Busbar and NEC 2008

FL SUNFL SUN Solar Expert Posts: 94 ✭✭✭✭
A few questions regarding Article 690.64 of the 2008 NEC.

1) Article 690.64(B)(2) "The sum of the ampere rating of circuits supplying power to a busbar shall not exceed 120% of the rating of the busbar."

Take a 200 amp panel with a 200 amp main service breaker. According to the rule, I can backfeed the load side of that panel with up to a 40 amp circuit breaker from a gridtie inverter?

2) Article 690.64(B)(7)"...a connection in a panelboard shall be positioned in the opposite (load) end from the input feeder location or main circuit location."

So the output of a gridtie inverter's backfed breaker goes to the last connection of the bussbar if the main comes from the top? Is this to help distribute the electricity more to the loads in the building than pumping directly back into the grid?

Lastly, if my continuous output rating from a gridtie inverter is 25 amps, my backfeed breaker would be sized for 125% of that value or 31.25 amps. I interpret the rule to go up to the next size breaker, not down. So I would choose a 35 amp breaker. Is this correct?

Comments

  • EcnerwalEcnerwal Solar Expert Posts: 101 ✭✭
    Re: Busbar and NEC 2008

    The opposite end of the busbar is to reduce the situation where the busbar is carrying more load than it's designed for, but it comes to the same thing (feeding the PV into local loads). If the feed and the PV are at the same end, you might have 240 amps feeding from the PV position down to the end of the bar. If the PV is at one end, it will feed the loads closer to it, and the main will feed the loads closer to it, up to the point where the loads are less than the PV out, and then the PV will feed the loads and the main. No part of the busbar will see more load than the main would supply.
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Busbar and NEC 2008
    FL SUN wrote: »
    Lastly, if my continuous output rating from a gridtie inverter is 25 amps, my backfeed breaker would be sized for 125% of that value or 31.25 amps. I interpret the rule to go up to the next size breaker, not down. So I would choose a 35 amp breaker. Is this correct?

    I've installed between 60 and 70 service panels, and I've no idea how many sub-panels.

    I've never seen a 35a breaker. I'm not saying they don't exist, but they must not be very common...
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Busbar and NEC 2008

    NEC 240.6(A) indicates that standard breaker ratings includes 35 A.

    HTH,
    Jim / crewzer
  • FL SUNFL SUN Solar Expert Posts: 94 ✭✭✭✭
    Re: Busbar and NEC 2008

    Here is the link to a 35 amp SD breaker. http://ecatalog.squared.com/datasheet.cfm?partnumber=HOM235

    Question is, since I am using an inverter that outputs 25 amps, 125% rule would require a 35 amp breaker. What has been standard in the field for this application, a 40 amp breaker? It just does not follow the NEC, and picky inspectors trying to go by the book on this "new" solar thing would be quick to notice.
  • FL SUNFL SUN Solar Expert Posts: 94 ✭✭✭✭
    Re: Busbar and NEC 2008

    SG, this thread is referring to the SB 6000U to clear the confusion. Rated for 25 amps continuous output.
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Busbar and NEC 2008

    No, as I said - I'm sure there are such things as 35a breakers.

    But I've never used one, never even seen one.

    So sure, they are out there, but what are you going to use it to protect - #9 wire?

    The breakers are to protect the wire and with a 35a breaker you're going to end up using #8 wire anyway.

    I don't have a copy of the code handy - nor have I studied it in relation to PV or GT systems (yet, I wanted to thoroughly understand how PV and GT systems *work* before I start applying the code to them) - is that 125% "required" or is it "allowed"?

    I.E., you are "allowed" to put up to 125a worth of breakers in a 100a panel - but you aren't "required" to.

    If "required", then it would need to be "at least" 31.25a (by your calc), so you could probably upsize to a 40a breaker since you'll be using #8 anyway.

    If "allowed", then it would need to be "at most" 31.25a in which case you would use a 30a breaker and #10.
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