Continuous Duty - Disconnect vs. Breaker

Status
Not open for further replies.

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I think the burden of proof is on you to show that there IS a derating for continuous loads required for a non fused safety switch ;) Note that the NEC 80% continuous derating we are familiar with is for OCPD's branch circuits, feeders, and service entrance conductors. I think you will find them in 210.19(A)(1), 210.20(A), 215.2(A)(1), 215.3, 230.42. It does not apply generally to all equipment. Class 320 ("400 amp") meter sockets are derated 80% for continuous use, but that is a product standard or UL thing not an NEC requirement.

How about posting the code section that requires derating switches?

Are the parts of a switch that carry current not 'conductors'? 705.60(B) requires continuous derating for conductors. It has an exception for "an assembly together with its overcurrent device(s) that is listed for continuous operation at 100 percent of its rating". It seems to me that could apply to a fused disconnect but not an unfused switch.

I don't why you guys are asking me to argue for a point I didn't wish to be true, but at this point I'm playing devil's advocate in hopes you can supply me with the right argument against an AHJ who doesn't agree.
 

Carultch

Senior Member
Location
Massachusetts
Are the parts of a switch that carry current not 'conductors'? 705.60(B) requires continuous derating for conductors. It has an exception for "an assembly together with its overcurrent device(s) that is listed for continuous operation at 100 percent of its rating". It seems to me that could apply to a fused disconnect but not an unfused switch.

I don't why you guys are asking me to argue for a point I didn't wish to be true, but at this point I'm playing devil's advocate in hopes you can supply me with the right argument against an AHJ who doesn't agree.

I don't see a definition of "conductor" in Article 100. I see sub-definitions of bare/insulated/covered conductors, but they all depend on the word conductor. So does "conductor" specifically mean a wire of some form or another? Or any material that can conduct electricity, like it does in Chemistry/Matsci?

If what you are saying is the case, that the integral OCPD also has to be continuous duty for any device to be rated as such, then there would be no reason to list an unfused disconnect for continuous duty.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Are the parts of a switch that carry current not 'conductors'? 705.60(B) requires continuous derating for conductors. It has an exception for "an assembly together with its overcurrent device(s) that is listed for continuous operation at 100 percent of its rating". It seems to me that could apply to a fused disconnect but not an unfused switch.
The unfused discos made by Square D and others are so listed. The fused discos are not, but since the minimum fuse size is calculated at 125% of the inverter output and you cannot put a fuse in a disco that has a higher rating than the switch itself, the point is probably moot.
 
Can you give me details on what equipment you installed for what application and what aspect of the inspection you failed? I assume (I hope) that your being nailed was an inspection failure and not a catastrophic system failure.

I am not looking to simply win an argument here; I just want to design safe and code-compliant PV systems. I have had hundreds of systems pass inspections in several jurisdictions and this issue has never come up.

Sure. This was a rooftop system, 300 something KW, 277/480 service. 600 amp PV feeder/combiner stuff. The inverter output circuits came off the roof into a combiner panelboard, then a 600 amp feeder fed the safety switch, then inside to a breaker in a MLO switchboard (supply side). There were 8 of these systems, but all of them were all a little different: some had a service buss tap to a fused disco inside, a few had a bus tap to a circuit breaker, but they all had the unfused disco on the outside which was a utility requirement. I only remember the SCCR of the disco being in issue at one site due to a variety of factors. The system had not been energized.

I have a real hard time believing that a unfused disco will blow apart at say 50k when it doesnt have a certain class of fuse ahead of it, but thats not my call to decide and it is what it is, I think this is an oft overlooked thing. 110.10 is the code section. IIRC, this section, or perhaps the mentioning of SCCR is new in the last few code cycles.
 
Are the parts of a switch that carry current not 'conductors'?

Well that could be argued. We have discussed that on here in the past in regards to the bussing in a panelboard and whether a panelboard can carry full continuous loads. I guess if you take the definition of feeder completely literally, and dont think that a piece of a equipment having its own article matters, then I guess the panelboard bus is a conductor that is subject to 215.2. Likewise for a switch.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I don't see a definition of "conductor" in Article 100. ...

Yes, I was aware of that when I asked the question. Can't really settle the argument there.

Well there are not a type listed in article 310 so if you call them conductors we have a problem.

I guess I just don't see how that wins the argument either.

The unfused discos made by Square D and others are so listed. ...

We use Eaton switches. None of the readily available documentation I found says they are so listed. I sent them a question about it. We'll see if they reply.

Is this something where the UL standard could be referenced? i.e. if the switch is listed to the standard then it's listed for continuous duty?

Well that could be argued. We have discussed that on here in the past in regards to the bussing in a panelboard and whether a panelboard can carry full continuous loads. I guess if you take the definition of feeder completely literally, and dont think that a piece of a equipment having its own article matters, then I guess the panelboard bus is a conductor that is subject to 215.2. Likewise for a switch.

Thank you for agreeing with me that it could be argued. That's all that really matters, right? :lol:

From my point of view, one lost argument with an inspector, reinstall of the switch, re-inspection labor and fees, opportunity cost, etc,... It wipes out the savings on at least 10 other projects where I might substitute a 30A switch for a 60A switch. I feel I've got to have an ironclad argument and documention if I'm going to bat better than 90% on this.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
From my point of view, one lost argument with an inspector, reinstall of the switch, re-inspection labor and fees, opportunity cost, etc,... It wipes out the savings on at least 10 other projects where I might substitute a 30A switch for a 60A switch. I feel I've got to have an ironclad argument and documention if I'm going to bat better than 90% on this.

Last time I looked I found the Square D documentation. I've been looking again (in my spare time and I'm awful busy, so no breath holding, OK?) but I haven't found it yet. I spoke with Eaton on the phone a couple of years ago and they told me the same thing.

I have never had an inspector challenge me over this issue.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Found it. Excerpt from Square D tech bulletin 311DB0401R0410 dated 06/2010:

• Non-fusible safety switches may carry 100 percent of the nameplate
current rating.
• Fusible safety switches may carry 80 percent of nameplate current
rating (continuous use).
 

iwire

Moderator
Staff member
Location
Massachusetts
I don't why you guys are asking me to argue for a point I didn't wish to be true,.

Because it was you who put forth the idea that non-fused disconect switches where subject the 80% rules of feeders.

And you are the only person I have seen make that claim and the only code reference you have provided for it is the article 100 definition of feeder.


I guess I just don't see how that wins the argument either.

Well there is no argument, disconnect switches are not feeders.

If we went with your theory that they are feeder conductors than every piece of electrical equipment that carries current would be subject to the 80% rules of feeders and branch circuits and that is far from reality.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
...
I have never had an inspector challenge me over this issue.

Good to know.

Because it was you who put forth the idea that non-fused disconect switches where subject the 80% rules of feeders.

And you are the only person I have seen make that claim and the only code reference you have provided for it is the article 100 definition of feeder.

I thought I was being clear that I'm playing devil's advocate. If I can come up with these questions, then so can an AHJ.

We'd be great if you were my AHJ, but you're not. :D
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Well that has been debated, and several prominent well respected forum members do not agree with you. I do, but I'm just saying the issue is not definitive.

What are we debating, exactly? The Square D documentation is very straightforward. If you are interconnecting an inverter with Imax = 25A through a fused disco, it must be a 60A switch. If it is through an unfused disco, it can be a 30A one.
 
What are we debating, exactly? The Square D documentation is very straightforward. If you are interconnecting an inverter with Imax = 25A through a fused disco, it must be a 60A switch. If it is through an unfused disco, it can be a 30A one.

A reasonable argument can be made that the nec calls the bussing in a panel board or unfused disco a feeder, and thus it needs to be derated for continuous loads, regardless of if the manufacturer says its ok. That would be an "Antonin scalia" esqe interpretation
And it's what the wording says, but I don't think it was the intent.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
What are we debating, exactly? The Square D documentation is very straightforward. If you are interconnecting an inverter with Imax = 25A through a fused disco, it must be a 60A switch. If it is through an unfused disco, it can be a 30A one.

From my point of view, what we're debating is whether, say, 1 out of 10 AHJs can raise enough a reasonable argument against using the smaller rated switch as to make doing so entirely not worth my while.

If I were using Square D switches then from my point of view we'd be done. But I'm using Eaton switches, and would have to put some time into finding out if I can get the same competitive pricing on Square D. If Eaton gets back to me with documentation to cite then I'll say we're done. :)
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
From my point of view, what we're debating is whether, say, 1 out of 10 AHJs can raise enough a reasonable argument against using the smaller rated switch as to make doing so entirely not worth my while.

If I were using Square D switches then from my point of view we'd be done. But I'm using Eaton switches, and would have to put some time into finding out if I can get the same competitive pricing on Square D. If Eaton gets back to me with documentation to cite then I'll say we're done. :)
I got the same info from Eaton, and I had it in writing at one time, but I've changed jobs twice since then and the documentation I had went with the computer I turned in when I left. I happened to still have the SqD info but I'll leave it to you to find the Eaton equivalent. Good luck!
 
Status
Not open for further replies.
Top