My plan is to run 240 volt 100 AMP power to detached garage. I have 200 AMP Panel in basement of house. Main question is wiring for the run. 50 feet of the run will be in house crawl space/basement. 50 feet will need to be buried 24" in ground. Can I run direct buried cable for the whole distance if so what size wire? Or should I install PVC conduit and pull individual wires and if so what size wire to pull? I would prefer aluminium wire for the cost savings.

Additional Details.

  • Main House Panel is a Siemens/ITE Load Center G4040MB1200.
  • Plan is to add a Q2100 100-Amp Double Pole Type QP Circuit Breaker to Main Panel.
  • House is all electric, 2 Heat Pumps, 1 Electic hot water heater, electric dryer and electric stove.
  • Main panel is full so plan to add 2 double stuff breakers to make room for the 100 AMP Breaker in Main panel.
  • Garage is 28x40.

    • Will be installing 30AMP rv plug for occassional use of RV AC for guests.
    • Will be installing Window AC unit in garage 20AMP circuit and a couple 20 amp circuits for power tools, and a couple 15 amp circuits for lights.

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Outdoor HVAC Unit Subpanel

  • How many BTUs is the window AC? Can you provide photos of the nameplates on the heat pumps, as well as photos of the main panel? Last but not least, how many square feet is the house, and how many kitchen receptacle branch circuits do you have? Commented Aug 19, 2019 at 23:02
  • Window AC will be 20,000 BTU. Not sure how to post photos. Heat pumps have mininmum AMPS of 16.9 and 21.4 on the labels. House is 3,500 sqft. 5 kitchen circuits Commented Aug 20, 2019 at 0:01
  • You should be able to hit "edit" in "share edit..." on the bottom left of your post, then use the button that looks like a stylized mountain-and-sun painting in the edit toolbar to add the picture to your post. Commented Aug 20, 2019 at 0:26
  • Thanks for picture adding instructions. Odd that you can't add as part of the post but only after posting and then have to edit. Pictures added to original post. Commented Aug 20, 2019 at 1:10
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    Camera is good but I have to compress pictures to be able to upload. I Uploaded all new new photos. I tried compressing them less, hopefully you can read them better. Thanks for your time. Interested to hear your calculation results. Commented Aug 20, 2019 at 2:28

4 Answers 4



Your existing panel is listed and labeled as a Circuit Total Limiting panelboard, which means that it can only use double-stuff/tandem breakers in spaces that are marked and designed to accept such. However, the label on your panel designates no spaces as usable with tandem breakers, only standard breaker poles, rendering your panel-stuffing plans a violation of the panel's listing, and thus of NEC 110.3. Furthermore, space 34 already has a tandem breaker in it, which means that your panel is overflowing right now!

This means that you'll need to change out your main panel for a 42-space, 225A, main breaker loadcenter, at minimum, in order to do this; a 54- or 60-space panel such as the Siemens P5470B1225CU would be my recommendation, even, given that you already are up to 43 spaces' worth of breakers and how useless double-stuff breakers are in this day and age. The reason for the 225A panel (vs. 200A) is explained below, by the way.

and Overloaded

Applying NEC 220.83 to your current situation, consisting of:

  • 3500 ft2 at 3VA/ft2
  • 5 kitchen SABCs and a laundry SABC at 1500VA apiece
  • 3 more 1500VA allowances for fixed kitchen appliances
  • a 12kVA range allowance
  • a 5kVA dryer allowance
  • and a 4.5kVA water heater
  • and your HVAC system consisting of 38.3A of heat pump outdoor unit as well as 111.9A of strip heat

puts you at 45500VA of non-HVAC load, taken at a 40% demand factor over 8kVA for 23000VA of factored non-HVAC load. To this, we add the 38.3A (16.9 + 21.4) of heat pump outdoor unit draw at full rating and the 111.9A (26.3A + 29.2A + 2.5A from one unit, and 53.9A from the other) of heat pump indoor unit draw at a 65% demand factor to get 8809+16729 = 25538VA of HVAC load at a 230V utilization voltage. Adding those two together and dividing by the 240V service voltage gets us a total load of 48538VA, or 202A, which is actually slightly more load than what your 200A service can handle already!

This forces us to bump the service to 225A to have enough headroom for the new garage. This requires a pigtail of 250kcmil Al to be added at each end of the existing service-entrance wiring (connected using Al9Cu Polaris connectors) to allow the existing wiring to run at 90°C and thus have sufficient ampacity for a 225A service, as well as a 225A panel, such as the one proposed above.

Wedging the garage in there

Now we can move onto the garage, which consists of:

  • a 28'*40', or 1120ft2, space at 3VA/ft2
  • 2 1500VA allowances for power tools
  • and a 9.6A/230VAC (representative 20kBTU, 10.4EER air conditioner from here)
  • OR a 30A/120V TT-30 receptacle supplying a RV, treated as a single 3600VA RV site as per 551.73(A) and the associated demand factor table

By itself, the garage draws 8568VA, or 35.7A at 240V. However, we can apply the 40% demand factor from 220.83 to the receptacle loads in the garage, meaning that the garage only adds 4752VA, or 19.8A at 240V, to the service load. (The RV receptacle, which would otherwise put you over even with the bump to 225A, is being treated as a non-coincident load with the rest of the garage loads as permitted by NEC 220.60 given that I doubt you'll be working on stuff in the garage while you have guests over at your house :)

...and getting it hooked up

Once the panel is replaced, getting the garage hooked up is the easy half of the enterprise. I would use 1.5" schedule 80 PVC for the entire run; in the basement/crawlspace, you can strap it to the bottom of the joists with an expansion joint in-line with the run to keep it from doing the worm on you, and then just transition it directly to the outside run with a pair of 45° sweeps to bring it down to burial depth (24" is fine) and 90° sweeps at each end to bring it into the panels. Inside this conduit, you'll be running 3 1AWG Al XHHW-2 conductors along with an 8AWG bare copper ground; this takes up nowhere near the fill space availabe in the conduit at 270mm2 used vs. 442mm2 available while providing a fully rated 100A feeder to the garage.

At the house end, a Q2100 is indeed the correct breaker choice for the job provided that the replacement house panel is Siemens as well, while I would in fact take the old 200A panel that got obsoleted by the house panel upgrade and put it in the garage; the main breaker is needed to serve as a local shutoff for power to the garage (and nothing more), while 40 spaces should be more than enough for even the most extravagant garage plans. If you wish to buy a new panel for the garage, the minimum I would get is a 100 or 125A, main breaker, 24-space panel, with a larger panel than the minimum highly recommended.

In it, you'll want 2 15A breakers for lights (and only lights), 2 20A GFCI breakers for receptacles (possibly combined as a 20A multi-wire branch circuit using a 20A 2-pole GFCI breaker to allow for 240V power tools), a 15A or 20A 2-pole breaker for the AC (depending on your AC) using a single NEMA 6-15R or 6-20R at the load, and a 30A breaker for the TT-30 RV receptacle.

Of course, when making all these connections, you'll need to use an inch-pound torque wrench and/or torque screwdriver in order to meet the tightening torque requirements of 2017 NEC 110.14(D). Even if your AHJ has not adopted the 2017 NEC, you'll want to do this anyway, lest your electrical system lose you the race!

  • Thanks for your detailed response. Not what I wanted to hear but I sort of figured the current panel might be to small. But I had a local electrical contractor give me a quote and they felt no problem adding the 100AMP breaker and doubling up a couple of the light circuit breakers. How can I tell what AMP service Duke Energy is providing me. Does that matter. Finally, to swap out the panel I assume Duke has to be called to pull the meter while panel is being swapped out and I assume licensed electricain will be needed to hook up the new panel. NC has adoped the 2017 NEC Commented Aug 20, 2019 at 4:20
  • @PeteDonaldson -- you'll definitely need to have the utility pull (or "pull", many utilities can do this remotely nowadays thanks to smart meters) your meter for the duration of the main panel changeout. Whether an electrician is needed for that depends on your local codes and AHJ, though. As to the size of the service, 200A vs 225A is something you'll have to discuss with your utility. Commented Aug 20, 2019 at 4:33

A different slant on that panel.

Bigger service, seriously...

First, at 110A you have enough emergency heat for a Connecticut Yankee. Typically all-electric houses with that much emergency heat are supplied by "320"/400A service, powering dual side-by-side 200A panelboards.

That is one way to go. Leave this panel right where it is, and lay another panel right next to it, and have the power company provision 320/400A service.

... Or downsize emerg heat + house subpanel

Another way to go is downsize that emergency heat. For that you'd need to talk to the manufacturer about whether the coils could be split or reduced. One extreme way to reduce them is power them with 120V instead of 240V with a breaker of half the ampacity, however this would reduce their power by 75%, which might be too much to be effective in a cold snap. Or not, an HVAC expert in your area would know.

However, downsizing the emergency heat will free up 0 or at most 2 spaces in the panel, and 1 is needed to de-double-stuff the existing double-stuff. So you will need a subpanel. Let's revisit the "2 panels next to each other/400A service" concept, and put the subpanel exactly where the 2nd panel would be in a dual-panel/400A service. That way you can make an easy upgrade to that should the need arise. (Which it's pretty close to arising now!)

And in fact use a panel with a 200A main breaker, or that can fit one. (Main breakers are not needed in subpanels in the same building as the main).

I am skittish of changing the main panel, because it is a ton of work and cost, and there isn't really anything wrong with it.

Garage subpanel

I hardly need to add to other coverage on this. I'm writing this with one purpose: To resonate with ThreePhaseEel that a 24 space is the smallest to even consider, and a 30 space is entirely reasonable. The reason is that spaces are dirt cheap, right now, but they will not be cheap later when you wish you had more spaces - a situation you are experiencing right now in the house.

Not lightly do we spend your money. We are space-crazy because we have seen far too many questions of "my panel is full, now what?" That could have been trivially avoided at panel purchase time. (Your builder is not guilty of this; he got the largest panel possible at the time. Your in-the-house needs simply exceed any 40 space panel, to say nothing of the garage.

Watch those stab limits

Each Space in a panel has a bus "stab" blade that it shares with the breaker to its left or right. So for instance Space 1/2 share a stab. Note the breakers are 50A and 60A, meaning that stab is carrying 110A. You can also see where stabs 5/6 are carrying 90A and stabs 9/10 are carrying 100A. You get the idea.

This is a very common situation when the electrician decides to stuff all the 2-pole breakers at the top of the panel, as if it were a "rule of six" panel or something.

Double-stuff breakers share a stab, so a 20/20 is putting 40A on the stab.

Stabs have limits, stated on the panel label, of around 125A give or take. I don't like to push it.

This is why I vastly prefer to run the big breakers down one side of the panel, and have plain 120V breakers across from them.

The reason I'm saying that is because you're about to add a 100A breaker to this panel, and that has to be across from 15A or 20A breakers, not even a 30A. And if you follow my house-subpanel plan, you may be adding another 100A.

Double-stuffs are nope

This panel is not listed for double-stuffs. So the one you have in there has to go. Not only that, you'll need to free up 2 more spaces for the garage sub, that is why I recommend a subpanel in the house. (Or the dual panel/400A setup).

Watch those multi-wire branch circuits

Sometimes, 2 circuits are wired using a single /3 cable and shared neutral. When this is done, you must be very, very careful with "double-stuff" breakers. The two hot wires must be on opposite poles (fed from adjacent spaces). This assures the neutral only carries difference current and not the sum of currents (which would overload it).

Further, modern Code requires they have a "handle tie" for common maintenance shutoff, and the easiest way to do that is use a 2-pole breaker. For now, the main thing is to make sure the "double-stuffing" done to date hasn't accidentally put any MWBCs in the same space. There must be 240V between the two MWBC hots.

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    Thanks for your comments and suggestion on reducing HVAC EM Heat. In my 25+ years in the house I have only used the EM Heat once or twice so will look into that option. I just contacted Duke Energy to find out options for upgrading house service and if that is not an option I guess my fall back option is to just skip the house and have them install a separate meter at the garage. Commented Aug 20, 2019 at 13:31
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    @PeteDonaldson since you use the EM heat so rarely, I would seriously consider my "convert to 120V" trick. That will reduce your emerg heat from 27kw to 7 kw (when it runs, it will run continuously), but will give you back 20kw to use for the garage sub. That'll be plenty.) Anyway, I would expect Duke to be very friendly to big service since they are glutted with generating capacity - tell you a secret, they don't actually upgrade your service drop until their smart meter telemetry tells them that you need it, so it costs them much less than you'd think. Commented Aug 20, 2019 at 21:41

If you have to go digging to lay direct burial cable, might as well run conduit so you can make any future repairs easier. Do not use EMT [thinwall] in the ground as it will rot out prematurely - pvc [sch 80] is cheap/easy to work with but some bends may need some heat applied to form moderate bends, and use long radius 90's where things turn up to make the pull easier. You need 1 1/4" conduit, and don't have more than 360 degrees of bends between pull points. Run 2awg wire [2 for phases and one for neutral] and 8awg for ground. 3awg would work, but sometimes that's a special order/not usually in stock [most cases] Minimum temperature rating for wire needs to be 75c, but if you think you need more headroom for motor loads, go with 90c as starting current adds heat.

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    I think you mean 1 AWG aluminum, alternate 0 AWG. Your numbers would be fine for copper but he wants Al. Commented Aug 20, 2019 at 0:10
  • Right you are. I missed that on account of personal prejudice over the years - I never run aluminum for anything. Ever. Most state building codes have done away with it I think [except for temporary construction], and it can be a problem for your homeowners insurance regarding coverage. Here's a study that's more in depth [note the 1969 UL/NEMA survey on page 25]: nvlpubs.nist.gov/nistpubs/Legacy/IR/nbsir75-677.pdf
    – Flinch
    Commented Aug 20, 2019 at 0:49
  • Harper: Why schedule 80 vs 40. Flinch. I can see not using Aluminum for receptacles etc. I would guess improvements have been made in technique and wiring since 1974. Seems that almost all of the opinions I have reviewed recommend Aluminum on longer sub panel runs. Aluminum is what is the Panel coming from the Meter and feeding all of my HVAC equipment. Commented Aug 20, 2019 at 1:44
  • @Flinch -- most AHJs are fine with aluminum these days, given that in-building wiring types are generally required to use AA-8000 series alloys (vs. the finicky AA-1350/EC that got aluminum its bad rap) by NEC 310.106(B). Furthermore, the 2017 NEC took a major step towards addressing the true cause of termination burnouts by adding the 110.14(D) torquing requirement for all places where termination torques are marked on equipment labels. Commented Aug 20, 2019 at 1:46
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    Note the abstract, which discusses branch circuits. This is feeder. Yes, that is outlawed for all interior wiring. Now you must use the AA-8000 alloy, and nobody's going to use it for branch circuits if they like to ever sell their house, but it's fine for feeder. Commented Aug 20, 2019 at 2:03

The problem is we don’t know your loads or total load for example if you have a max load of 75 amps #2 al wire would provide a voltage drop of 1.83% but if you want to get crazy and have 100 amps available now you are up to 1.73% voltage drop with 1/0 wire. South wire has a nice voltage drop calculator. The only part not intuitive about this is the parallel wire sets change from 0 to 1 (you have to have 1/0 or larger to parallel)

  • You know, I don't even check the voltage drop calcs until at least 150'. I used to check at 120', but then I realized the whole 125% thing and a 100A panel shouldn't really pull more than 80A if it's provisioned right. Commented Aug 20, 2019 at 0:30
  • I would agree with you 100% but my AHJ has pulled that card because the plans were not specified or no load data was provided, the same AHJ has approved a 100 amp panel with 10/3 w-g for the same reason we always say go big with the sub or go home, you can always up size the wire but it has to be based on load calculations that are smaller.
    – Ed Beal
    Commented Aug 20, 2019 at 0:47
  • I looked at Southwire calculator and wondered how accurate it was as when I used the maximum distance calculator and input 1 AWG AL and get a warning that 1 AWG will not carry 100 AMPs but when you use the Minimum conductor it says 1 AWG will work Commented Aug 20, 2019 at 1:55
  • @PeteDonaldson It depends if you're able to pull out of the 60C or 75C column. You don't get a free upgrade to the 75C column until you're above 100A. Commented Aug 20, 2019 at 2:28
  • @Harper -- or if you're landing on 60C/75C terminations at both ends (generally true for feeders, but not for branch circuits) Commented Aug 20, 2019 at 2:52

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