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Powerwalls and fire codes

hughw | Posted in Building Code Questions on

We’re in the process of designing a new home for a client on Martha’s Vineyard. We’ve been told by the solar panel provider that under present fire codes only a single powerwall (generically known as an Electric Storage System ESS) can be installed in a dwelling without triggering various provisions that would be onerous to meet in a dwelling. They say that  for systems over 20Kwh (a single power wall is rated at 13.5kwh), the ESS must be enclosed with two hour construction, have a sprinkler system, ventilation, fire alarm, and battery arrays must be mounted 3′ off of walls and 3′ apart. 

For any but the smallest home, a single powerwall is not sufficient for backup power except for relatively short period of time, and certainly not for a larger home in the dead of winter. Tesla has been extremely unhelpful regarding any limitations in the number of powerwalls that can be installed together. They say talk to their permitting division, but then indicate that you can’t talk to the permitting people without already having placed an order.

My research has found  that  under NFPA 855 , Chapter 15 , a stationary ESS of aggregating not more than 50KWh in a basement or 80KWh is allowable without special measures. But this is part of the electric code, not the fire code. See attachment.

We would appreciate any guidance regarding building and fire code requirements  relating to stationary ESS systems  exceeding 20KWh (i.e. multiple powerwalls).

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  1. MAinspector | | #1

    My suggestion would be to contact your Building Department and Fire Department to get their interpretation of the governing codes. They will be the authorities having jurisdiction over whats required in regards to your specific install...everything on this forum will just be an opinion.

  2. Expert Member
    BILL WICHERS | | #2

    Talk to your local building department. You may need to work a bit to find the right guy to talk to — this is likely to be a subject they aren’t very familiar with.

    I design facilities with large battery systems commercially. The codes are usually concerned with the amount of hazardous “stuff” contained in the batteries. In my systems, this is usually sulphuric acid and I have to list the number of pounds of the stuff in the system. I’ve run into issues with hydrogen accumulation concerns too. The Tesla system should be better about gas releases compared with the VRLA batteries I normally work with.

    Two hour fire ratings are usually for things like fuel systems. It’s not hard to build (two layers of 5/8” drywall on both sides of the studs). If that’s all you have to do it’s not a big deal. You could wall off a small “battery closet” and build it as a 2 hour room.

    Note that if your intention is to use these power walls for long term battery backup durin power outages, they really aren’t suitable for that — the capacity is much too low. You’d be much better off with a generator, which will also be a LOT cheaper.


  3. walta100 | | #3

    What is written in the national model code is interesting but also meaningless. What matters is what code your local elected officials voted to enact most have chosen a few year old version of one of the models. Sometime local inspectors convince elected officials to add requirements not found in any model. Some inspectors have been known to enforce rules of their own making. Fighting the made up rules is a risky game in that if you call their bluff and make them back down they are likely to find other things to complain about.

    If you watch a few YouTube videos of burning lithium batteries you are likely to decide what the inspector is asking for is very reasonable.

    Burning lithium produces very toxic smoke making the fire wall, sprinkler, fire alarm and ventilation seem reasonable. The batteries generate a lot of heat when charging and discharging and will last longer if you give then sum space to cool and 36 inch inspection and maintenance aisle sound about right.


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