I have a few questions I'd like to lump into this one, so please bear with me.

I am planning to completely rewire my main level, moving my laundry room up from the basement in the process. I decided that I want to put a sub panel in the laundry room that serves as the home run for the entire main floor. In looking for what gauge feeder wire I need, I ran across a confusing entry in the table that I am not sure what to do with.

  1. What does "Ampacity Dwelling" mean?


Allowable Ampacity 60C (Amps):  165
Allowable Ampacity 75C (Amps):  200
Allowable Ampacity 90C (Amps):  225
Allowable Ampacity Dwelling (Amps):     225
  1. I am assuming that a 150A sub panel refers to the 240V, so would that mean it could technically handle 300A worth of breakers in 120V circuits?

  2. Would a 3/0-3/0-3/0-1/0 SER copper cable be the right thing to run from my main panel to supply the sub panel for the main level? I do not mind paying extra to oversize my cable for larger derating and less wasted power from wire resistance. (I also cannot have aluminum wires in my house for unfathomable religious reasons.)

This is all part of a bigger renovation of this old house from 1978. I'm planning to upgrade the house from 150A service to maybe 300A at the meter so I can plan for the future where I need to charge vehicles. I also just got into welding as a hobby, so my 120V Harbor Freight welding setup may eventually grow into something more serious in my workshop (woodworking, 3D printing, electronics, etc.). When I upgrade my main service, I plan to have it wired to support a whole house generator, and the main panel will only supply power to the garage, HVAC, and fully-finished basement level (two bedrooms, home theater, etc.) in addition to the sub panel upstairs, of course. My house was built on a hill, so the garage comes in at the basement level, and the main level with the master suite, living room, kitchen, guest bedroom/office is above.

I want to do this once, and I want to do it right. I'm planning to have a dedicated 20A circuit for each bedroom in addition to a dedicated 15A circuit for each bedroom because you never know if you might decide to throw crypto mining rigs in every room one day. :)

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    Yes. The imperial director in charge of banning aluminum wire is named John Long. The ban only officially applies on my property, but...
    – John Long
    Feb 19, 2021 at 0:25
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    @JohnLong -- Class 320 service adds significantly to upfront costs due to the practical need for a 400A meter/main under most utility rules when combined with the current NEC. Also, many people think they need it because of outsized peak loads when they should really be designing those loads (instantaneous electric hot water, heat pump backup strip heat) out of the system Feb 19, 2021 at 3:04
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    @ThreePhaseEel One more thing to level-set: I sold my old house for a tidy profit. I was prepared to have a house built, but the idiot builder refused to install anything other than CAT5e. When I pressed the issue, he gave me back my $10k deposit. I ended up finding a MUCH better existing house with really good bones for over $175k less. So, I'm using that extra savings to make this my dream house/forever home. I want it to be beyond awesome when I'm done.
    – John Long
    Feb 19, 2021 at 3:24
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    Vote John Long out of office. He's obviously read some nonsense on the Internet that gravely misunderstands the concerns about aluminum wire, and is about to waste a fortune of your money. This guy would have you put copper wire on aluminum lugs, for Pete's sake! I get the whole idea of trying to do an A-#1 class job... however, I advise actually doing that and not doing a newbie's impression of what that is. This will require you to skill up. Feb 19, 2021 at 19:27
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    But you don't actually need to do good work to have satisfaction. A lot of novices are very fastidious about how they do wiring - unfortunately they also have very poor data collection and vetting practices, so they do the wiring to their own satisfaction and not at all to the inspector's satisfaction! LOL! End of the day, satisfaction is just a made-up emotion. What I value is a) complies with Code and b) has the flexibility to do anything we want it to. Electricity does our bidding. We're the masters here. Electricity should do everything we want. It works for us! Feb 20, 2021 at 5:59

3 Answers 3


"Ampacity Dwelling" doesn't apply to your situation

The "Ampacity Dwelling" figure you see is based on the 83% "uprate" that the NEC permits on 100-400A feeders or services that feed entire dwelling units. This is set out in NEC 310.15(B)(7); some sources provide a table instead of the rule, as this information has historically been conveyed in the NEC in tabular form, although the application conditions have not changed. As a result of that "entire dwelling unit" criterion, though, that number is useless when putting in a subpanel within a house as you are doing here.

150A refers to 240V, yes, but adding up the breaker ratings isn't what you do here

You are correct in so far as the 150A figure you're considering for your subpanel refers to a 240V rating. However, you don't figure the load on a panel by adding up the breaker ratings; instead, you need to do a load calculation using rules found in Article 220 of the NEC to figure out how much load you're putting on said panel. This requires the square footage of the area served, which is used to compute general receptacle and lighting loads, while not-so-generic loads such as kitchen/laundry small appliance circuits and heavy appliances either get their own allowances or are computed from their nameplate ratings. To top this off, generic-ish loads have a load factor applied to them past the first 3000VA of load to account for load diversity (you aren't running everything plugged into the walls in your house at the same time, after all).

Subpanel selection advice: GO BIG OR GO HOME

However, what I said above about load calculations doesn't mean you need to put some itty-bitty subpanel in. Having more busbar capacity than needed is not a large deal for reasonable values of "more", and more importantly, the rise of AFCIs means you can't rely on "double-stuff"/"duplex"/"tandem" breakers to bail you out of a shortage of breaker spaces. As a result, given that you don't want to rip anything out and replace it in the future, I'd use a 40- or 42-space, 200 or 225A, main lug panel as a minimum specification for your new panel. If you can get a 54-space or 60-space panel for a reasonable price, that would be a worthwhile upgrade, even.

Don't Fear the Aluminum (but get a torque wrench to keep the reaper at bay!)

Your "no-aluminum" policy is also not grounded in what we know today about electrical installations; aluminum wire was only ever a problem because AA-1350 "EC grade" pure aluminum wires were landed on steel wiring device (switch, receptacle) terminal screws with inadequate testing to smoke out the connection problems that happened due to thermal expansion, creep, and so on. However, distribution equipment uses plated-aluminum box-lugs instead, which provided they are correctly torqued, are not prone to such failure modes. (Burnups observed in the field are generally the result of inadequate torque, not any inherent property of the modern AA-8000 series aluminum alloy wires that are required by the NEC for most customer-side aluminum wiring applications.)

So, I'd go with 250kcmil Al SER cable instead of blowing a bunch of money on copper, and use the money you saved on that to do something more useful, like getting a big panel as I mentioned above, and an inch-pound torque wrench for that matter. However, even if you insist on the 3/0 copper (either as SER or as individual wires in 1.5" EMT or RMC -- 2" conduit simply doesn't fit in 2x4 stud walls), you'll still want that torque wrench, so that your electrical system doesn't give you the loose lugnut!

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    @JohnLong -- ever considered that copper is harder on the schmuck who has to go run the wire? (It's both heavier and stiffer than aluminum by a considerable margin) Feb 19, 2021 at 2:58
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    If you want to do it "the best way" run conduit (EMT, unless you're really going to run the "cost no object" thing to the ground and use RMC) and pull THHN or XHHW wires, rather than running cable. Of course, the best wire is really silver if cost no object .vs. better electrical properties (for the same size) is your watchword. Engineering-wise, appropriately-sized (or even oversized) aluminum will meet or beat copper performance at far less cost, but suit yourself.
    – Ecnerwal
    Feb 19, 2021 at 4:13
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    3 - 3/0 (since metallic conduit is the grounding conductor) will fit in 1-1/2" EMT (at just under 40%, and because it IS conduit, not cable, you can actually use the 75C rating if the terminals are rated for it, and they usually are...) - 0r 1/0 copper if mad for copper and that fits in 1-1/4 EMT (again using the 75C rating if the terminals are so rated) XHH buys a few percent over THHN (and is common on that size, IIRC)
    – Ecnerwal
    Feb 19, 2021 at 4:23
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    Yeah, we need a whole FAQ on "Hi, I'm a novice, and I want to DIY my electrical (okay so far) and I want to do it absolutely top-shelf, not to Code minimums" (correct: code minimums are for slumlords). "What should I focus on and what not?" Feb 19, 2021 at 19:34
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    @JohnLong -- you don't need the 1/0 EGC in the EMT; in fact, if you try to put it in, it forces you to 2" due to fill limits, which no longer fits in a 2x4 wall Feb 20, 2021 at 15:29

What does "Ampacity Dwelling" mean?

It means nothing at all, because it's coming from a wire salesman's website. They don't write the electrical code and they don't interpret it. Wire sites are chock full of lies. One site I like tells me #6 is good for 105A. (I know where they got that number, but it's disgraceful to publish it.)

Only the Electrical Code matters. If it's #10 or smaller you go to 240.4(D). Anything larger, you're at 310.15(B)(16), "The Big Table". You look up your cable type in the top row, that tells you the thermal limit of the cable. You're also limited by the thermal limit of connectors, so even if the wire has a 90C limit, you're pushed down to 75C or even 60C. I've never seen a case where someone got away with using 90C numbers in residential. You wouldn't want to anyway if you're talking about upsizing for voltage drop. NEC is concerned with not burning your house down, not your pocketbook.

Anyway, once you're in typically on feeder this size the 75C column, you work down to your target ampacity, 200A. Oh, look at that. 3/0 copper or 250 kcmil aluminum. That was easy. Rounding up or down didn't even come up!

Now, if your feeder size is anywhere near your service capacity, you make one more pass, using NEC 310.15(B)(7). Your service wires (which carry all the current downstream of the meter get a favorable 83% derate. And this is saying that your feeders never have to be bigger than your service wires (because that would be stupid). So take your 200A service size, multiply by .83, and get 166A. Back to table 310.15(B)(16) and we find that 2/0 copper @ 175A is legal as a 200A service wire, so no feeder downstream of that needs to be larger than that.

So I guess the numbers on that site aren't wrong, but the point is, you cannot trust them as they are not a proper source of data.

Regardless, I strongly advise Buy the wire LAST. Seriously. It's the least returnable thing in your project. Buy it only after you have finished learning, and haha, joke is, you never will be! So buy it as late as possible, so you will benefit from as much information as possible.

"Is my 150A panel good for 300A @ 120V?"

Yup. Kinda. It's actually NOT good for 300A. It's good for two banks of 150A. Here's what I mean. Suppose you have a machine that draws 32A actual and you must derate it 125% to 40A, as you usually must do. How many of those "40A" provisions can you put on 150A @ 240V? Seven of them (280A)? NO! Each 150A bank can support only three (120A) each. So six total.

That said, that only applies to loads you PLAN to run simultaneously by design such as banks of grow lights in a shed. General loads in your house, such as your 2 circuits per bedroom, are counted a totally different way, that allows for the fact that you won't be running everything at the same time. There's a whole section in Code on doing load calculations for a whole house, and that decides your minimum service size.

An interesting side effect of those rules is each kitchen and bathroom receptacle circuit adds 1500 VA (think watts) to your load calculation, but other circuits don't. So your additional bedroom circuits are "free".

Is 3/0 SER right for 200A to a sub?

Depends. If your service is 200A it's larger than need be. If your service is larger than 200A it's the minimum allowable size. Voltage drop is not a factor over such a short distance. SER is not a choice I would make, since inside conduit it's hard to pull, and without conduit, it is not upgradeable or replaceable.

What I would do: Panels.

First, let's talk about your ambitions. They are quite correct. "I want to do this once, and I want to do it right" - yes, that's how we work! Being picky about what's installed: with ya there. Not trusting anyone else to get this right - oh yeah. "I don't trust anyone else to do it the "right" way" - absolutely - so true. "Most people you hire will do things to the minimum code requirement." - yup, can't agree more. NEC minimums are for slumlords. Yet a lot of people follow it like it's the Bible - someone recently reported having a 5-bathroom house with all bathroom receptacles sharing 1 circuit, technically legal in NEC, but man, I hope that guy enjoyed the latté they bought with the "big saving" on wires.

Picking up on the idea of "200A now, more later", I'd take full advantage of the way "400A" (Class 320) service is currently actually implemented in North America. 400A breakers are prohibitive. So they install TWO 200A main panels - just run-of-the-mill, common-as-dirt, $130 panels from the box store. Most 400A meter-mains are specifically designed for this configuration, and are lugged for dual 250 kcmil wires (that's 200A in aluminum). Since they're not rated for paralleling, this is the only way they can be used.

However, NEC 2020 requires the main disconnect must be outside. It's by far the cheapest to integrate "main disconnect" and "main breaker", so now, "Meter-Mains" (meter and main breaker in one enclosure) are the option of choice. How is that handled in 400A? By having meter-mains with dual 200A main breakers.

They also sell "meter-main-panel" that combine all of them in one. These are a bad idea except for what are called "farm panels" which provide 8 breaker spaces and then thru-lugs to carry power onward to your primary panel (the one with most of your loads in it). I like them. The 400A versions of these are still the same deal, two 200A main breakers, but the internal panel is only fed by one of those breakers.

The second 200A breaker isn't even provided - that's so you can fit up a 100 or 200A breaker depending on if you have 300A or 400A service.

So for me in your shoes, there are two ways to go with this, both involve installing a 200A "panel inside the house" for now. On one path, you simply install a 200A meter-main and be ready to trash it later, but you keep your "panel in the house" and it becomes one of your two 200A panels. On the other path, you install a 400A meter-main now, and simply do not populate the second 200A breaker. When you take the upgrade, you populate the other breaker and feed that to a 2nd "panel in the house". In fact you could wire up this second panel today even; just feed it from the first panel.

Of course the panels would be enormous. Running out of breaker spaces is the most unnecessary crisis we deal with on here. Breaker spaces are dirt cheap. Honestly, what gets my goat is the endless parade of people who come on here, they spen, and that shouldn't be a problem if you're sympatico with copper feeder, but yet, so many of the novices we see who are out of spaces chintzed out on the panel and spent on copper wire. Just don't do that okay? :)

What I'd do: Conduit heck yeah

You know the builder who wouldn't deal with you because you wanted better than Cat 5? Are you kidding me!!?? Fine, you get Cat 7, what happens 5 years from now when you never see your kids - they're over at the neighbor's house watching his holographic TV because your ratty old Cat 7 can't support it? Honestly if you built your house 20 years ago, you'd have cat 3 buried in your walls and no 4K TV for you. Enough. Chasing trends is a loser's game. The only constant is that houses live much longer than tech.

What we do know, for sure, is that signals will travel over something long and thin - might be fiber, might be coaxial, might be spaghetti. But it'll be long and thin. So I would install conduit and call it a day. Not only does it allow you to change wire, it allows you to add wires.

Conduit is more initial work, but obviously that's not a deal-breaker for someone who wants the best.

Further, conduit really lends itself to the novice. And here I'll throw in a pitch for EMT metal conduit. Because it assembles like an Erector Set and if you mess up (which is understandable as you are learning), you can dismantle it and salvage all the fittings and most of the pipe.

Yes. EMT for telecomm. Least, better shielding, plus the Erector Set thing.

I haven't even talked about running AC Power in conduit, but that works even better. I would definitely do it for runs such as panel-to-workshop or panel-to-garage where you are likely to want to add or change things.

The best way to do that is run THHN individual wires (i.e. a black wire and a white wire, loose in the pipe).

The gotcha with AC power in conduit is you have circuit limits. There's at least 4 Code sections that interlock to mean in US 120/240V power, "4 circuits per conduit if the breakers are 30A or less". (There's a corner case involving a 28-30A non-continuous load with 4 circuits in a pipe, but never mind that). For >30A, you need to do the math with 310.15(B)(3)(a) "multiple circuit derate", but it eases the pain that you're allowed to derate off the 90C column. (which is why it's a non-factor for up to 4 <30A circuits.)

I myself prefer multiple 1/2" conduits anyway; it's easier to work with, and the sweep radius is smaller (easier to build around). You can have up to 4 sweeps between access points, and the access points must remain accessible forever.

After all, you pull the wires last, so you can build the conduit and then leave the actual pulling to the electrician. Many of them object to doing only part of a job, but don't mind so much if the part you did was competent conduit.

What I'd do: Many circuits.

2 circuits per kitchen is one of those slumlord bare minimums. The fact is, any common heating appliance is 1500W, because that is the maximum UL will approve for the US market. And if you're following the bouncing ball, you know that's 12.5 amps i.e. you can't run 2 of them on the same circuit. That means 1 heat appliance per kitchen circuit, and suddenly 2 isn't really enough.

And bathrooms! Holy smoke! 2 circuits per bathroom is again not excessive, at least for the master suite. Bathrooms sharing a circuit is right out!

Now let's talk about bedrooms - and pause to ask yourself: "Where did the idea of "1 circuit per bedroom" even come from in the first place? I sure don't know, it sounds like it's something that somebody made up. Well, whoever they are, they clearly never wired a house. Do they really mean to circumnavigate a bedroom with wire, sharing each wall with a different circuit? Ridiculous!

I say Run the wires down a wall, and serve rooms on both sides. If you run 1 circuit per wall, that means each bedroom has access to 4 circuits! (In practice you bend that a little bit because of physical layout, but the general concept holds. YOu can put 60-80A in any room, (though not in every room at once obviously).

No 15A circuits. Every circuit that is allowed to be 15A is also allowed to be 20A. I don't even own any #14 wire. (but I own 10 colors of #12, because I take color coding in conduit seriously, and since I work in EMT, green is not one of them.)

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    Thank you. You put a lot of effort into that answer, above and beyond. You pretty much covered the direction I'm heading in. I will definitely consider your suggestions here.
    – John Long
    Feb 20, 2021 at 23:32

What does "ampacity dwelling" mean?

That's just that vendor's way of notating that the allowable ampacity of a given conductor will vary depending on usage. From another vendor (https://www.usawire-cable.com/pdfs/NEC%20AMPACITIES.pdf):

*For dwelling units, conductors, as listed below, shall be permitted as 120/240 volt, 3 wire, single phase service-entrance conductors, service lateral conductors and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panel board(s) and the feeder conductors to a dwelling unit shall not be required to be larger than their service entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors provided the requirements of Sections 215.2, 220.22 and 230.42 are met.

USAWire then lists the allowable ampacity of various sizes in the context of the above quote.

Nassau Electrical just includes that as an additional line in their spec, after the 60C, 75C, and 90C lines.

If you are interested in the section of code that discusses this ... From https://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=63.0:

Section 310.15(B)(7) contains rules on calculating the conductor size for 120/240-volt, 3-wire, single-phase dwelling services and feeders. In the 2011 NEC, a simple table existed showing the service size on the left and the minimum size conductor required to supply the service on the right.

In the 2014 NEC, the table has been deleted and replaced with a simple 83% which results in the same final value as if the table had just been allowed to remain.

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