Switchboard with busing and no MCB - tap rule?

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Anode

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We have a project where we are planning on future expansion in our run to the point of interconnect. Despite the system size only being 10kWdc, we are putting in infrastructure that will accommodate maximum utilization of the roof, for future system expansion.

Our intent was to tap feeder conductors in switchboard with insulated piercing connectors to each phase. We had not confirmed what was inside the switchboard. Today we removed the cover off the customer side of the switchboard, which is on the right side, and saw busing fed from the left (utility) side. On the lower third portion of the customer side, there are the breakers for distribution. My question is, since there is no main circuit breaker in the switchboard, are we still bound to 120% of the bus bar rating above the distribution breakers? Or can we tap the bus above all those breakers and circumvent being held to 20% of 600A rated busing?

Picture of the diagram of the switchboard, and a crude rendition of what the busing looks like on the customer side.

Untitled.jpg2.jpg

Side note: MFG is fine with taping the busing on the customer side, and entering the switchboard on the customer side.

Your help is greatly appreciated.
 

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This is a no main device SES, you can back feed any of the allowed six overcurrent devices (switches or CBs) up to the rating of the device and the service. But the limit is six switches, an external switch would be a 7th handle. Some AHJ may allow an external fused safety switch as a second service.

Tapping of the busses usually violates the UL listing of the SES, but the manufacturer may have some provisions such as a special kit.

One possibility to consider is transferring one of the feeders presently served to a sub-panel or external fuse and using the original switch to feed both the sub-panel and the PV fused disconnect. I did this with a similar SES; The switched fuse was 200A, the PV fuse was a 100A fused safety switch and the cable from switched fuse to each of the other devices had an ampacity of 300A for the situation. In this case the existing 200A cable was protected by a 200A pull-out fuse (due to space restraints, a 200A fused safety switch could have been used. We had no problem with the AHJ or utility over this.
 
Anode said:
We have a project where we are planning on future expansion in our run to the point of interconnect. Despite the system size only being 10kWdc, we are putting in infrastructure that will accommodate maximum utilization of the roof, for future system expansion.

Our intent was to tap feeder conductors in switchboard with insulated piercing connectors to each phase. We had not confirmed what was inside the switchboard. Today we removed the cover off the customer side of the switchboard, which is on the right side, and saw busing fed from the left (utility) side. On the lower third portion of the customer side, there are the breakers for distribution. My question is, since there is no main circuit breaker in the switchboard, are we still bound to 120% of the bus bar rating above the distribution breakers? Or can we tap the bus above all those breakers and circumvent being held to 20% of 600A rated busing?

Picture of the diagram of the switchboard, and a crude rendition of what the busing looks like on the customer side.

View attachment 12793View attachment 12794

Side note: MFG is fine with taping the busing on the customer side, and entering the switchboard on the customer side.

Your help is greatly appreciated.
Click to expand...

The busing in the switchboard are "service bus bars" or service conductors so that would be a supply side connection, no 120% rule. If there are already holes where a lug can be bolted on to, you should be good.
 
What you're proposing is a classic supply side connection where the 120% rule doesn't apply.

You are lucky that the manufacturer is agreeable to your proposal and you should get as much documentation of their agreeableness and recommended methods as you can to show to your local building officials.

In my opinion since your tap creates a new set of service entrance conductors and leads to a disconnect in a new enclosure then you are allowed another six handles. See here. But I would run this by the AHJ before you install 10k of modules on the roof, not after.
 
BillK-AZ said:
This is a no main device SES, you can back feed any of the allowed six overcurrent devices (switches or CBs) up to the rating of the device and the service. But the limit is six switches, an external switch would be a 7th handle. Some AHJ may allow an external fused safety switch as a second service.
Click to expand...
This is good news. I'd imagine that landing on a OCPD instead of tap conductors to the bus, would make the manufacturer/inspector/utility happy, but would cost more. Just to be clear, my 175 or 200Amp 3P circuit breaker would be okay on a 600A rated bus because of no 120% rule. That is refreshing in this case.

Can you site any code to help me out with that?
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electrofelon said:
The busing in the switchboard are "service bus bars" or service conductors so that would be a supply side connection, no 120% rule. If there are already holes where a lug can be bolted on to, you should be good.
Click to expand...

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There are holes in the busing. Same response as above.​




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jaggedben said:
What you're proposing is a classic supply side connection where the 120% rule doesn't apply.

You are lucky that the manufacturer is agreeable to your proposal and you should get as much documentation of their agreeableness and recommended methods as you can to show to your local building officials.
Click to expand...

​
Well the manufacturer didn't seem to think it would be an issue, but said she wouldn't provide anything in writing. In this case, it may be better to land on a CB instead, pay the extra cost, and avoid the hassle? I would still need a fused disconnect in this system configuration, yes?
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Once again, thank you all very much. :thumbsup:
 
Anode said:


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Well the manufacturer didn't seem to think it would be an issue, but said she wouldn't provide anything in writing. In this case, it may be better to land on a CB instead, pay the extra cost, and avoid the hassle? I would still need a fused disconnect in this system configuration, yes?
​
Click to expand...

If you are backfeeding a breaker, many AHJ's will still require a lockable disco, but there is no reason for it to be fused. Many unfused discos are rated at 100% so you may be able to use a smaller one than if it were fused.
 
Anode said:
Our intent was to tap feeder conductors in switchboard with insulated piercing connectors to each phase.
Click to expand...

Make sure you anticipate the number of parallel conductors on each phase. 600A = 2 sets of 350 kcmil Cu, or 2 sets of 500 kcmil AL.

In any case, do not do anything that would break the symmetry of the parallel conductors. If you make a tap onto one set, you have to make the same tap onto all of the sets.

One way to do this, is for your interconnection tap to contain the same number of parallel conductors as the service, and make an insulation piercing connector equidistant from the service conductor terminal. Another way to do this, is to use multitap connectors, regrouping the service conductors in order to make your tap with a dissimilar number of conductors.

Technically you can argue that you meet the NEC if you insulation pierce just one conductor in parallel, since you still technically keep the parallel service conductors "the same length" and "terminated in the same manner". However, if you break the symmetry this way, there is the possibility of overloading the set you tapped. See below for a discussion of such an example:
http://forums.mikeholt.com/showthread.php?t=170330&page=3
 
It may be a little late, but here is a link to an article published in the Electrical Currents newsletter by the WA state chief electrical inspector. It covers how L&I interprets the line side tap rules.
 
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