Medium Voltage Switch sizing

[electrofelon]

Once we connected a PV system with (I'm just making up the numbers for illustration because it was so long ago) where the inverter output current was 98A. We interconnected in an MDP with a 125A backfed breaker per code, and we figured that we could use a 100A unfused disconnect since it was rated higher than the inverter Imax. An inspector called us on it and said he would only allow it if we got it in writing from the switch manufacturer that it was OK. The manufacturer told us that the switch must be rated equal or greater than the breaker.

I'll just add that the manufacturer's position was understandable, since in the event of a fault on the inverter side of the switch, the OCPD would allow 125A to flow through the 100A switch.

I remain skeptical. I mean who did you talk to exactly? Werner Von Siemens or Joseph Eaton directly? I would want to see a NEC requirement or such language in the instructions telling me that the switch must be protected at its rating by an OCPD. We can install NF switches as service equipment with NO OCPD after all.

 

[ggunn]

I remain skeptical. I mean who did you talk to exactly? Werner Von Siemens or Joseph Eaton directly? I would want to see a NEC requirement or such language in the instructions telling me that the switch must be protected at its rating by an OCPD. We can install NF switches as service equipment with NO OCPD after all.

I can only relate what happened to us. No, I didn't speak to Werner or Joseph directly, but we got an official response on the company (whichever it was) letterhead that we could not do it. And yes, of course with service equipment things are different.

 

[electrofelon]

I can only relate what happened to us. No, I didn't speak to Werner or Joseph directly, but we got an official response on the company (whichever it was) letterhead that we could not do it. And yes, of course with service equipment things are different.

I did some digging and I do see that language in both the Siemens and Eaton safety switch catalogs. So I am almost going to say you are correct, however they state this in their SCCR ratings section, so it is not clear to me if that statement is just for if/when you want to use the series combination ratings to get a higher than base 10k SCCR. Also while I sort of agree with your statement "of course with service equipment things are different" where is that stated? I combed thru the catalog and although they say SUSE, they dont state any exception to the must be protected by a breaker wording. ITs all confusing. Then we have what SQ D says (which I have complained about before and makes no sense).

"a Non-Fused Safety Switch by itself does NOT provide any short circuit current rating because it does not contain any overcurrent protection. HOWEVER, any brand of circuit breaker or fuse not exceeding the ampere rating of the switch may be used ahead of a non-fusible safety switch when there is up to 10,000A MAXIMUM of short circuit current available."

Of course the "does not provide any short circuit current rating" makes absolutely no sense, but they also dont see to offer any allowance to use it as service equipment then. So V confused.

 

[ggunn]

I did some digging and I do see that language in both the Siemens and Eaton safety switch catalogs. So I am almost going to say you are correct, however they state this in their SCCR ratings section, so it is not clear to me if that statement is just for if/when you want to use the series combination ratings to get a higher than base 10k SCCR. Also while I sort of agree with your statement "of course with service equipment things are different" where is that stated? I combed thru the catalog and although they say SUSE, they dont state any exception to the must be protected by a breaker wording. ITs all confusing. Then we have what SQ D says (which I have complained about before and makes no sense).

"a Non-Fused Safety Switch by itself does NOT provide any short circuit current rating because it does not contain any overcurrent protection. HOWEVER, any brand of circuit breaker or fuse not exceeding the ampere rating of the switch may be used ahead of a non-fusible safety switch when there is up to 10,000A MAXIMUM of short circuit current available."

Of course the "does not provide any short circuit current rating" makes absolutely no sense, but they also dont see to offer any allowance to use it as service equipment then. So V confused.

So, from the experience I had to this day I have always used an unfused switch that is rated at least as high as the OCPD ahead of it when there is one. I don't recall ever using one as service equipment, so I guess that would be all the time.

 

[electrofelon]

So, from the experience I had to this day I have always used an unfused switch that is rated at least as high as the OCPD ahead of it when there is one. I don't recall ever using one as service equipment, so I guess that would be all the time.

I'm having lunch today with Werner and Joseph, I will ask them for more information. That is if I can even get a word in: James Hardie and Robert Bosch are coming to, and when those two get arguing about politics, not much other discussion can happen.