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Overview: My question ultimately boils down to figuring out how many wires I can properly stuff down an existing conduit.

There are many questions pertaining to wiring and grounding sub-panels, but I'm not sure if the addition of the metering aspect makes it trickier or not.

Backstory: I am upgrading my service to 200 amp. Down the road, I may get an Electric Vehicle, and our utility has a special pricing if you charge during certain hours of the day, and in order to be on that program, you need a separate meter for that. So, when I replace the existing meter and breaker panel, I want to make sure I've set myself for success if/when I add the new meter down the road.

I currently have a subpanel in my detached garage. If I was to do this specialized meter, that panel would become a main panel fed by the EV meter. However, I have a light on the garage, that is switched from within the house.

The following is an illustration of how I think the system should be setup. What I am uncertain about, is connections of the grounding between each meter system. From my reading, what I have drawn is legit, and that it's fine to bond the panel grounds together even when on different meters. Others online have said that each 'service', the House and EV meters, should have their own independent grounding wiring systems, meaning 2 connections to the water meter, and 4 ground rods by the house. Personally that seems nonsensical, but I want to be sure. The main area that result in a problem is filling the conduit.

When charging an EV, I'd want to pump as much juice out to the EV panel as practical. If I do indeed have to run two 8ga ground wires in the conduit, I may be only able to do 50-60 amp continuously, instead of 75 or 100 as I'd like. 1-1/4 conduit isn't huge, and with 6 current carrying conductors in there I have to derate the capacity by 80 percent. With 3 14g wires, 3 2g wires, and a 6g wire I'm getting pretty close to the 40% max fill limit.* When all the temp and fill deratings are computed I might have to downsize my EV conductors.

Diagram of expected wiring

So what is it? Can ground be shared by everything, or do I need to have multiple grounding systems for each metered system?

*I am looking into a smaller neutral wire, then I'd be able to put two 1g wires in for 110 nominal amps, but haven't done the math yet on whether that would be up to code, but in that scenario I'd be a razor thin amount under the 40% fill rule.

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Use bare copper ground wires.

Grounds don't need insulation, so don't waste the fill.

Consider using copper feeder.

Copper? For feeder!!?? Weird, I know. Copper is expensive and a poor conductor by most measures. But when conduit fill is the scarce commodity, volume is the measure that counts - and by that measure, copper is the best conductor on earth by volume because it is so very dense. (well silver is like 2% better, a negligible gain for extreme cost, so not worth mentioning, just as we don't mention sodium is 5% better than aluminum and also explodes).

For thermal derate, only count wires that count.

For fill, all wires count. But thermally...

If 2 wires together, by design, cannot flow more ampacity than 1 wire is rated for, they count as 1 wire:

  • You're not allowed to intentionally use ground for current, so grounds don't count.

  • In split-phase or MWBC, neutral only handles differential current. Neutral current + the lesser hot conductor must = the greater hot conductor. So neutral doesn't count in these.

    • Unless it's a "2 hots out of 3, + neutral" circuit in 3-phase power. There, the neutral has some current with 2 hots maxed, so neutral counts there.
  • The 2 travelers together, only 1 is hot at a time, so the 2 count as 1.

In almost all cases, this boils down to 2 wires per circuit.

Derate off the highest the wire can run.

There's a table, 310.15(B)(16), which gives wire ampacities based on thermal ratings. Different wires have different ratings, and so do terminations.

  • Say you come off a 75C breaker using 90C wire to a 60C NEMA socket. What ampacity can you breaker for? The 60C number, because the socket is the limiting factor.
  • But when you're doing conduit fill derate, what ampacity do you derate off of? 60C? No, the socket is not inside the conduit. 75C? Ditto. You derate off 90C.

So let's say you have #3 Cu THWN with 6 conductors that count. 80% derate off 115A (90C) = 92A derate. Round up to the next size breaker, that's 100A breaker. Suppose your EVSE is 79 amps, 92A can definitely carry 79A. You must derate the EVSE 125% so 98.75A, and boom! 100A breaker. Everything is happy.

Don't run neutral to things that don't need neutral

EVSE's don't need neutral. At least if an EVSE needs neutral, don't buy it. Because that's just super lazy on their part, they're saving 50 cents buying a 120V-only controls power supply instead of a 120+240V one.

Garage lights also don't need 120+240V split-phase. If you really have a 2880 watt light, run 240V only and evict the neutral from the pipe.

"Oh, but I'll have a subpanel there" -- then you need to use good "design language" to communicate to subsequent morons that there is no neutral here. First, add an accessory ground bar to the subpanel, and use it for grounds and not the factory supplied neutral bar. Second, I would make a sign out of heavy cardboard and cut it with little comb fingers that can go into the neutral bar, and the sign says "No 120V loads! Neutral Not Available In This Panel" or something like that. For bonus points, remove all the lug bolts from the neutral bar. You've done your part. If someone insists on sticking a 120V load in that panel, not your problem.

What's the "main" panel doing so far from its meter?

I am wiggy about the idea of coming off the meter with a long, unfused run to the EVSE main panel in the garage. I can't cite chapter and verse but I bet that is not Code. Putting the EVSE main panel at the meter will cause this run to become a branch circuit and not a service, and the no-neutral 240V makes more sense. At that point you don't go to any panel, you simply go to the EVSE directly. (unless you want to stop at a fat surge suppressor first).

I am also uncomfortable mixing service 1's service wires with service 2's branch circuit wires. That seems like it may be a codevio, but again I can't quite chapter and verse.

Multiple services can use the same Grounding Electrode System.

I have 2 services sitting right next to each other that grab the same Ufer ground. Their EMT conduit web connects not only to each other but to the 480V service as well. And they're on a metal building. I couldn't separate them without really trying, and if I tried, all I'd do is create shock risk across the insulating pads.

Dump the lighting branch. Power it out of the existing garage sub.

That little light switch is a lot of trouble. You need 3 wires (2 travelers 1 return) to power that thing. But that's what it takes before smart switches came along.

So convert the garage light to a smart switch (out at the garage). Use a partner smart switch inside the house that uses either wireless or powerline signaling (make sure it's fed by the same pole in your panel).

If that doesn't work, go to a "Plan B". Replace the three #14s with three #18s, and control the light from a GE RR7 relay in the garage. Now you can wire it in the conduit in #18 wire (though still NM-B outside the conduit). The switches get replaced with momentary-on switches. Pushing up throws the relay "on". Pushing down throws the relay "off". Do not use standard 3-ways, which are on 24x7 and will overheat the relay.

Here's the trick. This is low voltage (24V) wiring. You're not allowed to put low-voltage wiring in the same pipe with mains wiring unless all of the low-voltage wiring is contained within Class I wiring methods rated for 120/240V. That means you can throw #18's in the conduit, but you can't run 18/3 thermostat cable outside the conduit to the switches - for that you must switch to 14/3 NM-B, which is a Class I wiring method.

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  • Having the EV outlets as just outlets, fed by a panel by the meter, and not running a neutral for them at all, is an option I had not considered. Removing that neutral may open up enough space to have 100A to EV chargers, allowing me to power the subpanel with maybe up to 50A for other stuff, an attractive option. I'll have to check the math but I am quite interested in the idea. – whatsisname May 8 '20 at 18:31
  • The smart switch option is a good idea as well, it's something I thought "there must be something there" but the specifics of the low voltage stuff is an area I'm unfamiliar with. Might be good idea to do both the smart switch and the "240 only outlet" concept together for maximum juice. – whatsisname May 8 '20 at 18:35
  • Your codevio spidey sense re: the commingled conductors is dead on the money BTW (230.7 is the Code section you were looking for, but couldn't find) – ThreePhaseEel May 9 '20 at 20:32
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One mast, no duplex box, two meter-mains, no problem

The primary problem with your proposal as it stands in your post is that NEC 230.7 prohibits service and non-service conductors from being commingled in the same raceway or cable:

230.7 Other Conductors in Raceway or Cable. Conductors other than service conductors shall not be installed in the same service raceway or service cable in which the service conductors are installed.

As a result of this, you'll need to provide a service disconnect at the meter for the EV feed. The simplest way to do this is to make it a meter-main as well; although one could use a separate breaker enclosure for that, that would just take up space and create more issues with service entrance bonding and grounding. However, since you don't want to use a "duplex box" type multi-meter packaged assembly, this means we'll need two separate meter mains, and a way to make the tap connecting the two. Normally, Xcel does this via separate service masts fed from the same service drop using overhead taps; however, we can use slightly fatter conductors to accomplish this with a single mast and the correct metering parts.

In particular, since your utility accepts Milbank metering hardware, we can assemble things so that the service-entrance conductors feed the house meter-main, then are tapped, either now or at a future time, to feed the EV meter-main from below, as if it were an underground service, via a conduit off the bottom left-hand side of the house meter-main, where it ties into the house meter-main's utility-side underground service gutter.

This means you'll need a Milbank U5871-XL-200-5T9 for your meter-main instead of the Eaton MBX816B200BTS your electrician originally suggested, with a set of Milbank K4977-EXT tap lugs slid into the meter-main's line positions and an A7517 2" hub. This gives us the ability to accept SECs up to 350kcmil, with a maximum of 1/0 for the service tap to the EV meter. It also provides 8 breaker spaces that can use basically any 1" breaker you can find on the shelf, and a set of 200A feed-through lugs for the feed out to the house distribution panel.

With this, we use a Siemens MM0202L1200JLX for the EV meter-main, with a field-fitted Q280H for the main breaker. This is then fed from below using a 1.25" LFNC from the bottom right corner of the house meter main, curving upward to go into the meter compartment of the EV meter main from below while passing over the 1" PVC riser from the T-body into the EV meter main. Furthermore, we can fit the tap lugs for the EV meter now, but leave the EV meter-main and service conduit unmounted for the time being.

Physically speaking, this puts the EV meter-main on the left and the house meter-main on the right, mounted so that the EV meter-main's customer-side bottom KO aligns with the existing 1" PVC and the meters are at the same height, somewhere between 4' and 6' off the ground to the center of the meter. A 2" RMC mast with 350kcmil Al XHHW-2 wires in it is used for the main service-entrance conductors, going down directly into the top of the house meter-main via its hub fitting From there, 4AWG copper THHN wires (two hots and a neutral) are run down through the house meter-main's underground service gutter and through the 1.25" LFNC for the service tap from the house meter main to the EV meter main. Finally, the grounding electrode conductor lands in the house meter-main, as depicted in your picture.

Sidebar: if you really want to use somebody else's meter main

If your electrician is dead-set on using the Eaton MBX816B200BTS, the concept proposed above isn't completely nullified, thankfully. Provided the utility is OK with it, one can use a pair of insulation piercing tap connectors (Ilsco IPC-350-4/0 or equivalent) to tap the hot conductors in the meter-main, then use the utility-side grounding lug on the meter-main to land the tap neutral.

Now that we are properly served...

Now that we have the service-entrance hardware sorted out, we can tackle the feeder conduit to the garage. The existing conduit gets routed through a T body and some more 1" Schedule 80 PVC up to the EV meter main, past the right side of the house meter main. You'll need a set of raintight reducing washers (Bridgeport 1073-RT or equivalent) to go from the 1.5" or 2" provided by the knockout on the bottom right of the EV meter main to the 1" of your conduit, in addition to the obligatory PVC male adapter, by the way.

With that out of the way, we move onward to the garage end, where a 100A, two pole, NEMA 1 (indoor), unfused safety switch gets fitted next to the existing panel, connected by a 1" rigid nipple. This will provide the required disconnecting means for the EV charger branch circuit, while the panel main breaker does the same for the feeder.

Once this is done, we remove the old wires from the conduit between the house and the garage, unwiring the outdoor light switch in the house at where it ties into the conduit run. We then ensure that the bonding screw in the garage subpanel has been removed at this point, in addition to making sure that the garage has its own ground rods, connected to the grounding bus at the garage subpanel with a minimum of 8AWG copper.

With that verified, we can pull the new wiring: two 4AWG copper THHNs for the EV charger, three 10AWG THHNs for the feeder to the garage subpanel, three 14AWG THHNs for the floodlight switch, and an 8AWG bare copper ground wire for all of this. The 10AWG and 14AWG wires get pulled through to their existing destination inside the house (where they transition out of the conduit, or into boxes), and then connect to the garage panel and garage floodlight circuit, the same way the existing feeder and floodlight wires were connected. The 4AWG copper THHNs and the bare 8AWG copper grounding wire terminate in the EV meter-main at the house, end, then, with the grounding wire landing in the grounding bar of the garage subpanel and the two 4AWG hot wires getting pulled through the garage panel and the nipple to reach the line lugs on the safety switch.

Last but not least, the existing grounding wire from the panel to the conduit entry point is connected to an insulated 10AWG grounding conductor run from the EV meter main, through the conduit body, to the conduit entry point inside the house. This is done this way to avoid sending any current on the EV meter-main's neutral out to the garage and back as objectionable grounding current or splicing in the new T conduit body (which may cause serious box fill problems even if the body is marked with its volume and thus legal to use as a housing for splices). Note that if objectionable grounding current does become a problem here, you are permitted by NEC 250.6 to disconnect and remove this insulated 10AWG wire and thus break the parallel path between the two bonded meter-main enclosures.

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