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Key question: Can I split 300 AMP service to 100 amp and 200 amp panels? What’s involved as far as the meter, wiring requirements, etc.?

I think we got a bit away for what was asked? Lots of people asking if the upgrade is necessary, etc. The upgrade IS necessary and is happening. I’m just trying to decide if I can keep and reuse the existing 100 amp panel for my garage or if it’s going to be not possible.

Thank you everyone for your input and help.

I am looking at upgrading my service. Would I be able to run a 100 AMP panel (Garage) and 200 AMP panel (house) off of 300 AMP service?

Does anything need to be connected in any special/ particular way?

I have heard of say, 400 AMP service being split to 2x 200 AMP panels, but I am not certain how it is done for my instance. I assume I will probably need a special double-tap meter?

Edit: I am currently on 100 AMP, but my panel is getting very full. I plan to move to at least 200, but my utility provider mentioned that if I am upgrading service, there is no additional cost to go to 300 vs 200.

I DO have a lot of power-hungry devices: -2 air compressors -4 large servers (and growing) -2 ceramic kilns, 240volt/50 AMP (I haven't used at home yet, but would like to). -I also have a welder I intend to use. -I am looking to install a mid-size 240v milling machine.

I don't have one yet, but EV vehicle is down the road.

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    Is this for EV charging? You never need a service upgrade for that. Unless the EV is an old forklift lol. But even then we could figure something out. Sep 2, 2023 at 1:12
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    Not to be all “xy problem” but hat are you trying to power in that garage? 200amp sevice with 60amp subpanel in garage covers a lot of use cases Sep 2, 2023 at 1:34
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    Do you know how much power your house pulls already?\ Sep 2, 2023 at 5:19
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    Who is your electric utility for that matter? Sep 11, 2023 at 11:44
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    @threephaseeel We have Duke Energy for power
    – Jozef Xie
    Dec 15, 2023 at 18:20

2 Answers 2

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Some assumptions here:

  • You currently have 200A service feeding a 200A panel
  • You want to add a 100A panel for the garage, presumably for EV charging or use as a workshop (welder, big table saw, kiln, whatever) or conversion to an apartment (Accessory Dwelling Unit)

Assuming that's the case, you very likely don't need a service upgrade at all. The first step is to figure out (a) how much electricity you currently use and/or are expected to use based on your existing stuff and (b) how much additional electricity you need.

The proper way to do this is a Load Calculation. One for the existing feed and another for the new stuff. A Load Calculation takes into account:

  • Size of your house
  • Standard amounts for kitchen, bathroom and laundry circuits
  • Oven, cooktop, etc. if applicable (you might have gas)
  • HVAC
  • Water heater if applicable (you might have gas)
  • Clothes dryer if applicable (you might have gas)
  • Any other known fixed and/or large loads

It is not simple "add up the loads", because there are adjustments built in based on "not everything runs at the same time". It is not exact because everyone's usage is different, but it is a standard guide and it generally works quite well.

There is a rough alternative if your utility provides either a peak demand value on your bill (typical on commercial bills, but varies by utility; if you have peak demand in kW on 240V service then divide it by 240 to get Amps) or downloadable detailed usage (available both commercial and residential, typically in either 15-minute or hourly increments; divide hourly kWh by 240 or 15-minute kWh by 60 to get Amps).

The "clamp meter" method is reliable in the long term - that's essentially what the utility usage detail does. But in the short term it is useless because it is too easy to go high (run the HVAC in the maximum mode, turn on the dryer, turn on the oven and 4 burners, etc.) or low (harder to force the electric water heater to turn on when you need it, or "emergency heat", etc.).

Once you have good load calculations (old and new) or at a minimum historical peak usage + load calculation for the new panel, you see what you've got.

If the old is already over 200A then you are living on borrowed time and need to look at cutting usage or a service upgrade. But that's not likely.

If the total can be handled by 200A then you're done, no upgrade needed.

If the total is close to 200A then take a serious look at what you are adding. Two key things to seriously think about (if they are on your plan) are:

  • Tankless electric water heating. If you are turning your garage into a small apartment (ADU) then you might think that will save a few square feet. But it is usually not worth it because of the HUGE demand on the electricity supply. You can get a small tank and hide it in a cabinet in the bathroom or kitchen.
  • EV charging. 30A is enough for almost everyone. 20A is even enough for most. So if your load calculation is a little bit over 200A but includes 50A or 60A for EV charging, lower the charging rate (it has to be installed that way, not "I'll remember to run it at a low rate") and you're done.

Beyond that, there are a number of new ways to manage power to significantly lower the need for > 200A to a typical house, but we need more details to help.

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  • I added some clarifying details. Would you take a look again? This doesn't quite answer the question asked.
    – Jozef Xie
    Sep 26, 2023 at 14:48
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I would recommend getting a clamp meter and taking a reading on the service wires coming into the house while you have most things turned on (dryer, oven, elec. Heat, common lights) you most likely only need a 200 amp service with a 100 amp sub panel in the garage. If you are drawing more than 150 amps or so you'll need to either upgrade to a 400 amp service or you can do two separate services, a 200 amp service on the house and a 100 amp service on the garage. (Assuming the garage is detached, if the garage is connected to the house most utilities won't bring a separate service to it so 400 amp service would be the way to go)

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  • Your input is very constructive and brings up some important considerations, but I am looking to see if it is possible to run a 100 panel AND 200 panel off 300 service, and how this needs setup, like say what connects where and what gauge wiring is needed, etc.
    – Jozef Xie
    Sep 3, 2023 at 6:28
  • I added some clarifying details. Would you take a look again? This doesn't quite answer the question asked.
    – Jozef Xie
    Sep 26, 2023 at 14:49

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