Background: I have a decent skill set; I've worked in commercial HVAC and facilities maintenance for over 20 years. I'm considered "electrically qualified" at my work and I have a state ticket as an apprentice electrician. I have found a master (with a licensed & bonded company) and a journeyman who are willing to pull permits and work with me, allowing me to do as much of the work myself as possible.

My home is not overly large, about 1200 sq ft with a 720 sq ft attached garage with a 2nd floor guest quarters above it. It was built in 1930 and moved to its present location in the mid-60s, after which the garage and apartment were built as an addition. The main service entrance (200A) is located at the L where the garage joins the home; the drop runs alongside the garage on 2 poles I am responsible to maintain and then turns 90 degrees to enter the main panel. The wiring runs so close to the house that it is impossible to clean or maintain gutters or paint fascia without touching the 240V lines.

The main panel is a modern GE board and most of the wiring was redone by the previous homeowner; he did the work himself but it looks as if he did it right. On the other hand, the garage and apartment are fed by two sub-panels which look as if they were installed by Dr. Frankenstein...there's not even a way to shut power off to them; they're wired direct to the main lugs of the service entrance.

I'm planning to develop a home-based internet business which will entail a server rack and 3000 VA (30 amp) UPS. Also, thinking ahead, there are other potential high-demand items that I may want to install, such as a swimming pool, woodworking shop, and (if she ever moves in with me) my mother's glass fusing kiln (which draws 46 amps at 240V). I could also use that last outlet for a welder or possibly for charging an electric car. I will also want, eventually, to install a whole-house generator to back up the online business.

Bottom line is: I want to upgrade the main service entrance to 400 amps and relocate it to the back corner of the garage where it can be fed from a direct drop from the utility pole. I want to put a new main panel in the garage which will service the circuits there and have a 200A feeder to the existing main panel. I want to make provisions for sub-panels for laundry equipment, the woodworking equipment and kiln, and the possible swimming pool which, along with the water heater and the A/C units for the home and apartment, could be managed by load control relays so that I can keep the size of the generator to 20KW/200A or so.

To place the new main panel in the most convenient location for future expansion I will need to bore through a number of 2x8 joists. That limits me to 2" conduit. After doing a "dream sheet" load analysis, I come up with a need for 400kcmil feeders, a #1 neutral, and a #3 ground. If I use IMC conduit and THHN wire it looks like that fill will be workable...just barely.

Anyhow, I will be discussing all this with my electricians, of course. But I want to have my ducks lined up before I shoot. Can anyone make any suggestions of things which should be considered now, before I start into this? As I said, I've worked in commercial facilities for quite some time but this is the first time I have addressed a project of this scope as a homeowner. Thanks for any help.

EDIT TO UPDATE: Been a long time since I've looked back in here; time to revisit with some updates. I still want to upgrade the service entrance but I'm not in a hurry. I have (with the help of an electrician) rerouted the feed to the two Frankenstein subpanels so that they come off an actual breaker at the service entrance; they're no longer tied direct to the main feed. These panels have the neutrals and grounds commingled which is, I've found, a no-no (although ca. 1965 maybe nobody cared), so they're not long for this world. I'm planning to replace both of them with new 125A GE panels (TLM1212CCUGK).

The plan now is that when I do upgrade the service, I'll get a 320 amp distribution panel and install it at the back of the garage. Preferably with underground conduit going back to the power pole like the neighbor behind me has instead of a "drop". I'll run conduits from the distribution panel to the existing main panel and the two subpanels I'm replacing, and simply install 125 amp breakers to feed each panel. When/if I ever get around to adding a swimming pool I'll put in a fourth branch panel for the pool & spa equipment, fed the same way.

The subpanels I'm replacing will be "convertible main breaker". I'm going to leave the designated main breaker slots unused; if at any time in the future I want to meter the house, apartment, and common areas (laundry, garage, etc) separately all I will need to do is put in the requisite meter cans at the new service entrance, install the main breaker conversion kits, and tie the feed wires in accordingly.

I know I'm moving slowly, but I've had a stretch of unemployment and my current job has been hit hard by the virus panic. Thanks to all of those who gave me ideas and good information.

  • In your third to last paragraph and in the front matter describing the other Frankenstein installs, what is your approach to rerun their main feeds to the new main garage box? You mentioned the 2 service poles, are you going to bury/reroute with new wire. Maybe add some pictures so you get some additional ideas?
    – noybman
    Sep 16, 2017 at 16:22
  • 1
    What state or country do you live in? Are you rural or more urban? Sep 16, 2017 at 17:22
  • 1
    The Frankenstein sub-panels are in the SW corner of the garage and are connected to the main panel by 1" EMT. The plan is to put the new service drop and 400A ATS at the outside NW corner of the garage. The new main panel will be placed very near the sub-panels it is replacing and the 200A feed will be new conduit back along the route of the existing sub feed. The two existing service poles will be removed. Maybe rather than pushing so much wire through a 2" conduit running through several joists it might be best to run a couple of smaller conduits and balance the load. Suggestions?
    – ehbowen
    Sep 16, 2017 at 17:29
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    Suburbs of Houston, Texas (within city limits).
    – ehbowen
    Sep 16, 2017 at 17:29
  • After doing some more online research, it looks to be difficult to purchase a 200A branch breaker unless you go to a commercial product...and I need arc fault protection since this is a residence. I'm thinking now that I'll put the 400A panel at the new service entrance (exterior) and run feeds direct from it to: Existing main-125A; Garage sub-125A; Water heater-50A; Laundry equip-50A; Future woodshop/kiln-100A; and future pool panel-100A. I think that, after I take the load off the current main panel, a 125A feed will be sufficient.
    – ehbowen
    Sep 16, 2017 at 18:09

2 Answers 2


How I'd do this

Article 220 is where I would start as well. Since I don't have your utility bills in front of me, I used the standard Part 3 procedure for ampacity calculation as that allows me to use assumed loads for the "three sisters" (water heater, dryer, range), vs the alternate procedure (220.82/83) which requires nameplate loadings for these appliances. Furthermore, I assumed that:

  • Your servers will not need dedicated cooling, and all server loads are on the 3000VA UPS.
  • The house has 4 kitchen branch circuits, while the apartment has 2.
  • 3 1500VA load allowances are sufficient for the garage shop atop the standard lighting load and the EV/kiln/welder receptacle, which is allotted a full 12KVA.
  • A pool heater is not needed (it's Houston, you probably want your pool water cool more than you want it warm) and a 1HP pump suffices for the pool.
  • A heat pump with a low ambient kit suffices for HVAC, and emergency heat is not needed (I used Trane XL16i 's as representative units, with 3tons for the house and 1.5tons for the apt. as "rule of thumb" sizes. A full manual J sizing would be needed for a precise load calculation here.)
  • Both the house and guest suite have loads alloted for them for electric hot water heating, an electric dryer, and an electric range not exceeding 12kW. (This treats the guest suite as a complete dwelling unit, which may be a bit generous in its facilities.)

With these assumptions in hand, we come out with 148A@240VAC for the house, 87A@240VAC for the garage/shop, and 84A@240VAC for the apartment. This allows us to do a fairly neatly balanced split, with a 200A feeder from the new service equipment to the house, and 100A feeders each for the garage and the apartment.

Running the feeders

While running 2" conduit for the feeders is a good idea, the 400kcmil wires are grossly oversize for a 200A feeder. Between 75°C terminations and the adjustment provisions in 310.15(B)(7), we can use 4/0 Al instead. Furthermore, XHHW-2 is the insulation of choice for such large wires (vs. THHN), and large aluminum conductors are compact stranded as a rule, which lowers conduit fill significantly. A single 4/0 Al compact stranded XHHW-2 only takes up just under 180mm2 of space, meaning that without a neutral downsize, the over 800mm2 a 2" conduit can provide in usable fill is more than plenty for the 540mm2 of fill the wires take up.

Likewise, we can use 2AWG Al XHHW-2 for the 100A feeders, although you might as well use 2" conduit there as well so that you only have to buy one size of conduit for your feeder runs.

400A service is hard...

As to the service equipment itself, this is where things get tricky. A typical solution at the 400A level splits the service equipment across two or three panels; however, this is probably not the greatest solution insofar as it requires all the panels to be in close proximity and also has no single point of shutoff. Another possibility would be to use a 400A meter main type device, but 400A meter mains are typically not used with overhead service conductors. While it'd be possible to use a couple of elbows (and a drainage hole) to fix that, your AHJ and/or utility may not approve of such.

A third option is to use a 400A loadcenter with a standard meter socket, but those are only available in large slot counts, which is actually somewhat inappropriate for the application at hand, and not everyone makes them. The fourth and final option, and one I'd be partial to if not for the high price tag, would be to use a 400A enclosed breaker as the service entrance device, with a distribution block in the enclosure (or associated gutter) for tapping the feeders to the various subpanels. This provides a single point of shutoff and minimizes the length of unprotected conductor, but is costly as 400A enclosed circuit breakers are commercial parts, basically. It also invokes the feeder tap rules, which limits the length of the runs from the main to the subs to 25' unless they are run on the outside of the building.

...but not impossible

Thankfully, it turns out, Square D's meter-mains can be used with an overhead gutter accessory (the OCK400) -- this allows an underground-only meter-main/CSED to be converted to work with overhead service. Furthermore, the configuration of the Square D QU12L400CL is ideal for this application -- the 200A factory main can be used to feed the house subpanel, while the other two feeders can be provisioned using QO2100 breakers in the side-panel section. Of course, this meter-main becomes your service equipment, with 4-wire feeders to all subpanels (or feeders in metal conduit).

As to the subpanels themselves

I would go with 42-space panels each for the garage and apartment, and one of two configurations for the house:

  • If you value having everything (that isn't on the generator) in a single panel, I would practically require getting a 54 or 60 space, 200A panel for the house, as a 42 slot panel sounds rather...tight, given the amount of electrical stuff roaming around your house.
  • If you don't mind having two panels for the house, then instead of a traditional main-panel/subpanel setup, I'd use two 42 slot/200A panels set up in a daisy-chain using a set of subfeed lugs in the first panel to feed the main lugs (or a backfed set of subfeed lugs, which may be preferred for wire-bending reasons) in the second panel. This gives you a minimum of 74 slots spread across the two panels, while allowing to use identical panels for the house, garage, and apartment/guest quarters.

Also, depending on how many outdoor loads you have, you may wish to have an extra panel for the garage that is dedicated to those. In that case, I'd use a 200A, 8/16 space "mobile home" type loadcenter with factory-fitted feed-through lugs that has been coverted to main lugs from a main breaker configuration (or is factory fitted with main lugs), connect it in line with the garage feeder (the main lugs connect to the feeder to the meter-main, while the feed-through lugs connect to the feeder off to the garage), and mount it to the outside of the house in a convenient place. That provides a nice place for circuits to outside loads to go (such as the pool pump, or outside lights and receptacles).

Backup power complications

I would not use a 400A ATS and try to jigger a zillion load relays here -- that sort of complicated load-shedding orchestration is not a particularly good idea from a "number of things that can break" standpoint. Instead, I'd split the standby loads out into a subpanel of their own, with a smaller ATS feeding that subpanel in addition to a feeder from either the meter-main or the house panel. If you desire standby loads in the garage or guest quarters, then you can have an additional ATS tapped from the generator feeder conductors feeding a second standby subpanel.

If you want the apartment separately metered...

Some AHJs or electric utilities may insist on separate metering for the apartment here, which complicates matters as most gang-metering setups are designed for a relatively symmetric split between the units, rather than the 300A for the house+garage+pool and the 100A split meter for the apartment. This means that you will need to have a utility sealable trough that connects to a 400A meter-main for the house/garage and a separate 100A or 125A meter-main for the apartment using conduit nipples; this does obviate the need for the OCK400 to bring the overhead feed down to where the meter-main can get at it, though.

Your choice of meter-mains on the 400A side in this case will depend on if you want to have the house use a 200A panel (or panels) with a parallel 200A panel in case more ampacity is needed, or if you wish to use a single 400A panelboard/loadcenter. In the former case, the same QU12L400CL from above will do the trick -- the 2 200A breaker positions provide parallel feeders to the house, while a QO2100 or QO2125 protects the garage feeder, and the remaining 125A position is left open to provide expansion space.

In the latter case, though, things get interesting. The meter-main becomes a Square-D QUM400CL with a BMK2Q400 mounting kit and a single QDL22100 or QDL22125 branch breaker fitted; however, the jumpers in the BMK2Q400 are not used to feed the branch breakers, but 6-wire, double-entry, Al7Cu mechanical splice connectors rated for 250kcmil wire -- one connector for each leg, with both jumpers going into the connector. From there, 3/0 copper pigtails are led off to the line lugs on the branch breaker, forming a feeder tap under 240.21(B)(1) rules. The feeder wires, which are 250kcmil aluminum with 2 wires in parallel for each leg and the neutral (this is fat enough that 310.10(H) permits parallel wires), also go to the splice connectors, one pair of hots on each connector, with the neutrals landing on the center set of neutral terminations. This fat feeder requires a 1AWG aluminum EGC (or metal conduit instead), and all this wire is wrapped up in a whopping 3" conduit. You'll also need to use a 400A loadcenter/panelboard or paired 200A loadcenters here, with the latter subjecting you to the feeder tap rules again (25' tap rule, 240.21(B)(2)).

Once that is done, the garage feeder can be run in a more normal fashion, landing its hots on the load lugs of the meter-main's branch breaker. The apartment meter-main can be any suitable 125A class unit with a 100A main breaker, and the apartment and garage panels and feeders are as above, or as suited if you decide to bump them to 125A in this case.


Sorry for the delay but since I asked a question I felt I should respond with an answer. The largest meter base you can install for a single family residence is 320A single phase base. Unless you get into special exceptions. In that case you need to work with a good contractor or engineer. Most contractors will install a minimum two 200A MCB panels to that meter. You will not have to size the wire to the meter as the Utility Provider will put in what they want.

Now NEC Article 220 covers calculations for sizing electrical services, because, especially in residential, it is not concerned so much what your connected load is but what is the actual demand. Since you already have the house built you can forego the calculations for building a new one and use the information the utility provider can give you. It's probably already on the invoices you receive from them. You need the peak winter and summer loads for the last year. If it's not on your invoice you can call them and get it from them. You can consider the existing load to be the highest peak load plus an extra 25% of the peak load. By the way all of this is in KW.

Now that you have a working load for the existing. You can work with the new loads you are installing. Take into consideration how many machines will possibly be running at one time. If you are installing servers you may have to upgrade some cooling for them. If it's just 3000VA then you probably don't need to upgrade.

This is where a good contractor can actually be saving you some money. Rather than DIY installing to a connected load (very expensive). They can help you determine demand load and reduce the material and labor into a realistic budget.

Hope this helps, good luck.

  • It does help. Frankly, I thought that the 400 amp calculation was overkill, especially since a lot of those loads would not be used often. As an example, I wanted to install outlets for electric clothes dryers "just in case"...but currently I use a gas dryer and have no plans to switch. The whole idea of the thought exercise now, though, is to do it right the first time...I don't want to have to do it over in future because I cut corners now!
    – ehbowen
    Sep 20, 2017 at 3:39
  • Where is that 320a rule is it a state thing? I had a true 400 amp panel at my last home and just installed one on a residence my company owns. This is a True 400 CT based feed for the meter because to go to a true 400 the meter needs to be a CT type. Straight snap in 30/40 meters are as you say not continuous 400 but CT can be and I know of more so where is this rule?
    – Ed Beal
    Mar 9, 2018 at 0:36
  • A 320A base is what is used on 400A service -- the discrepancy is one of rated vs continuous currents, and has to do with breakers requiring that 80% continuous load derate while a meter pan is rated for a given continuous current. Apr 19, 2018 at 0:17
  • @EdBeal and others: Yes in Texas the largest Residential meter you can install and remain within the residential classification is a 320A, because in Texas we are not a public utility but use private providers. This does not mean we can't install a larger service at a residence but the meter with a ct will be classified as a commercial service. Meaning you will be charged at a different rate. This includes penalties on your winter and summer peak usages and they are ridiculous. You might be better off setting two meters but you may need to divide up your billing into two addresses. Apr 19, 2018 at 20:57

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