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Thread: Battery Combiner OCPD Necessary?

  1. #11
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    Quote Originally Posted by jaggedben View Post
    Okay, but what about conductors in raceways connecting two battery units? Last one I did had about 20ft of EMT in a garage.
    I would consider that two separate battery banks connected together, and those connecting conductors leave their respective battery banks and need to be protected at the point they leave each bank. The unprotected conductor allowance only applies inside a battery bank enclosure. I can't have a site with 20 different interconnected battery bank locations around the campus and say it's really one battery bank.

  2. #12
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    Quote Originally Posted by PWDickerson View Post
    I get that unfused battery conductors are subject to high fault currents that exceed their rating, and I try to design battery systems with fuses located as close to the battery terminals as possible to mitigate the risk. The point of this post is to consider a different risk that I had not really considered prior to designing systems with many parallel 48V batteries. The fact that each of the battery in question has a built in 80A breaker eliminates the risk of a fault in an unfused conductor, but when many such batteries are bussed together, a whole new (to me anyway) issue arises. I spent some time yesterday calculating the short circuit current in a 15-battery system with 20' #6 battery conductors. The 14 non-shorted strings would each supply about 150 amps to the shorted string. The non-shorted batteries would probably take 10-15 seconds for the breakers to trip given a typical response time. Until that happens, you would have over 2100 amps passing through a single #6 wire. That would do a lot of damage.

    The 150A output for a shorted string seems very low since batteries are usually great sources of fault current. I looked over the battery data sheet and I did not find a fault current rating for this battery. Usually, there is enough fault current to trip the CB faster than that.
    Last edited by pv_n00b; 12-31-18 at 11:36 AM.

  3. #13
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    Quote Originally Posted by pv_n00b View Post
    The 150A output for a shorted string seems very low since batteries are usually great sources of fault current. I looked over the battery data sheet and I did not find a fault current rating for this battery. Usually, there is enough fault current to trip the CB faster than that.
    The battery side of the shorted string conductor will experience up to 1260 amps, which is the battery's short circuit current rating (in the manual, but not in the spec sheet), and will immediately trip the battery's breaker. However, the bus side of the shorted string conductor will receive current through the bus from the rest of the batteries in the system, and the short circuit current will be evenly divided between them. If the string conductor is long enough and only sized for the 30-40 or so amps that it will carry, it might limit the short circuit current within it to a value that would not immediately trip the breakers in the other batteries.

  4. #14
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    Quote Originally Posted by PWDickerson View Post
    The battery side of the shorted string conductor will experience up to 1260 amps, which is the battery's short circuit current rating (in the manual, but not in the spec sheet), and will immediately trip the battery's breaker. However, the bus side of the shorted string conductor will receive current through the bus from the rest of the batteries in the system, and the short circuit current will be evenly divided between them. If the string conductor is long enough and only sized for the 30-40 or so amps that it will carry, it might limit the short circuit current within it to a value that would not immediately trip the breakers in the other batteries.

    At 160A the 80A breaker will trip in a few seconds unless they are some strange slo-blow CB. But it comes back to the same reason, protected conductors inside battery banks are considered relatively safe from faulting. There is nothing stopping you from putting in more fuses if you want, just like there is nothing stopping someone from putting in fuses when doing a feeder tap. The NEC just says that you don't have to. If it really bugs you that the conductors are unprotected then fuse away.
    Last edited by pv_n00b; 01-03-19 at 08:45 PM.

  5. #15
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    Fused Battery Combiner1.pdf

    Here is a simple schematic showing the issue. The fuse on the battery end of the shorted conductor will blow eliminating current from the battery to the short. The other 23 batteries that aren't shown will supply current to the short through the bus. The fuse at the bus will protect the "bus side" of the shorted conductor. If there were no fuses at the positive bus, the other 23 battery breakers could supply 1840A (23 x 80) to the conductor without ever blowing a fuse. The resistance in the wire would limit the short circuit current to approximately this amount

    In this type of installation, all the battery conductors need to be the same length to keep the current the same in each battery circuit, and so the battery conductors tend to be longer than one might expect. The are typically routed through gutters/conduits, and are not all within a single enclosure.

    As I said in my original post, I don't understand why fusing wouldn't be required in this case when it is required in a PV DC combiner panel in order to protect a shorted string from current supplied by the other strings connected to the bus. Seems like the same thing.

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