[RE-wrenches] backfeeding "six disconnect" panel

Tue Nov 2 15:07:59 PDT 2010

Why should the Code be practical :-)?

In a plain vanilla electrical system everything downstream of a breaker can be sized assuming that the breaker will protect things. There is only one source of energy, and the breaker is the gatekeeper which limits how much energy can get through. When you add a backfed solar breaker, however, you are introducing a second source of energy. Typically in a panel the sum of all the load breakers is greater than the rating of the bus bars, so you can't rely on the load breakers to protect the busing and the conductors. To keep things in check, the Code limits the total of all sources.

With the load center you have, the bus bars in the top section are not protected by anything other than the short-circuit capacity of the utility transformer. The idea is that the load breakers--all labeled as a service disconnect--provide a limit to the maximum current. You wouldn't want to put six 100 A breakers in there, because then you could potentially draw 600 A and exceed the capacity of the bus bars. When you backfeed solar onto this bus you're not really adding any significant amount to the transformer fault current.

So maybe to more directly address your question, where you backfeed doesn't make any difference to the bus bars in the top section of your panel, but when you feed into the bottom section you have to worry about the rating of the bus bars down there.

Scott

On Nov 2, 2010, at 1:59 PM, Dana Brandt wrote:

> Thanks. That's really helpful. I think the Code Corner quote will help in dealing with the AHJ. 
> 
> From a practical standpoint, though, I don't see why our PV input power should limited by the 690.64 (B) (2) calculation if there's a main breaker present, but if there's no main breaker we can feed in 100% of the bus rating. 
> 
>  "The basic restriction (which will be in the  2011 NEC) that would apply to this type of main service panel is that the sum of the overcurrent devices from the PV inverter(s) should not exceed the rating of the panel bus bar or the rating of the service entrance cables."
> 
> Is there something special in the busing on these main-lug-only panels that means we don't have to be so conservative or is it a matter of the code not really being consistent?
> 
> Thanks,
> 
> Dana
> 
> Dana Brandt
> Ecotech Energy Systems, LLC
> www.ecotechenergy.com
> dana at ecotechenergy.com
> 360.510.0433
> 
> 
> On Mon, Nov 1, 2010 at 10:01 PM, Philip Boutelle <philboutelle at gmail.com> wrote:
> Dana,
> I'll echo Scott's earlier comment: connecting to an empty 'main' breaker position that is on the top bus will be a supply-side connection. From Code Corner #126:
> 
> Some dwellings have main-lug-only panels. There is no 
> single main breaker feeding the panel, but up to six main 
> breakers are allowed. Empty breaker positions can be used 
> as supply-side connections. The basic restriction (which 
> will be in the  2011 NEC) that would apply to this type 
> of main service panel is that the sum of the overcurrent 
> devices from the PV inverter(s) should not exceed the 
> rating of the panel bus bar or the rating of the service 
> entrance cables.
> 
> -Phil Boutelle
> Real Goods Solar
> 
> 
> On Mon, Nov 1, 2010 at 6:55 PM, Dana Brandt <dana at ecotechenergy.com> wrote:
> Hi everyone,
> 
> I just noticed my last email didn't go through because I'd attached pictures of the panel. If you're interested in the pics of the panel please contact me off-list. 
> 
> Thanks for the feedback. To clarify, the upper part of the bus is lug-fed directly from the service meter - no overcurrent protection. I have this same situation in two homes - one has a Bryant panel, not sure about the other. 
> 
> It seems possible connecting to the top bus could be considered a supply side connection. It's done through a panel bus, though, which seems like maybe it wouldn't qualify.
> 
> It's true that the 60A breaker feeding the 100A bus fulfills the calculation requirements. However, the calculation has to follow upstream to the bus supplying that 60A breaker, so again we have a potential problem since there's no overcurrent protection for that bus.
> 
> So, it seems the real question is about that top bus. Can we consider a connection there a supply side tap? If not, who do we do the 690.64 (B) (2) calculation without a main breaker? 
> 
> Thanks!
> 
> Dana
> 
> Dana Brandt
> Ecotech Energy Systems, LLC
> www.ecotechenergy.com
> dana at ecotechenergy.com
> 360.510.0433
> 
> 
> On Thu, Oct 28, 2010 at 6:37 PM, Matt Lafferty <gilligan06 at gmail.com> wrote:
> Hi Dana,
>  
> "Sounds like" you have a lower-bus rated at 100A being fed by a 60A breaker... Yes?
>  
> If so, technically speaking but not necessarily the right thing to do, you have a SWEET SETUP! In 690.64 terms, you are legally allowed 120A to feed that bus. Which converts to a maximum 60A PV interconnection breaker located at the opposite end of the bus from the 60A supply breaker. This is a panel configuration that I have wished was still made for many years.
>  
> The downside is... It's probably older than me. With really wimpy buses. And really bad options for replacement breakers. And is full of 2-wire homerun circuits.
>  
> If it doesn't have a perfect-in-every-way bus, it's time for a new panel. You also need to be able to buy real... Modern.... And New UL Listed breakers. Those gray looking replacement breakers in the home improvement store? They're NOT OK. In any way.
>  
> Wadsworth? FPE? Bulldog? &...??? These panels are NOT OK for interconnection at this point. Time to upgrade. Yes, really. Zinsco? Depends. Any panel with signs of arcing around the bus anywhere? Better have a damned good explanation before I'd be tying in some PV. I know of a couple. Warped breakers? Change it!
>  
> I just got done doing a 90% rewire on a friend's house and it tore my heart out sending the panel to the recycler. (This friend is into long-term thinking and sustainable everything. I would have left it if I could and so would they.) The bus was pristine. It is rumored that Al Capone used to stay in this place when he was in the area. The panel was giant, with lots of room. This panel had a coolness factor off the charts. But... I couldn't reliably get trustworthy breakers. And it was my friend's house. Somebody I care about. I couldn't leave it in there. I use the same judgement whether my customer is a friend or not.
>  
> If what you have is a modern panel, share! I know I would personally appreciate it if you were able to share mfr, model, pictures, etc. Whenever we are upgrading/swapping/moving a service panel, we need to be installing something that provides the best opportunity for interconnection of distributed resources. We don't have those at this point. And we need them. A split-distribution (not just split-bus) service panel is one way to accomplish this. The trick is to limit the possible size of the breaker feeding the "2nd" bus (the lower bus in this case). It's not rocket science.
>  
> I'd be happy to use and publicize a product that a mainstream manufacturer came up with to deal with this. How about you?
>  
> $0.02001,
>  
> Solar Janitor
>  
> PS - Home rewiring for friends only. I love doing it, but I do it for love. There ain't no amount of money that makes me want to do it just for money!
> From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Dana Brandt
> Sent: Thursday, October 28, 2010 5:19 PM
> To: RE-wrenches
> Subject: [RE-wrenches] backfeeding "six disconnect" panel
> 
> Hi everyone,
> 
> I'm working on two systems for homes that have the old style breaker panels with no single main breaker. Instead, they have six breakers in the upper part of the panel each labeled "main breaker," which satisfies the six disconnect rule. The bus bar is split, with one of the six upper breakers (60A) feeding a separate bus in the lower half of the panel. It appears that the upper bus is rated for 200A, and the lower at 100A.  The sum of the main breakers does not exceed the 200A rating of the upper bus. There are spare breaker slots on both the top and bottom buses. How do I deal with 690.64 (B) (2) when there's no main breaker? 
> 
> Thanks!
> 
> Dana
> 
> Dana Brandt
> Ecotech Energy Systems, LLC
> www.ecotechenergy.com
> dana at ecotechenergy.com
> 360.510.0433
> 
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