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Thread: Residential Battery Backup

  1. #1
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    Residential Battery Backup

    Hello,

    We are installing a home battery backup system in combination with a solar array. This particular house is small with very few electrical loads.

    Normally we would seperate out circuits from the main panel and create a "backup loads panel" but in this case we are putting the unit right in-between the service conductors and backing up the whole house.

    Here are the details: House is fed by 4/0 AL from a 200 amp breaker at a mer/main in the road. The 4/0 lands in 200 amp main panel on 200 amp man breaker. 200 amps is beyond overkill for this house.

    The backup unit is limited to 60 amps output (there is a 60 amp breaker installed) even in normal pass thru mode while grid is up. So our plan is to cut the 4/0 feeders, splice them down to 6AWG in a box, land in the unit, come out of the unit with 6AWG, splice back onto 4/0 feeders that feed the panel.

    This seems strange but it is all protected by the 60 amp breaker.

    I am wondering if the ground wire has to remain the same size as it is with the 4/0 feeder or if it can be sliced down to #10 in the same box because it is now protected by a 60 amp breaker?

  2. #2
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    On the supply side, the 6awg after the splice would be a tap. The EGC can be downsized to the same size as the tap conductors, which in your case is no downsizing. See 250.122(G).

    On the load side of your 60A breaker, the EGC can be downsized to 10awg cu.

    You must leave the existing EGC in place where it runs with the 4/0 conductors. 250.122(B).

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    I know you said they don't have very many loads, but I'm wondering if changing the service to effectively be a maximum of 60 amps is a good idea. If they decide to add a few load later, they are going to have to undo what you did.

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    I thought to mention that the minimum service disconnect for a dwelling is 100A. Since there's a 200A service disconnect, that requirement is met, technically, but maybe not in spirit ...

    I will say that I don't think my house has ever used more than 25A since we've been in it.

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    Quote Originally Posted by JoeNorm View Post
    Hello,

    We are installing a home battery backup system in combination with a solar array. This particular house is small with very few electrical loads.

    Normally we would seperate out circuits from the main panel and create a "backup loads panel" but in this case we are putting the unit right in-between the service conductors and backing up the whole house.

    Here are the details: House is fed by 4/0 AL from a 200 amp breaker at a mer/main in the road. The 4/0 lands in 200 amp main panel on 200 amp man breaker. 200 amps is beyond overkill for this house.

    The backup unit is limited to 60 amps output (there is a 60 amp breaker installed) even in normal pass thru mode while grid is up. So our plan is to cut the 4/0 feeders, splice them down to 6AWG in a box, land in the unit, come out of the unit with 6AWG, splice back onto 4/0 feeders that feed the panel.

    This seems strange but it is all protected by the 60 amp breaker.

    I am wondering if the ground wire has to remain the same size as it is with the 4/0 feeder or if it can be sliced down to #10 in the same box because it is now protected by a 60 amp breaker?
    It seems to me that it would be simpler just to use the MDP as a disconnect and feed a protected loads panel with the battery backup in the usual way. You could move the few existing loads to the protected loads panel and still have room for more (unprotected) loads in the MDP, and you wouldn't have to worry about conductor sizing. You could even add more solar to the MDP without having to worry about solar/battery balancing.

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    Quote Originally Posted by Coppersmith View Post
    I know you said they don't have very many loads, but I'm wondering if changing the service to effectively be a maximum of 60 amps is a good idea. If they decide to add a few load later, they are going to have to undo what you did.

    I agree. I don't think its a great idea either but that is how it was designed(not by me). In any other house I would voice my concerns louder but this place has the least amount of loads I've ever seen in a residence.

    This is the most cost effective route for the customer, adding a backup loads panel in time consuming.

    Thanks for the replies, I mainly wanted to make sure it sounded up to code with the wire sizing, sounds like it is if I run a #6 ground.

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    Quote Originally Posted by JoeNorm View Post
    Hello,

    We are installing a home battery backup system in combination with a solar array. This particular house is small with very few electrical loads.

    Normally we would seperate out circuits from the main panel and create a "backup loads panel" but in this case we are putting the unit right in-between the service conductors and backing up the whole house.

    Here are the details: House is fed by 4/0 AL from a 200 amp breaker at a mer/main in the road. The 4/0 lands in 200 amp main panel on 200 amp man breaker. 200 amps is beyond overkill for this house.

    The backup unit is limited to 60 amps output (there is a 60 amp breaker installed) even in normal pass thru mode while grid is up. So our plan is to cut the 4/0 feeders, splice them down to 6AWG in a box, land in the unit, come out of the unit with 6AWG, splice back onto 4/0 feeders that feed the panel.

    This seems strange but it is all protected by the 60 amp breaker.

    I am wondering if the ground wire has to remain the same size as it is with the 4/0 feeder or if it can be sliced down to #10 in the same box because it is now protected by a 60 amp breaker?
    what did you say the calculated load came to for the single family dwelling

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