switch will interfere with that function. After all, it basically applies to just about every service, that the fuses limit current, but the switch still comes first for disco and re-fusing.
 

charlie b

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I don't think 240.21(C)(4) would allow termination on the disconnect first.
That is the essence of my question.


There is no difference in the physics of the two configurations. Suppose the conductors land on the switch, and the fuses are downstream (within the same enclosure). With the switch closed, the conductors are essentially connected directly to the fuses. With the switch open, the fuses are out of the circuit. But the fuses cannot protect against a failure within the secondary conductors. So why should it matter?

By the way, the circuit is 480 volts, so the disconnect has to be upstream of the fuses. If I can't land the conductors on the switch, then it will have to be a breaker, as you say.
 

david luchini

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There is no difference in the physics of the two configurations. Suppose the conductors land on the switch, and the fuses are downstream (within the same enclosure). With the switch closed, the conductors are essentially connected directly to the fuses. With the switch open, the fuses are out of the circuit. But the fuses cannot protect against a failure within the secondary conductors. So why should it matter?

I agree the configuration really wouldn't matter physically, but as you point out the code requires the secondaries to terminate at either a single c/b or single set of fuses. And the OCPD is required to be integral to a disconnecting means, or immediately adjacent thereto. I read that as either fuse-switch in a fused disconnect, or fuse-adjacent switch in a separate configuration. It seems silly, but that's the code language.

By the way, the circuit is 480 volts, so the disconnect has to be upstream of the fuses. If I can't land the conductors on the switch, then it will have to be a breaker, as you say.

I'd say go with the breaker anyway, easier maintenance with no fuses to replace.
 

LarryFine

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I agree the configuration really wouldn't matter physically, but as you point out the code requires the secondaries to terminate at either a single c/b or single set of fuses.
I still believe that the switch doesn't change the fact that the termination is the single set of fuses.

I feel the intent is a single fuse set, not only the fuses. Multiple fused switches would be prohibited.

What would be the big concern about having switch blades ahead of the fuses? Services certainly do.
 

jim dungar

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240.21(C)(4) requires the conductors to terminate on a single set of fuses, either in an integral disconnect, or located adjacent to a disconnect. I don't think 240.21(C)(4) would allow termination on the disconnect first.

You literally cannot terminate directly to "listed" fuses, so we must infer the intent of the code when it tells us to do so. You can only terminate to fuse holders or disconnect switches with integral fuse holders.

If 240.21(C)(4) is applicable only if you terminate on the fuses, then it can never be applied on circuits greater than 150V to ground.

The OP did not specify the voltage. If the circuit under discussion is not 120/240 1PH 3W, 208/120 1PH 3W, or 208Y/120 3PH 4W then a supply side disconnect must be used.
 

Volta

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I like Larry's take on this, we can really have no reasonable argument against the disconnect on the line side of the fuses. The disconnecting means is mentioned in half of the sub-sections- (3) and (4).

Terminating at a single set of fuses does not disallow the fuseholders, the reducing pins or lugs one might use, splicing devices, or the disconnecting means, as far as I can tell.

It disallows multiple sets of OCP devices in parallel.
 

david luchini

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I still believe that the switch doesn't change the fact that the termination is the single set of fuses.

I feel the intent is a single fuse set, not only the fuses. Multiple fused switches would be prohibited.

If you are terminating on a switch then you are not terminating on an OCPD. If it is OK to terminate on the switch side of a fused disconnect, then is it OK to terminate on a disconnect switch and then route to a set of fuses that is mounted adjacent? Or is it OK to terminate on a switch and then route to a c/b that is mounted adjacent? In either case you're not terminating on a c/b or set of fuses, as 240.21(C)(4) requires.

What would be the big concern about having switch blades ahead of the fuses? Services certainly do.

There is no concern, except that the code tells you that service conductors should terminate at a disconnecting means and it tells you that outside secondary conductors should terminate at a single OCPD.
 
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