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I'm doing the panel schedule for a 400A residential service, but running out of space on the 40-circuit main panel. Here's a cartoon of what I would like to do.

Is it possible and/or advisable to tap multiple subpanels off the mains line (after the meter and 400A switch/breaker)? Do professional electricians do this?

Ofcourse, wires would be appropriately sized for the amperage, and load calculations check out for the incoming 400A service.

enter image description here

Edit: I'm looking at using the Square-D Homeline 400 Amp 30-Space 40-Circuit panel that's readily available at Home Depot. Model: SU3040D400CB.

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    That's not how 400A panels are wired internally. What is the model number of your meter-main? Jul 27, 2021 at 6:41
  • Wires are not cheap. Consider just one subpanel near main with all additional breakers. Or replace the main panes with bigger. Just the cost is matter.
    – user263983
    Jul 27, 2021 at 11:18
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    @manassehkatz-Moving2Codidact In either case, the OP is going to have to go with a 400 amp meter, they are expensive unless CT metering is used. In my neck of the woods, to get to a true 400 amp service, a CT (Current Transformer) box that provides a small signal (think 12 ga wire) to a simple, inexpensive meter. I've never installed one so I have no idea what the cost of the CT box is, or if the PoCo supplies it. Jul 27, 2021 at 16:01
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    Also, to simplify/ reduce the electrical demands, ditch the on-demand electric water heater for a traditional tank type electric, or a heat pump style. On demand electric WH have HUGE power requirements. Jul 27, 2021 at 16:05
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    @Shubham -- 1) why the tankless HWH? 2) how important is it to have a panel, not just a disconnect, at the meter? 3) you're still in the Bay Area right? (it looks like Cali will be adopting the 2020 NEC next year or so at the latest) Jul 27, 2021 at 23:01

4 Answers 4

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400A service actually works quite differently

Because 400A breaker and 400A buses are basically unobtanium. You can buy them, but only out of the industrial catalog and you'll spend $5000 by the time you're done. Every once in awhile we get a novice who insists on forcing that because they don't know any better, won't listen, and just thinks everyone pays that. No, nobody does lol.

Anyway, "normal in residential" is 400A service being rendered as dual 200A panels.

Now, states are slowly starting to adopt NEC 2020, and some had local rules older than that, which require "outside disconnect switches" (so the fire department can quickly de-energize the panel). A main breaker is also a disconnect switch, so the preferred method is a "Meter-Main", or the meter and main breaker(s) all in one chassis.

In the case of a 400A meter-main, that means two 200A main breakers, which then go on to feed two separate subpanels, presumably inside the house out of the weather.

However, also popular is the 400A "ranch panel". This provides the above, plus right in the chassis, a subpanel to support branch circuits. It is attached to ONE of the two 200A breakers. Most support 4 or 8 spaces for branch circuit breakers. You picked one that supports 30. It's possible that your 30-space panel does not have "thru lugs" to go onward to another subpanel. That won't be an impediment for you.

enter image description here

Above: How a 400A ranch panel is normally set up.

enter image description here

Above: How a 400A ranch panel works with two subpanels. In your case, the left side subpanel is "built in" to your meter-main. Also note the two subpanels have main breakers; YOU don't actually need them, I just put them there because I have more layers in that drawing that add some generator interlocks.

Remember: The internal subpanel is attached to ONE of the 200A breakers.

To use the other 200A breaker, you must add an additional subpanel elsewhere. In fact, the second breaker position won't even be populated - you get to choose a 100A or 200A breaker there depending on if you have 300A or 400A service.

Subpanels are for more than one circuit

I want to mention that, because your use of subpanels is rather odd. What you drew is legal - you can indeed "daisy chain" them like that if you're willing to spend that much on feeder cable.

However, it's normal to have more than one load per subpanel. So any of those loads (except the ADU) would simply be a branch circuit coming out of the nearest convenient subpanel which had the needed ampacity to support them. That way, the loads are downstream of a much smaller breaker, and so the wires can be smaller.

Breakers protect wires. So if you come off a 200A breaker to serve a 40A subpanel with a 40A load, your wires need to be sized for 200A (because that's the breaker). For instance if that's going to an heat pump disconnect, you'll find that 40A rated disconnects cannot take 250 kcmil (200A) wire. That forces you to install a 200A subpanel there for one 40A breaker.

That was the fatal flaw in your plan as-drawn, all those subpanels would have to be 400A-rated, so $3000 each since we're in the industrial parts book at that point.

You shouldn't be cramming anyway

You are trying to do the panel schedule (which circuits on which breakers) and are running into breaker space scarcity. That should not exist. You should get yourself plenty of spaces -- panel spaces are CHEAP. Not in that particular panel, but in most panels, and subpanels plural will save the day here.

Certainly you will need to populate the second 200A breaker in the meter-main-combo panel. Then, you will use 250 kcmil feeder to take that to a subpanel. (I strongly recommend subpanels indoors, as it is less wear and tear on the expensive AFCI and GFCI breakers). From there you can either daisy-chain 250 kcmil to more 200A subpanels, or you can use a smaller feeder to bring up to 125A (economically) to other subpanels.

Be liberal with subpanels. Don't take branch circuits long distances.

Notably, one of the things you want to avoid is having a massive bundle of 14/2 and 12/2 cables going 75' across a huge, spready house simply to reach an available breaker space. #1 Spaces shouldn't be in short supply and #2 use subpanels to have spaces close to where they're needed. Then, you have one heavy feeder supplying the subpanel... for instance a 90A/240V feeder (180A@120V) costs less than two 12/2 cables. "That's a no-brainer!"

By smartly placing subpanels, you can keep the cable runs short, reduce voltage drop, and save a lot of money on wire.

And I have things for you to spend the savings on, believe me!

Those loads need to be re-sized.

First, bone up on heat pumps. You really want this technology in a new build. An "air conditioner" is simply a heat pump with the $10 reversing valve deleted so that it can't heat. Heat pumps are more money right now, but they're a really smart hedge against all the crazy stuff happening with regulation of fuels, and the mostly-favorable changes happening in the electric market. You don't want to be 15 years down the road paying 4x the gas price while cost of electricity is half what it is now (when used at favorable times). Anyway...

80A for an air conditioner: I'm not sure where that figure came from. Either this house is a mega-mansion, is poorly insulated, or using very inefficient units. They can't even sell units below 13 SEER anymore. The most efficient heat pump units are 24-26 SEER. That gives a whole lot of BTUs for not many amps. Also on such a large installation, consider having two units - that way you have some A/C if one unit goes down.

60A electric tankless: That's enough for one shower. This sounds like a rather large house which will have a lot of water demand. Under-sizing is the #1 blunder made with tankless heaters, and it's a project killer. Your family will be highly dissatisfied with the result, and you'll be forced to tear out the tankless and fit tanked heaters. For a successful tankless, figure at least 120-150A. Or consider multiple smaller tankless heaters at/near the point of use - greatly reducing the wait for hot water. Multiple small tankless can be "oversubscribed" somewhat.

50A for a future ADU -- check with your AHJ but I think they will require more than that.

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  • 50A for a future ADU - if it also has a tankless heater then it will definitely need a lot more. Jul 27, 2021 at 20:57
  • Thanks for the great answer. I think I understand what to do now. 400A unit datasheet connection diagram shows a 200A QDL breaker factory installed, and another 200A breaker is to be "field installed", wasn't clear what this meant earlier until I saw your diagram.
    – Shubham
    Jul 28, 2021 at 23:29
  • It's a 5-ton AC unit for a 3000sqft house, I was assuming I need to size the breaker for the starting load. Some quick googling said about 70-80A starting load for 14seer units. Sounded reasonable to me? I have a 3 ton 13seer unit at home right now, breaker is sized for 40A I believe.
    – Shubham
    Jul 28, 2021 at 23:31
  • The 60A for tankless is going to only power the 13kW unit under the kitchen sink. I'm going tank-type to supply all the 4 bathrooms, that one only uses about 5kW on a 20A breaker. The reason I split it was because the kitchen is pretty far away from the tank-type WH, but maybe I don't even need this? It would be cheaper to run 75' of hot water pipe to the kitchen than 60A wiring with no actual benefit.
    – Shubham
    Jul 28, 2021 at 23:33
  • The instructuons will tell you the minimum breaker to use, and the maximum. One advantage of subpanels is you can run far cheaper aluminum wire to them. So making a large A/C disconnect a subpanel can save serious coin. 13-14 SEER is basically the bare minimum. The government won't let them sell less efficient units. A little savings now (or not, given the higher provisioning), but you'll pay for it every summer thereafter. Jul 28, 2021 at 23:34
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It is not advisable. You technically could, if you used a panel with feed through lugs (generally on the bottom of the busing), but the feeder wire to the additional disconnects would have to be rated for 400A (and the 83% derating wouldn't apply) and then if the taps to the disconnects were reduced size you would have specific length and overcurrent protection requirement. Really that's an expensive and impractical method. Just the wire connectors for each of the taps above the remote panels you show are about $100 each. (Connectors linked are if you used expensive copper wire, aluminum would be bigger and cost more.)

The normal methods would be to have two panels. Either using a 400A as shown with a single (200A?) breaker in the 400A panel feeding a (200A?) sub-panel, or if planned from prior to construction a "Class 320" electric service.

A Class 320 is a single meter that feeds two disconnects/panels that are rated for 200A each adjacent to the meter. You can choose to use fused/breaker disconnects at the meter and then run 200A feeders to remotely located panels that are located convenient to loads, or you can mount panels directly at the meter location and run all branch circuits back to the service location.

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  • The 83% derate wouldn't apply to the panels. It would apply to the wires, so 600 kcmil Al wires would be "alright" by which I mean wildly inappropriate but legal. Jul 27, 2021 at 20:00
  • Even if you bought the $100 connectors and the fat wires, could you use the subpanels as junction boxes for them or would you also need a huge box next to each subpanel to do the splicing ?
    – jay613
    Jul 27, 2021 at 22:35
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The diagram (and description, I think) is based on tapping the main feed just after the 400A main breaker. Instead, run each subpanel from a breaker in the main panel - e.g., 80A or 60A etc. as listed in the diagram.

But I would modify that slightly. Put your really big loads, like the tankless hot water heater, directly in the main panel. Put mostly smaller loads into the subpanels. Those subpanels can themselves be quite large (in spaces) but should be kept to (typically) 100A or less in total current in order to keep the wiring (and the matching breakers in the main panel) smaller. Then those subpanels don't actually have to have main breakers themselves, as the breakers in the main panel will protect the wires and provide a convenient disconnect for each subpanel (since they are next to each other, you don't need a disconnect in or next to each subpanel like you would for a separate building).

In fact, if you do this, you likely won't need 4 separate subpanels - probably 1 will do just fine, though perhaps for logical separation another subpanel for the future ADU would make sense. Your main panel (even if 30 spaces as in the image rather than the 40 listed in the description) can easily hold:

  • 2 - subpanel 1
  • 2 - subpanel 2 (adu)
  • 2 - AC
  • 2 or 4 or 6 - hot water (some of these use multiple large circuits)
  • 2 - cooktop
  • 2 - oven
  • 2 - EV charger
  • 2 - dryer

and still have several spaces left over.

Your nice big (42 space?) subpanel can then handle all the lighting and receptacle circuits, but it can include 240V loads too. Just do a rough load calculation to make sure the feed to the subpanel is large enough. I would expect that with all the major 240V loads in the main panel, 100A would be more than adequate.

What you describe appears to be a 400A Meter Main. In a quick search, it looks like some of those are 30 space/40 circuit or similar. Keep in mind that when it comes to (a) high-current breakers and (b) AFCI or GFCI, spaces count. "circuit" only works with half-size breakers, generally limiting what you can install. So just look at spaces. That really doesn't change my answer - 30 spaces is plenty. But keep in mind that subpanels (whether "main panel installed as a subpanel" or "designed as a subpanel") have the same issue, and it matters even more for "lots of little circuits", because in new installations you need AFCI and/or GFCI for most 15A and 20A circuits. For example, if you want to install a separate receptacle circuit for each room instead of "half the house on one circuit", the number of spaces needed (1 space per AFCI circuit) goes up pretty quickly.

You may also find (I did in a quick search, but that was big box, not real supply house) that 400A Meter Mains with lots of spaces may be hard to get. If so, you can go with a smaller (e.g., 8 space) Meter Main and an extra subpanel if needed. An 8-space Meter Main can support 4 subpanels, with the only possible complication being the on-demand hot water heater (as it may require more than 100A all by itself, but using multiple circuits so won't have room for it in an 8 space Meter Main.)

As noted by Harper, you really don't want AFCI & GFCI outside. Yes, you will need a Meter Main, but you can get one with a lot fewer spaces and put everything in subpanels, so that your Meter Main can be outside (it will almost certainly have to be outside, both so the utility can access the meter and so first responders can access the disconnect (main breaker(s))) and nearly all your wiring inside. Once you do that, no need to have the subpanels next to each other, though that may still make sense depending on the layout of your house.

And one more thing...HOT WATER:

You may want to rethink the on-demand electric hot water heater. That uses a lot of electricity. Overall usage should be comparable to tanked heating (the same water is getting heated, just at different times, and tanked heaters are very well insulated these days). If you can get natural gas in your area, an on-demand gas water heater can make a lot of sense. But if not, consider that:

  • If you ever get peak demand billing then the on-demand water heater is going to increase your peak demand. A lot.
  • You may be able to use smaller service (= cheaper panel/etc.) without the on-demand water heater. Seriously.
  • If you have power outages then (a) with a tanked heater you have hot water to use for a while, (b) if you install a generator or PowerWall for backup power it won't (realistically) be able to support a large on-demand water heater.
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    I didn't see your post before my comment above, GREAT ADVICE. I actually hate on demand electric WHs, they take a LOT of power. With cities banning natural gas, going all electric, and electric vehicles (EVs) coming and being charged.....I just don't know how the electric grid can support all of that. California can barely keep the lights on now. What happens when everybody is charging EVs, on demand electric WH, electric range, electric clothes dryer, etc. Yeah I'm ranting and this comment may get deleted, but just had to say. Jul 27, 2021 at 16:15
  • And I saw your comment after my update. The whole "ban natural gas" thing is crazy. I mean, conceptually it makes sense, but only if the "average" electric kWh is being produced more efficiently, but we're a long ways from that in the US. Natural Gas for electricity or more residential use is still a hydrocarbon, but much cleaner (except for "carbon") than oil, coal, etc. and unless (ha ha ha!) nuclear has a big boom (pun intended) it will be a long time until we have non-carbon source for most US electricity. The house I (sort of) grew up in was built in early 1970s when there was a Jul 27, 2021 at 16:19
  • local ban on natural gas due to the oil crisis (at the time supply & political, not "carbon"/environmental). But the result was horrible expensive resistance heat (always cold in the winter), all electric appliances, etc. When I moved out (1.5 miles away) I got wonderful natural gas heat and when I redid my kitchen I got a natural gas cooktop. Eventually rules changed and my parents got natural gas installed in the 90s - it was literally 50 feet away for 20 years. Jul 27, 2021 at 16:21
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    While we might get snipped for a "conversation" WOW...we really agree. Natural gas electric turbines are about 40% - 45% efficient in generating electricity. Natural gas delivered directly to the point of use (cooking/heating) is about 80-85% efficient. Some people (politicians) just can't comprehend the "big picture" and make stupid, woke decisions with no context, no science, no studies. Jul 27, 2021 at 16:41
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All the time. We have off 2 split off 300A on Main

  1. Garage 100A - a) 30A for RV off house,
  2. House 200A - a) 30A for AC; B) 30 A Air Handler, dehumidifier & furnace; C) 60A Attached garage converted. D) 60A in (converted fuse box) into Shed. (Chicken coup)

Point is, on the Main 200A the runs are on a total breaker on main panel like the chicken coup/shed has A 20A on the main and then on sub panel a 10A and 15A

The point is total load will trip the main breaker in the main.

Husbands a Retired Master Industrial, Marine, Commercial, Residential.

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  • This doesn't clearly address the need to either run 400A rated wiring to all the sub-panels, or run the sub-panel off a smaller breaker (which it sounds like the OP wasn't planning to do)
    – izzy
    Jul 27, 2021 at 15:45

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