Nameplate Rating Bussing

[MJRobinson]

Please see attached picture.

label Information: "Through Bus" = 800A Section Amps = 600 Max.

Want to confirm for 120% rule that "Through Bus" = Nameplate Rating.

Thanks
MJR

 

[jaggedben]

I wouldn't confirm that. Ask the AHJ.

Looks like you've got a 'multiple ampacity' busbar which if in California under the 2014 NEC (equivalent) can be interconnected to under 'engineering supervision.'

 

[pv_n00b]

Through bus is the horizontal bus that connects the sections together. Section bus runs vertically in a section and is what supplies whatever is connected to the bus in that section. If you land a back feed breaker in the section then the bus rating is 600A for the 120% rule. For more info see this: http://electrical-engineering-portal.com/switchboard-construction-siemens-basics

Probably not what you were hoping for.

 

[kwired]

Through bus is the horizontal bus that connects the sections together. Section bus runs vertically in a section and is what supplies whatever is connected to the bus in that section. If you land a back feed breaker in the section then the bus rating is 600A for the 120% rule. For more info see this: http://electrical-engineering-portal.com/switchboard-construction-siemens-basics

Probably not what you were hoping for.

I don't do PV stuff, but agree - depends if you are feeding into the 800 amp main bus or into a 600 amp section bus.

 

[jaggedben]

I don't do PV stuff, but agree - depends if you are feeding into the 800 amp main bus or into a 600 amp section bus.

To invoke the 120% rule you have to be at the opposite end of the busbar, which arguably isn't possible when you have multiple busbar sections coming off the main bus, presumably without individual OCPD. (i.e. you can't be at the opposite end of all those busbars) I think best case is that the main switch gear OCPD is not more than 600A and the AHJ lets you tie into the opposite end of the 600A section that is at the opposite end. I assume there is a stipulation that each 600A bus section can't have more than 600A load, and if that's followed then the logic behind the rule would deem such an installation safe. But article 705 doesn't explicitly address that. Probably it just needs an engineer to sign off on the details that simple code rules (or not-so-simple code rules, as the case may be) just can't fully address.

 

[pv_n00b]

To invoke the 120% rule you have to be at the opposite end of the busbar, which arguably isn't possible when you have multiple busbar sections coming off the main bus, presumably without individual OCPD. (i.e. you can't be at the opposite end of all those busbars) I think best case is that the main switch gear OCPD is not more than 600A and the AHJ lets you tie into the opposite end of the 600A section that is at the opposite end. I assume there is a stipulation that each 600A bus section can't have more than 600A load, and if that's followed then the logic behind the rule would deem such an installation safe. But article 705 doesn't explicitly address that. Probably it just needs an engineer to sign off on the details that simple code rules (or not-so-simple code rules, as the case may be) just can't fully address.

I agree, once you start interconnecting to switchboards there are few simple rules of thumb anymore. Switchboards are too highly customizable and you need to understand how that specific switchboard is put together. I've seen weird stuff out there. I've talked to many contractors who just default to supply-side interconnections because they don't want to get involved in what is going on in there, I don't agree with that choice but many do it.

 

[ggunn]

I agree, once you start interconnecting to switchboards there are few simple rules of thumb anymore. Switchboards are too highly customizable and you need to understand how that specific switchboard is put together. I've seen weird stuff out there. I've talked to many contractors who just default to supply-side interconnections because they don't want to get involved in what is going on in there, I don't agree with that choice but many do it.

Why not? Supply side interconnections are frequently much simpler to design and install. They are just as legal and safe as load side connections. If the input OCPD in the switchgear happens to have space on its input bus to land PV connectors, it's easy-peasy.

 

[pv_n00b]

Why not? Supply side interconnections are frequently much simpler to design and install. They are just as legal and safe as load side connections. If the input OCPD in the switchgear happens to have space on its input bus to land PV connectors, it's easy-peasy.

How many have you done where you used a landing point that was labeled "tap" and not just some convenient hole or stud that happened to be there? How many people drill holes in bus to land PV conductors? It might seem easy to do, but it's not easy to do correctly.

https://www.ul.com/global/documents...newsletters/electricalconnections/april10.pdf

 

[MJRobinson]

Answer

Answer

Thanks,

Just to be on the safe side of the rule. I'll treat this as 600A.

I'll have to de-rate the main to get me under the rule.

I'll also look to the manufacturer to see if they have Tap Locations on the Main Bussing.

Michael

 

[ggunn]

How many have you done where you used a landing point that was labeled "tap" and not just some convenient hole or stud that happened to be there? How many people drill holes in bus to land PV conductors? It might seem easy to do, but it's not easy to do correctly.

https://www.ul.com/global/documents...newsletters/electricalconnections/april10.pdf

I have never drilled a hole in a bus to land PV connection terminals. I have found open holes just like the ones the service conductor terminals are landed on and gotten the OK from the switchgear manufacturer to use them for PV interconnection.

 

[pv_n00b]

I have never drilled a hole in a bus to land PV connection terminals. I have found open holes just like the ones the service conductor terminals are landed on and gotten the OK from the switchgear manufacturer to use them for PV interconnection.

I've talked to many switchboard manufacturers and have never found one that will approve landing at an unused hole that is not marked for taps. What manufacturers have you worked with who have approved it? Did they send you an approval letter or was it over the phone? I'd like to know who to work with.

 

[pv_n00b]

I've always thought that when 705.12(B)(2)(3) on busbar rating referred only to panelboards that it was just a typo since (B) says the provisions apply to all distribution equipment. But I've been asking around and someone on CMP4 told me that it was intended to only apply to panelboards because they have more restriction on loading than switchgear. I was also told that in 2020 this will be made more clear by stating that interconnections to switchboards and switchgear will require engineering supervision just as 705.12(B)(2)(3)(e) requires for multi-ampacity busbars. That's great on one hand since it removes all the busbar rule of thumb restrictions in (B)(2)(3), but requires an engineer to decide how much can be back fed.

 

[ggunn]

I've talked to many switchboard manufacturers and have never found one that will approve landing at an unused hole that is not marked for taps. What manufacturers have you worked with who have approved it? Did they send you an approval letter or was it over the phone? I'd like to know who to work with.

One was, I believe, GE, who sent over a rep and gave me a letter. The other I don't remember who it was. They were unresponsive for a long time, but finally one of their engineers told me over the phone "Of course you can; the holes are there for parallel sets, and if they aren't all filled you can use the open ones for PV." He kind of acted like I was asking a dumb question. The AHJ wanted it in writing; they accepted an email from the engineer.