Backfeeding FPE switchgear

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mdillonelectrician

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I recently went on a site visit to evaluate a service for 150 kw of solar to be installed. The switchgear was FPE ( federal pacific) and there were several open spots( fused switches) not breakers.my question is it possible to backfeed said switches or do i need to do a supply side tap? The space is limited for new gear. So hooking into the switches would be very convenient.Any tips or clarification would be greatly appreciated.
 
Good question. The master electrician I used to work with was pretty wary of Federal Pacific gear. But I seem to recall that he was most concerned about their circuit breakers.

I'd run it by your consulting engineer and the AHJ.
 
My concern would be possibly having the fuses live with the utility side turned off, even though the inverter is supposed to shut down when there is an absence of utility voltage. I would not be comfortable changing any of those fuses unless there was also a disconnect on the load side of that fusible switch.
 
If the question is simply whether you can backfeed fused switches per the code, then the answer is a simple yes.

Note that 705.12(D)(5) refers only to circuit breakers and not switches or fuses.

hillbilly1 said:
I would not be comfortable changing any of those fuses unless there was also a disconnect on the load side of that fusible switch.
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I would be. I would be doubly comfortable if the DC disconnect(s) were opened.
 
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You do not state the service involved. If the SES is 1000A or higher on 480V, there is likely ground fault protection (GFP) if there is main disconnect. GFP presents many problems.

If there is no main device, you have the 6-switch limit. While you can backfeed up to the service rating, most utilities require a single lockable AC disconnect safety switch and that in turn limits the PV to the highest rated fused switch the SES is listed for.
 
Right. Interconnecting via a fused disconnect is definitely allowed by Code. And often the 6-handle rule comes into play. [If not, then you're up against something else, like 705.12(D)(2) & (7).]

The potential grey area in my mind is whether working with the existing equipment triggers a service upgrade or equipment upgrade—either as a best practice or a requirement in the mind of the AHJ. And that's a judgement call that could be outside of the Code. It sounds like this may be 50-year-old equipment and we've definitely run into some installations of this vintage that we could not interconnect to without the AHJ requiring a service rebuild.

Also, there's the "once you touch it, you own it" factor to consider with vintage gear. How comfortable are you "owning" the existing installation? If you are not comfortable with it (or if you can't find a PE that is comfortable with it), then you probably want to rebuild it.
 
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hillbilly1 said:
My concern would be possibly having the fuses live with the utility side turned off, even though the inverter is supposed to shut down when there is an absence of utility voltage. I would not be comfortable changing any of those fuses unless there was also a disconnect on the load side of that fusible switch.
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I wouldn't worry about that. It's not just because of UL1741 that GT inverters shut down when the grid tie goes away, it's the way they have to work. GT inverters match voltage and phase with what they see on their output terminals.
 
ggunn said:
I wouldn't worry about that. It's not just because of UL1741 that GT inverters shut down when the grid tie goes away, it's the way they have to work. GT inverters match voltage and phase with what they see on their output terminals.
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But as with any device, mechanical or electrical, failure can happen. I just wouldn't want to be the one to find out it has failed.
 
hillbilly1 said:
But as with any device, mechanical or electrical, failure can happen. I just wouldn't want to be the one to find out it has failed.
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In general I agree, but I cannot think of a way in which a GT inverter can fail that would result in its powering up dead AC lines. It's not just a feature built into GT inverters that can fail, it's how they work. An AC disconnect, properly installed, breaks the conductors on the line (utility) side of the fuses and the load side is hard wired to the inverter. Some inverters have an integrated AC disconnect, some don't.
 
Theoretically you could have multiple catastrophic failures in the inverter that could result in the straight DC from the panels being put onto the AC lines (which would be obviously bad for all sorts of things). But having a grid tied inverter energize an open circuit with AC voltage (for more than a couple cycles) is even less impossible.

To put it another way, opening the required DC disconnect for the solar system is pretty much an absolute failsafe against getting shocked from the solar side while replacing fuses in a properly wired AC disconnect. There's absolutely no need for another AC disconnect on the load side of the fuses.
 
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