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I'm reading the US NEC 2020 article 706 and trying to understand the required over-current protection needed between the battery and the inverter.

My understanding is that a disconnect is required outside the house by the electrical grid meter that disconnects both Positive and Negative cables. But this disconnect doesn't necessarily need to provide over current protection.

Inside the house, I'll have a breaker to provide over-current protection, but I'm not seeing if that protection applies to both cables or just the positive.

I've seen that UL listed Lithium Iron Phospate (LFP) batteries have their own built-in breaker that only disconnects one of the cables. I'm thinking though, that if I had more than one battery, code requires a single over-current protection device for the whole (LFP) battery bank?

So to sum up: do I need an over-current device if the batteries already have one? And would it need to protect both cables or just one?

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  • What voltage battery bank are you using here?
    – ThreePhaseEel
    Commented Jul 17, 2023 at 22:41
  • The range is 44.8V-56.8V.
    – Mike Gray
    Commented Jul 18, 2023 at 5:46
  • Also: what do you have connected to the battery bank for charge sources?
    – ThreePhaseEel
    Commented Jul 22, 2023 at 1:25
  • Just the inverter/charger unit.
    – Mike Gray
    Commented Jul 23, 2023 at 4:36
  • Ah. What make/model is said inverter/charger?
    – ThreePhaseEel
    Commented Jul 23, 2023 at 5:12

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Here is my best interpretation of the code:

240.15: Current carrying conductors that are ungrounded (example, hot wires in a US AC circuit panel) must have overcurrent protection.

240.22: Current carrying conductors that are grounded (example, neutral wires in a US AC circuit panel) do not need overcurrent protection.

706.31(A): The same rules apply to DC wiring. Any current carrying conductor, negative or positive, must either be connected to ground somewhere, or have overcurrent protection.

If there is a ground connection in a DC system, it's usually the negative. But from what I've read, depending on exactly what make and model equipment is being used, grounding either conductor can mess up ground fault protection, which is also required by code (690.41) if you have a solar input. I'm not 100% sure on this part, but I think its correct. So it seems the best way around this issue is to put both the positive and negative on their own breaker.

If you have more than one battery, it gets a little more complicated. A bank of batteries in parallel should each have an overcurrent device on at least one conductor. I don't think it matters which. The batteries I've seen have the negative controlled by the BMS, while the positive has a breaker built into the battery. (I'm still looking for the code on this. It's got to be in 706 somewhere.)

From there, the best practice seems to be to put all the conductors on a set of buss bars (is it buss or bus?) and have a single set of conductors (one negative, one positive) going to the inverter. Each of the individual battery breakers would be rated for the max current of the individual battery. The big set of cables would then have their own overcurrent devices. One for the positive, one for the negative. These overcurrent devices would be rated for the maximum current of the combined battery bank. Or the maximum current rating of the inverter. Whichever is smaller.

250.4(B): Now, DC devices are still going to need a connection to ground even though the conductors are not grounded. I haven't figured that part out yet, but that wasn't my original question anyway. 250.122?

I hope this all makes sense and is accurate. Please correct me if I'm wrong.

Edit: I'm adding NEC 2020 code references where I can find them.

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  • I'm not doubting you, but your answer would be strengthened by including the code references (and quotes) that you're basing your interpretations on. That would allow the actual electricians here (I just play one online and in my house) to be able to more quickly and accurately assess your answer.
    – FreeMan
    Commented Oct 5, 2023 at 12:02
  • I hadn't considered that, but it's a good idea. I'll go back and edit my answer.
    – Mike Gray
    Commented Oct 6, 2023 at 1:19
  • Solar GFP is a solar system thing, not a general DC thing, and is better provided by a dedicated dual pole array GFPD instead of one of those "ground breaker" devices. (hint: think of what happens when a "ground breaker" GFP trips)
    – ThreePhaseEel
    Commented Oct 7, 2023 at 1:58
  • If it's what I'm thinking of, that's a breaker with a small 1A breaker tied onto it. So in the event of a ground fault of more than 1A, the ground breaker trips and also causes the regular breaker for the circuit to trip. Seems clever to me, but then the connection to ground is gone. Better than nothing, but I'd rather have a setup where grounds are never disconnected.
    – Mike Gray
    Commented Oct 24, 2023 at 21:06
  • @MikeGray -- the device I linked is a set of electronics that measures the difference between current on + and current on - flowing through it and trips its internal breaker when that difference becomes too large (IOW: it's a differential trip, making it analogous to a GFCI, just for solar DC)
    – ThreePhaseEel
    Commented Oct 25, 2023 at 1:53

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