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I have two large AC units with electric heat strips requiring several large breakers for a 200 amp service. This is new construction.

It’s actually cheaper to install a 100 amp panel as a main panel for the attic space to serve all the needs of the units and leave the 200 amp panel for the rest of the house.

The 100 amp panel is closer to service entrance then the panel box so wire by foot is also cheaper.

I’m referring to heat pumps in the air handlers. I was debating costs of running the wires for two units to main panel vs one sub panel. Also a sub panel means more space in the main panel.

The attic is conditioned space by spray foam and is accessible by pull down stairs as required by local codes. The entire attic has a subfloor which air handlers will sit on.

Can I install a 100 amp main breaker panel like this in an attic space?

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    Will the attic be finished? How accessible is it. Will you have stairs to it, or at least a pull down ladder? Code requires "easy access" to it. So I would think a sub-panel in an attic where you have to crawl over insulation and joists to get to it doesn't qualify as "easy access". Hopefully one of the "big 3" here will provide a more definitive answer. Aug 21, 2022 at 15:07
  • Unless you have a separate service, you can only have one main panel, plus sub panels fed off of the main. 100 amp will not feed a 200 amp. With that power to heat, it probably be much cheaper to get a heat pump.
    – crip659
    Aug 21, 2022 at 15:07
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    @crip659 Agree about heat pump. Absolutely crazy to put in toaster heat these days (actually, for a very long time in most places). But as far as the multiple panels - in many areas with new construction there will be a meter main that includes a small main panel feeding the 200A, 100A, etc. The 200A is then not technically the main panel (so ground and neutral separate) but would still practically speaking be termed the main panel since it has nearly all circuits/breakers in it. Aug 21, 2022 at 15:15
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    @GeorgeAnderson The OP might be referring to the air handlers which would/could have electrical resistance strip heaters. I've got them in my unit, used them once in two years.. lol
    – JACK
    Aug 21, 2022 at 16:00
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    Given that you're in Vegas, a modern cold-climate heat pump can handle down to well below your 99% winter design temperature (aka 17degF) without any supplemental heat -- so I suspect your HVAC contractor is giving you a bill of goods on your heat pumps instead of setting you up with the latest technology, and jacking up your energy bills in the winter as a result (because heat pump heat is always more efficient than toaster heat) Aug 21, 2022 at 16:21

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Here is the big IF, IF IF IF the location of the proposed sub-panel has "easy access", it's probably OK. BUT AND BUT AND BUT, you'll have to check with your AHJ to be sure.

Next, panels are not expensive. In your situation, I'd suggest a 200 amp panel for your A/C (maybe heatpumps?) and a class 320 service. That way you can supply both panels with 200 amps Since this is new construction, now would be the time to do it. A class 320 meter base is quite a bit more expensive than a typical 200 amp meter base, but it provides a lot of capacity. You'll need to check with your POCO to make sure it can and is willing to supply it. You'll need 500 MCM AL wire (I think, please double check) from the transformer to the meter base. It's clearly more expensive, but it sounds like you have enough power needs to justify this approach (class 320 MB with 2 200 amp panels). Also it would provide the capacity for EV charging down the road.

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  • Yes heatpumps. Yes it’s a conditioned space unused for our purposes in the attic. Also it’s easy access with stairs to it. Are you saying have a 200amp sub panel in attic space run off the main panel?
    – Jason
    Aug 21, 2022 at 16:08
  • @Jason -- see my post on your question -- with modern cold-climate heat pumps, you need neither the heat strips nor the big panel in the attic to power them Aug 21, 2022 at 16:24
  • @Jason There is so little cost difference between a 200 amp main panel and a 100 amp sub-panel (maybe about $100-$150 diff.) that it might make sense to go big, just to be safe. But to your point, The OP needs to know what the power requirements are and if he has the appropriate equipment. I'm currently installing a mini-split system in a rental house and it's crazy how low the outside temps can go and it still produces heat (like below 0 deg. F), so I get what you're saying. My answer directly addressed the OP's question and not making alternate suggestions. Aug 21, 2022 at 20:30

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