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I am working on adding a sub-panel to my main panel. I have 200 amp service, and I am putting in a 175 amp subpanel (using this: HOM2175BB) and running 4/0-4/0-4/0-2/0 AL SER approximately 60 feet (including all routing up and around) to the other side of my basement into a HOM4080M200PCVP panel. This might be overkill, but I'll be installing a 48amp EVSE (60 amp breaker) as well as adding a Heat Pump Hot Water Heater (30 amp breaker) and at least four 20 amp branch circuits soon and then in the future anticipate adding another EVSE (all right near the location of the new subpanel), so I wanted to have the flexibility to have close to as many amps as possible in this second subpanel location. Plus, if I ever decide that I need to upgrade to 400 amp service, hopefully, I can just reroute the extra 4/0 SER cable up to my meter and feed both 200 amp panels from there with minimal extra wiring.

Anyway, so to my question. The photo below is my current main panel (which was already so full that I had to add that subpanel to it just to free up a few spaces). The HOM2175BB has to be wired at an angle (see second picture below), so it will be installed in the bottom right side of the panel and come out the bottom of the panel. My plan is to route the 4/0 SER along the grey line I've drawn in the picture below out and around the panels, up the wall and around the top to then go across and be routed the rest of the way around to the other side of the basement.

Does this SER wire need to be protected (in this case I would assume run through something like 2" pvc conduit) for any of the run that is below 8 feet or below the panel or below the joists (which are just out of the picture at the top)?

Everything I've read in various code sections seems to just say "where subject to damage," but I can't tell if this area in the unfinished basement is subject to damage or not. Any help would be greatly appreciated. enter image description here enter image description here

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Yes, it needs to be protected. But that does not have to be conduit. With conduit you have to feed it through. My electrician struggled just to bend that stuff a bit getting it through the wall from the meter to the top of the panel. Looping it around like that and through a conduit? Crazy. But you can protect by running it and attaching it to the wall and then put some plywood in front of it. I think that would be a lot easier.

Also keep in mind that you are way overdoing this. You could probably do just fine with 90A and 2 AWG AL - much smaller and cheaper. That could be run as cable (protect the same way with plywood, though with much smaller cable, conduit is a more realistic solution than with the big stuff) or run as individual conductors (but then you need conduit the whole distance). The key is the EVSE. 48A/60A is huge. You could probably get away with 90A if you went with:

  • 30A 240V - Heat pump
  • 20A x 2 120V - Tools/etc. - you could actually, typically, put in at least 4 circuits (but figure out a useful combination of which circuits (for big tools) on each leg so things are balanced) and still call it ~ 20A.
  • 40A 240V - EVSE

40A at 240V is plenty for most people. Take a look at the Technology Connections EV video for all the details. But 40A should be plenty for most people, most cars, most of the time. And if you have two EVs, unless both are used > 100 miles per day on a regular basis, 40A is still enough and the cars/EVSEs will sort it all out automagically!.

If you really think you may need more some day, then use individual 2 AWG AL wires now but instead of using 1-1/4" PVC, which would work for now, go with 2" PVC and you will be OK all the way up to 4/0. Or you could go with 1" metal (smaller than the PVC because you don't need ground) or future-proof with 2" metal.

Wrestling 4/0 cable...not fun.

Also, have you done a load calculation to see whether you can add any or all of this? Do that before deciding how much EV you can handle.

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    For the very similar subpanel install in my house, the electrician used flexible metal conduit. It was an absolute wrestle getting it run. They sized it to barely fit over the SER, which I believe is a code violation but passed inspection just fine. I agree, this is wild overkill going with 175A. I did 125A and it's been more than enough for sensible levels of charging plus the workshop and the heat pump.
    – KMJ
    Commented Dec 22, 2022 at 18:33
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    Thank you for the reply! You make a good point about it being a pain to put it in conduit. The plywood method seems like it would be much easier. That said, if I ended up with the 2-2-2-4 wire that would be (a little) easier to route through conduit, would I just need to use conduit from the panel until the top of my picture above? Once the conduit reached the joists, can the SER wire exit the conduit (perhaps with a cable clamp at the end of the conduit?) since it is now in an area that is no longer subject to damage? Or if you start at the panel with conduit do you have to go the whole way?
    – will74
    Commented Dec 28, 2022 at 15:58
  • Conduit for a cable is protection only, so you only need to use it (or something) where protection is required. Once it is out of damage range (8' ?) you can either run it through the joists or, as I understand from another recent question, attached to the bottom of the joists (because it is 6 AWG or larger wires). If you use individual wires then they need to be in conduit the whole way. Commented Dec 28, 2022 at 16:02
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"Where subject to damage" is a judgment call

Everything I've read in various code sections seems to just say "where subject to damage," but I can't tell if this area in the unfinished basement is subject to damage or not.

Yeah, that will vary by the particular viewpoint of the AHJ. However if physical protection is required, you don't need to use electrical conduit, but if you do, make sure it actually provides the physical protection. Some types are not adequate. Nobody ever got red-flagged for using EMT.

You don't need a breaker actually

I am working on adding a sub-panel to my main panel. I have 200 amp service, and I am putting in a 175 amp subpanel (using this: HOM2175BB) and running 4/0-4/0-4/0-2/0 AL SER

NEC 310.15(B)(7) says no residential feeder ever needs to be larger than 83% of the SERVICE size. Your service size is 200A. 83% of that is 166A. 4/0AL is 180A wire.

That means 4/0 is adequate for your entire 200A service as well as ANY feeder to any panel running at any ampacity up to service size (200A).

Since you already have a 200A main breaker, you don't need another one. You can simply use a "Subfeed Lug Kit" (a breaker with no breaker) to tap the main panel.

Plus, if I ever decide that I need to upgrade to 400 amp service, hopefully, I can just reroute the extra 4/0 SER cable up to my meter and feed both 200 amp panels from there with minimal extra wiring.

Well, in that case, the 4/0 wire would not be sufficient. Remember your favorable derate comes off 83% of the service size, which will be 332A on a 400A service. Suddenly 4/0 wire is only good for 180A anymore. Easy answer: use 250 kcmil wire here.

to the other side of my basement into a HOM4080M200PCVP panel. This might be overkill,

No, not at all. We wish more people would get 40-space/200A subpanels.

Your new loads need to work in your Load Calculation

You need to follow NEC Article 220 and do a Load Calculation for the current and planned loads on your panel. Not some other method :)

Your service must be sufficient for the loads to be served, which is to say you're not allowed to overload the service based on the Article 220 Load Calc.

You need to do a Load Calc for each panel, but more importantly you need to do it for the whole service. I am concerned you are piling a lot of new load on, the load may not fit given your panels being full, and this may be unnecessary in any case.

60A is a misconception about EV charging

We get this a lot. You have every EV shipping with a travel charger intended to be used at RV parks, so it comes with an RV connector which is 50A. Then you get the Tesla Wall Connector which is dynamically programmable but maxes out at 60A circuit. And seems like everybody thinks they need a 50-60A circuit to have any hope of sensible level 2 charging at home.

This is simply not true.

I think people haven't earnestly thought about the miles they actually drive. Put it this way, a typical car lease grants 12,000 miles a year. Assuming they only drive weekdays, that is 48 miles a day. And that means you're spending 1-2 hours a day behind the wheel, in suburban traffic typical of areas with high EV adoption.

At 48 miles a day, you can replace that with level 1 charging with a regular old wall plug.

If we go to 240V/30A which is half your plan, we're now charging at four times the speed of level 1, 200 miles a day given 13 hours of charge, and it's vanishingly unlikely that you'll ever need more. And if you do somehow have two 300 mile days in a row, you can always hit a DC fast charger for like 10 minutes to close the gap.

Even this 240v/30A is overkill for a single car. You can get by with 240V/20A (2.6x Level 1). I'm mentioning 30A because you plan two cars.

With two cars, more magic happens

It's easy to get EVSE's that coordinate between each other. So they are together, sharing a single current allocation. Tesla Wall Connector v2 and v3 have this Power Sharing built-in, and other companies market it under names such as Share2. When one car is attached, that car gets all of it. When two are attached, they dynamically split. And when you have two cars, two funny things happen.

First, it is vanishingly unlikely that both cars need big charge at once. You're likely to have 170 miles and 30 miles. That's just the kind of thing that Power Sharing is really good at. Plug in both cars and walk away: They split 30A for a few hours until the 30-mile car is finished, then the other car gets all 30A. "That was easy".

But second, cars tend to arrive staggered. So your functional charging time is from when the first car arrives to when the last car leaves. That means you actually get longer, and get to charge more overall miles.

And then we get rid of the service upgrade.

EVSE's latest trick is to put a clamp meter (Current Transformer) on the house's service wires. The fact is, house draw is rather spiky. It's big bunches of it when the dryer is on, cooking is being done, water heater is cycled on or A/C is cycled on. But most of the time, the house is quiescent - drawing a piddling amount - only an amp or two. (houses average about 4 amps or 1.0 kW, so obviously they're well under 4 amps most of the time.)

By sensing amps on the service wires, the EVSE can know when heavy appliances are running, and simply slow down charge rate to stay within service limits. Since this isn't much of the time, it has little effect on practical charge rates -- but it has a HUGE effect on the need for service upgrades or rather, the lack.

This tech is far cheaper than a service upgrade.

This can combine nicely with someone's desire to have a high-amp charger. If you want to have dual EVSE's sharing 100A *and it has the sense clamps on the service wires, you can do that right now on the service you have.

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  • Thank you so much for the detailed reply! You mention a current transformer on the house's service wires - do you have a link to a product like that?
    – will74
    Commented Dec 27, 2022 at 21:49
  • evdutystore.elmec.ca dccelectric.com myenergi.com/zappi-ev-charger just a few random ones. An EV shop may be able to better advise. Commented Dec 27, 2022 at 22:50
  • Thank you, this is very helpful! You're right - something like this is certainly less expensive than a service upgrade. I've done a load calculation with Article 220 and I'm still okay with 200 amps even when adding in a heat pump hot water heater and the EV charger. When the house was being built, they spread the loads out across a lot of circuits, which is why the panel was full. Thank you also for the clarification on the wire size for 400 amp service. While I think load management makes it so I won’t need to upgrade service, I have a follow-up hypothetical scenario question.
    – will74
    Commented Dec 28, 2022 at 15:52
  • As you mentioned, 4/0 is acceptable for my 200 amp service because of NEC 310.15(B)(7). But, once you bump up to 400 amp service, now the derate is to 332 amps, making 4/0 only good for 180a. In this situation, if I upgraded to 400 amp service with a new meter with a small feed through panel next to it, could I install the 175 amp breaker (with the 4/0 wire) to the “sub panel” & use the feed through lugs with new 250kcmil wire going a few feet down into my current main panel? Effectively giving me 400 amp service with one 200 amp and one 175 amp panel instead of the typical two 200 amp panels?
    – will74
    Commented Dec 28, 2022 at 15:54
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    @will74 sure, if you can find a 175A breaker. Most of that kit comes with one 200A breaker pre-populated and the other empty so you can field-install a 100, 150, 200A to match the service size. Since no power company offers 175A or 375A services, that breaker might be a black swan with a black swan price. Price the difference between 4/0 and 250 kcmil wire and decide whether it's worth the risk. Commented Dec 28, 2022 at 21:45

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