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I am hoping ThreePhaseEel can chime in!

I am looking far ahead and planning the electrical for a home I am building. From left to right I have an electrical pole - 100ft then a garage - then another 30ft to the home. The POCO wants to put the meter in the Garage to avoid the need for a transformer upgrade. This is a 400amp service. My load calcs require ~300ish amps at the house, ~100amp at the garage, and 80A of solar will be on the garage roof.

The question: The NEC 2017 allows each building at a residential home to have one service conductor. How would I split the service conductor at the garage to go to the home and still feed a panel at the garage?

My plan is after the meter at the garage (Eaton 324C) to use a tap box and split the 400amp service - one set to the home and then the other to the garage.

  • At the garage: post tap box put a 100Amp service disconnect. then an interior subpanel
  • At the house: when the wires land at the house put a fused 400amp disconnect and feed 2 200amp sub-panels
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    @manassehkatz-Moving2Codidact its a lot but a big house all electric. Im really trying to get 400Amps over because that is typical. 2 AC,s with electric heaters, car charger, double oven, hot water heater, induction stove, dryer, heated floors.
    – Ryan Pond
    Oct 6, 2022 at 3:55
  • Who do you have for an electric utility? Oct 6, 2022 at 11:48
  • @ThreePhaseEel SCE in socal, nec2017
    – Ryan Pond
    Oct 6, 2022 at 15:21

2 Answers 2

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SCE may not let you cheap out on this

While NEC 230.40 Ex 3 would permit you to hang a socket at the garage and have separate service entrance lines from that socket to the garage and the house, that may not fly under the SCE ESR-5 section 3.0 and 3.1 rules on meter switches:

3.0 Meter Switch

For each and every meter, the contractor shall furnish and install a switch, or other approved disconnecting means with overcurrent protection. This is referred to as the “Meter Switch." It shall be installed at the same location and directly adjacent to the metering and service equipment. The meter switch shall control all of, and only, the energy registered by that meter. Where permitted by local code or ordinances, the meter switch may consist of a group of switches or breakers per National Electric Code (NEC) 230-71. A separate meter switch will be required for each separate service of any group where the loads are totalized by a single meter.

3.1 Number of Switches

Every meter or service shall be furnished with a meter switch as described in Section 3.0. The NEC allows up to a maximum of six switches or disconnects to constitute the main overcurrent device for a single commercial or industrial service. A residential service may have a maximum of two switches or disconnects for each single-family service.

As a result of that, we'll have to start with something SCE will definitively let you install, even if it is a vastly more expensive setup than what you were originally contemplating. We're also going to assume from your initial choice of meter socket that this is an underground service dip from the pole; if it's an overhead service instead, then you will need a unit that accepts an overhead conversion gutter, such as an Eaton HP40 or HP402442, or a Square-D CUM400CB.

The first component we'll need is a 400A, EUSERC-compliant, single disconnect meter main. This is a very expensive piece of equipment (as in several thousand $), but it is quite important in that it's the only way we can provide 300A to the house while meeting SCE's rules about meter switches and the NEC restrictions on multiple feeders to a single structure.

Once we have that, our path then is determined by whether the meter-main you selected has a loadcenter interior in it or not. If it does, then we can simply get two 100A breakers for it (or a 125A and a 100A, if your garage feeder turns out to be bigger than 100A) along with a matching pair of 225A subfeed blocks to provide an effective 400A tap from the panel busses to the house. In this configuration, the garage panel is a 200A main lug (or main breaker converted to main lug) unit, configured as a subpanel, with a 100-125A feed from the main panel provided to it, and the solar breaker lives directly in the main panel.

If it doesn't, then we'll need either a three-pole power distribution block (UL1953 listed) with 2 250kcmil line and at least 3 250kcmil load ports or a 400A 120/240V tap (terminal) box and a conduit nipple + fittings, along with more 250kcmil Al to jumper between the load lugs of the main breaker and the tap box or PDB, to tap the feeder to provide power to the garage. Since we are invoking the 10' feeder tap rule in NEC 240.21(B)(1) for the garage feeder in this case, we must run said feeder in conduit (using 250kcmil Al conductors and a 4AWG Al ground) instead of using a SE cable, and position the garage panel so that it is close enough to the main panel for said 10' rule to apply. Furthermore, we must use a 200A main breaker panel for the garage panel here, also due to the tap rule coming into play. This additionally means that the solar breaker must live in the garage panel.

Either way, from the garage to the house, we'll need to lay a 3" PVC conduit with 6 250kcmil wires (a pair for each hot and a pair for the neutral) and a 1AWG Al ground in it. At the house, we'll need a 400A, 120/240V, NEMA 3R tap/terminal box to combine the parallel conductors along with 2 200A main breaker panels, configured as subpanels. Furthermore, since we're again invoking the aforementioned 10' feeder tap rule for the connection from the tap box to the house subpanels, we again must run the tap connections to the breaker panels in conduit, and position the house panels accordingly.

Note that neither of these setups requires an emergency disconnect at the house since it's being fed from service equipment located elsewhere on the property. Grounding is not hard, either; the garage's ground electrode is wired to the main panel in this configuration, while the house's grounding electrode has a conductor run from it to the Intersystem Bonding Termination device at the house, which then has 4AWG tap conductors run from it to each of the two house panels.

Don't get gassed!

The LA Basin is an oil and gas producing area (still!), and one of the drawbacks of that is that you need to be attentive to the possibility that methane gas may try to seep into your electrical system via underground conduit runs, with obviously bad results. The normal solution is to use extended range explosionproof Y seals (Eaton/Crouse-Hinds EYSX) in between your terminal adapter/expansion joint threads and the panels or tap boxes themselves, so that these metal fittings get grounded correctly. Note that they need to be filled with the appropriate sealing compound to do their job!

Don't let your electrical system give you the loose lugnut, either!

If you're planning to install this yourself, note that you'll need an inch-pound torque wrench to meet the NEC 110.14(D) terminal torque requirements. This is a good idea even if your jurisdiction doesn't enforce this rule, since you really don't want your electrical system to do to you what Greg Biffle's loose lugnuts did to him!

If SCE wasn't so picky...

If SCE lets you apply NEC 230.40 Exception 3, then things become simpler. The meter box gets replaced with a Milbank U5056-O fitted with a K5049 triple tap kit, and the various 200A panels are fitted out as main panels instead of subpanels; also, there is no need for equipment grounding conductor runs from the meter to the panels, as they are all main panels now. However, this does introduce the need for an emergency disconnect at the house instead of a simple tap box. While one could use a fused disconnect for this, given the cost of dummy fuses these days, it's cheaper to use a non-fusible disconnect switch. That said, 400A NEMA 3R non-fusible disconnects are not cheap or easy to find any more -- the Square-D HU265R is the only one that I can get a reasonable in-stock price for, and that requires you to fit a H600SN neutral kit to it for your application.

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  • @TheePhaseEel based on your comment, and to avoid buying an expensive 400amp meter main. I think I can use a Siemens HF225NA beside a 324N meter box. Then a tap box at the garage to split out 600kcmil into 2 branches - 100amps to garage and 400amp kcmil to the house. Then at the house another tap box to split that for 2x 200amp panels. I’m worried about tap of a tap rule for this config. That’s violated correct? So then I would not tap the garage 100amps. I’d tap that at the house and come back to the garage with it.. correct?
    – Ryan Pond
    Oct 10, 2022 at 19:17
  • @RyanPond -- issue #1 is that the HF225NA is NEMA 1 (indoor only) not NEMA 3R (outdoor/rainproof) -- you need a HF225NRA instead. As to whether you're violating tapping a tap with your proposed arrangement...that's much harder to tell. I'd lean towards "no" since all the taps are coming off the full ampacity of the feeder, instead of being a "subset of a subset" if you will. Also note that modern fusible hardware can draw a :( from an insurance adjuster or home inspector who doesn't know how fuses have changed over the years... Oct 10, 2022 at 23:20
  • thanks for catching that. I was teaching myself more too. The fact that a tap is defined as an undersized conductor made me feel confident it’s not a tap as defined by nec. I have one last question. Pushing the solar back on the garage feeder or sub panel. I worry the feeder conductors can see 400amps + 100amps (80*1.25) backfed from solar = 500amps. What do you see as an option? I think a line side tap makes the most sense? Ideally I’d like to keep the solar after the ocpd for future transfer switch and utilizing solar as backup generation.
    – Ryan Pond
    Oct 11, 2022 at 4:12
  • it wouldn't let me edit my question but I wanted to add -- by design I think its ok to push the solar back through the 600kcmil conductors only because the total OCPD protection is 400amps (200*2) from the other panels. So there cant be any power overdrawn.. would the NEC/AHJ accept that?
    – Ryan Pond
    Oct 11, 2022 at 4:46
  • @RyanPond -- yeah, there is the question of the math for the solar feed...I'll have to get back to you on that one Oct 11, 2022 at 11:46
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See NEC section 240.21(B)

240.21(B)(2) So long as it less than 35' to the house panel you can use 300A cables from a fused 400A disconnect next to the meter. else use 400A cables

So long as it's less than 10' to the garage panel you can use 100A cables 240.21(B)(1) from the same 400A disconnect else you'll need 134A cable assuming less than 35' 240.21(B)(2).

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  • Thanks @Jasen. A detail I am struggling with - when the 400amp feeder lands at the house. Do I need an exterior disconnect at the house? Or can I use a tap box and go directly to 2 200Amp panels?
    – Ryan Pond
    Oct 6, 2022 at 4:37
  • If there's an external disconnect at the meter/garage that shuts everything down, you don't need another at the house, AIUI
    – Ecnerwal
    Oct 6, 2022 at 13:48
  • @Ecnerwal at that point am I not violating multiple feeders when I tap and split the 400amps at the house? At the garage, after the meter, where do I tap to keep my house supply a service conductor. And if I do that do I have to have a disconnect at the house?
    – Ryan Pond
    Oct 6, 2022 at 14:57
  • Jasen's approach sounds like one 400 A service disconnect at the garage, with the feeder being premises wiring. That is obviously the straightforward approach. It sounds like Ryan wants to utilize 230.40, Exception No. 3, in which each building effectively has a separate service that comes from the single meter. Even if that is legal, the utility may not like it. They generally get a veto on things that happen before the service disconnect (and things after it that could backfeed). Oct 6, 2022 at 15:10
  • @KevinCathcart my goal is to get get 2, 200amp subs at the house because 400amp panels are expensive and hard to find. Jansens solution is good but how about the next step about landing at the house with 2, 200amps.
    – Ryan Pond
    Oct 6, 2022 at 15:20

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