10

I’ve got a trivial problem that has become complicated by a non-trivial setup.

My goal is to install an 80amp (100amp breaker) EVSE to charge my electric truck. An incentive from my electric provider will pay up to $850 for the installation of an EVSE, and since the EVSE itself was provided with my vehicle, I figured it would be an easy way to get the wire paid for. My provider requires that the installation is performed by a licensed electrician. After calling a few electricians, I was able to get one to come out to install it (It is difficult for me to get anyone to come out to my property due to its rural location and far drive).

I was surprised when the electrician told me that it was impossible to install the EVSE at the rated 80 amps, he mentioned that the upgrades would not be feasible, and he wouldn’t take the job (Out $250 for the service call….)

The electrician gave three main reasons why this install was not possible:

  1. The wire feeding the sub panel was not large enough to handle the load and would result in unacceptable voltage drop.
  2. After performing a load calculation, the load on the sub panel is too large to handle the additional load.
  3. The service size (And utility transformer) is not large enough to handle the entire load of the property if I were to add the additional EVSE.

Absolutely none of this makes sense to me due to my understanding of the electrical infrastructure on my property. I fear I may be missing something.

The Details:

  • My property is served by a 600 amp service (50 KvA transformer)
  • 2 x 200amp panels to the house
  • 1 x 200amp panel to the shop
  • I’d like to install the EVSE in the shop
  • I have a 16kW solar array that is on the roof of the shop, but not directly attached to the shop electrical panel.
  • The wire from my meter to the shop is labeled ENCORE WIRE CORP 500 KCMIL AA-3003 AL
  • The shop is ~400 feet away from the main panel
  • The breaker at the main panel for the shop and solar is 200amp, but the shop panel itself has a 150amp breaker
  • The solar system that is connected has a utility mandated disconnect which has a 100amp fused disconnect.

Existing loads on the shop:

  • A whole bunch of outlets (Served by 2 double pole 20amp breakers), in practice not more than one is used at a time
  • A 15amp breaker feeding a hydronic heating system
  • A 50amp breaker feeing a whole bunch of outbuilding sub panels

Existing loads on the house

  • The house (400 amp) has a lot of electrical loads
  • 4900sq feet
  • 2x EVSE (50 amp NEMA 14-50)
  • Geo Thermal Heat Pump
  • Electric Oven
  • 2x Electric Clothes Dryer

When searching online to try and figure this out myself, all of the NEC load calculations don’t make sense for the “Shop” space that I have, so I don’t understand what is reasonable. I was planning on just going ahead and doing this myself (After wasting $250 for the service call), but I figured I’d ask the internet if my understanding is incorrect.

Questions:

  1. Is it reasonable to put an 80 amp (100 amp breaker) EVSE on my shop panel?
  2. Will I experience unacceptable voltage drop with my 500kcmil wire if I put the EVSE on it?
  3. The electrician was saying that the 80amps of solar counted AGAINST my load calculation, and the 80 amps plus the 80 amp EVSE plus the additional loads would put me over the limit for the wire to the shop. In my mind the 80amps of solar (Located at the shop) will help with voltage drop and shouldn’t be accounted for when considering loads on the shop panel.
  4. Is the size of my utility transformer (50KvA) upto the job?
  5. I don't want to break "Harper's Law", so any suggestions on the wire? Manual indicates that I need 3AWG copper. I am going about 30feet in an exposed area (In the shop)

I understand that my setup is atypical, so if you require any additional information, please let me know.

Appendix:

Additional information that isn’t relevant to the question but will hopefully dispel any “You don’t need this” type answers.

I 100% agree with both Harper and Alec that 80amps of EVSE charging is not necessary…….but……. My utility combined with my solar setup has me in a sort of loophole that I am trying to take advantage of. I am fortunate enough to live in a state that provides net metering on a basis of cost (Not KwH). This combined with Time of Day rates means that I make a lot of “Credit” during the expensive day time hours, and have a ton to use during the night time. I’ve got a 4 hour window where the electric costs are 1/3rd what they would be during the day. This combined with the very large battery pack in my truck means that I want as much as I can take during these 4 hours. I’ve had the truck for a while, and know that with my current use case it takes around 8.5 hours to charge @ 40amps.

Photos:

Wire to shop panel

Wire to shop panel

Shop Subpanel

Shop Subpanel Shop Subpanel breakers Shop Subpanel label

Main Panel

Main Panel Main Panel Breakers

One Line Diagram from solar install: (Should hopefully clarify a few things

enter image description here

4
  • 2
    Note that (there's a video essay making the rounds that says) you might not need 80 amps for EV charging. If you calculated that you really do need 80 amps then okay, but you might also consider seeing whether your 80A EVSE can be configured to lower amperage, with some hidden knobs or switches, and run the thick wire anyway, and then it will be really easy to upgrade your charging speed later if you happen to upgrade your service. Commented Feb 21, 2023 at 17:42
  • 1
    I did something similar to what user253751 says - I wired my EVSE for 48A as that was the biggest my car can possibly take, and did the load calc to confirm that it fit in the panel. However we're electrifying the house, so eventually we're going to need the room in the load calculation and I will have to turn it down to a lower amperage.
    – KMJ
    Commented Feb 21, 2023 at 19:55
  • 4
    See the "Appendix" section of my post. I am already setup with 40A EVSE charging, but have specific needs for the 80A.
    – cyclops
    Commented Feb 21, 2023 at 20:11
  • If the EVSE has capability to throttle charging based on current sensing on the feed wires. Then you can change the load calculation because the EVSE will prevent the car from overloading the panel when the other loads come on. However I'm having trouble finding confirmation of that capability in their marketing material. Commented Feb 22, 2023 at 11:13

2 Answers 2

10

This is about to go HNQ isn't it? Ahem...

Welcome HNQ visitors! DO NOT let this discourage you from getting an EV! The vast majority of people can charge just fine on circuits safely and easily achievable in your home today. Really, really. In this Q&A we are dealing with a freaky "edge of a corner" case with "double bonkers" charging needs because this person is quite knowingly pushing the extreme limits of what is possible with EVs. I wholeheartedly support that - but I wouldn't recommend this for a regular person who doesn't know they're an astronaut LOL.

But the rest of you, I really want to free you from the hobble of thinking you need some giant 50A circuit and a costly service upgrade. I bet we can squeeze usable charging into the service you already have. By the way, the mile numbers in the Technology Connections video are low. Tesla claims at least 50% more.

Now back to OP.

TLDR: Maybe, but you need to tidy up your Load Calculation from unneeded and superfluous things, notably those two 50A EVSE circuits you won't be using anymore, and the odd solar setup.

I 100% agree with the electrician that this is too big for your service based on your other loads, which sorely need tidying up. It's too big for almost anyone's service, but you're an exception because builders don't install 600A services, that's a labor of love. The claim about the solar subtracting from your ampacity has validity and we'll look at that.

Let's talk about your setup

First, the NEC Article 220 Load Calculations are serious business. The problem is, your motives are corrupt. You are hellbound to reach your "win" condition so you're too prone to twisting numbers. Seen it countless times before. So you need a neutral third party, and if that electrician took enough notes, they might be able to e-deliver it without a second site visit. There needs to be one for each subpanel, with any given subpanel's figure falling into the panel which feeds it, and finally those add up to a master Load Calculation for the entire service.

Note that EVSE's come in at 125% of programmed charge rate in the Load Calculation. They're a brutal factor since they're a brutal load.

Every panel must be sufficient for "The Load To Be Served" per the Load Calc.


NEC rates things in intermittent power (with continuous being 20% less). The power company rates everything in continuous power (with intermittent being 25% more). You say your utility transformer is 50 kVA. You can divide by 240 and get 208A (but remember, that's continuous; it's the equivalent of NEC 260A). The power company is counting on diversity of residential loads being unlikely to exceed 208A (hard continuous load) for long enough to burn up the transformer. You were saying you wanted to add 80A of hard continuous load?

You see, we are really in "the land of sharp pencils" here.

My local utility in my old near-suburban town has limits on its infrastructure and needs to know about EVSEs... I think your local utility would also need to be told about the EVSE.


The solar is a monkey wrench. Something must be done to assure on a max solar productive day, the loads inside the panel don't overload the panel. Imagine a 200A panel with 100A of solar going full tilt. What prevents the loads inside the panel from pulling 300A? Nothing. So this problem is moderated by saying "If the utility supply and solar are on opposite ends of the bus, then their breakers can sum to 120% of the panel's bus rating". I know this is complicated.

The HOMC30UC has a 200A bus (not 225A). 120% of that is 240 amps. So the main supply breaker + the solar breaker can sum to 240 amps if fed from opposite ends of the panel. What do you have? 150A supply breaker + 80A solar = 230 amps. That's why the 150A breaker; it was their attempt to comply with the 120% rule. But they made a mistake IMO.

The 400 kcmil wire can certainly handle 200A. So this limitation can be removed by having a "mini panel" with a 200A breaker coming into it*, then solar breakers ONLY in this panel, then a 200A breaker exiting it*. With no loads in this panel - well, work it out. Try worst-case current flows in all directions, it really can't go wrong.

* I'm saying it like that, because it doesn't really matter whether the 200A breaker is inside this panel, or at the other end of the feeder.

That's exactly the situation you describe in comments: "The wire from the main panel comes into the shop building, then splits off (Polaris connection in the large gray box at bottom) into the 100amp fused solar panel and the 150amp sub panel in the shop serving the loads."

I see breakers on all 3 sides - a 200A at supply, a 100A on the solar, and a 150A on the shop panel. I see no reason you can't enlarge the shop breaker to 200A - but it does need to have one for this to work.

So we have gained 50A of Load Calculation inside this panel by deleting the solar penalty. I don't know how much that'll help with the overall Load Calc in this panel because you say other subpanels also come off this. So you'll need to evaluate that, you're looking at the Load Calcs not me.

So how do we get there?

Based on the transformer capacity I see 0% chance of successfully installing this thing without talking to the power company to figure out whatever needs to be figured out at their end.

It's a shame you don't charge during the day. An Energy Management System could be used with a clamp meter on your feeder, so limits truck charging so it draws 0 amps from the grid during the day, effectively dumping all the solar into the truck. Morning solar is practically worthless during A/C season because A/C demand lags solarization by a number of hours (building's thermal mass).

But since you're supposedly running the EVSE in the middle of the night, a simpler answer may simply be load-shed devices added to some of the big spenders in the Load Calculation. The devices will interrupt AC power when the EVSE is charging. Water heater, dryers, range, let's see what other Big Spenders you've got... wait a minute...

2x EVSE (50 amp NEMA 14-50)

Wait. But you only have one EV.

Even if you had two... this is a job for Power Sharing! Grant a group of EVSE's a power allocation and they split it.

By setting these up independent and not power sharing, you are creating the worst-case scenario on your Load Calculation. These need to be provisioned 40A (if the travel EVSE) or 50A (if a wall-unit EVSE setup for 40A actual) each. And they are woefully underused - most of this provisioned 80-100A is wasted because the cars are not coordinating.

So yeah, I think we found the 100A you need. You say you only have one EV, well when a second one comes along, Power Sharing.

12
  • 1
    Due to time of day billing and the net metering based on cost (Not kwh) means that if I send my excess solar to the grid during peak hours, I can get 2x that back during non-peak hours (Sell for $0.164, purchase for $0.058). The whole point of this was to get my full charge in during my 4 hour off peak times.
    – cyclops
    Commented Feb 21, 2023 at 21:23
  • 2
    Well then, tear out the dual 14-50 sockets, and wow, we just found 100A in your Load Calc! "that was easy" Now does the 150A subpanel have a main breaker? I don't see why they limited the subpanel to 150A, it seems like they were doing the math I described but did not understand the reason for the rule. Note your system is complicated, and the scarce resource in the USA today is the brain power to understand all this stuff. Commented Feb 21, 2023 at 21:27
  • 2
    @cyclops maybe not at the shop. but I would think the two 50A EV charging circuits could be replaced with one 100A one. Worst case, add some load sheds to interrupt the dryers when EV charging. I can't imagine you do a whole lot of drying at 2 AM. Commented Feb 21, 2023 at 22:59
  • 2
    Harper, you are a great resource to the SE community, and I wish there was a way to DM you to give a personal "Thanks". I'll call back the electrician and see what it will cost to remove the 2 14-50 outlets and install the single 80amp charger. (At least some of it will be paid for my the utility incentive). I appreciate the thoughtful and detailed response.
    – cyclops
    Commented Feb 22, 2023 at 1:40
  • 1
    Posting as a comment, unsure if it should be an answer or not. I reached out to a different electrician to look at the situation. He performed a load calculation for the entire property, and determined that the only thing that would need to change (Without removing any existing loads) would be the utility transformer. He coordinated with my utility, and had them upgrade the transformer to 75KvA, paid for by the utility. He came back out to install the EVSE. The total cost of the electrician was also covered by the utility by their EVSE installation incentive.
    – cyclops
    Commented May 24, 2023 at 18:13
6

If I understand the terminology correctly, a 50 KvA transformer is (essentially, I know power factor, etc.) 50,000W. Divided by 240V that gets 208A. Which would mean your real total service is 208A, not 600A. Which makes a huge difference in how much power you've really got to work with.

Unless your utility actually tells you that you have 600A service, I have a feeling you really have 200A service. Just because it feeds multiple 200A panels doesn't mean you multiply the total service. (Same as you can add up all the breakers in a typical panel and the total is way more than the main breaker.)

Looking at the electrician's 3 stated issues:

  • The wire feeding the sub panel was not large enough to handle the load and would result in unacceptable voltage drop.

I think the electrician was wrong on this one. According to the Southwire Voltage Drop calculator, 500 AL at 200A (and really you can only do 150A because of the subpanel breaker, and realistically should then be at 120A for continuous derate) is only a 4% drop. And at 150A, 3% drop. So as far as I can tell, that is just not an issue.

  • After performing a load calculation, the load on the sub panel is too large to handle the additional load.

That's a little hard to say for sure. Your limit is 150A, so allocating 100A to EVSE means figuring out whether 50A is a realistic (formal load calculation or otherwise) number for the rest of the shop, including the outbuildings that it serves.

  • The service size (And utility transformer) is not large enough to handle the entire load of the property if I were to add the additional EVSE.

At 208A, powering house, shop, outbuildings, etc. you're lucky to get 40A for the EVSE and 80A/100A is simply not going to happen without a service upgrade.

I am not at all sure how solar fits into this. I know it affects certain calculations for the panel it is in, which may be why there is a 150A breaker instead of 200A. But even if overall it benefits you financially, the load calculation has to handle worst case, which would be at night, and you can't simply say that your other loads will be off at night - maybe you don't usually do welding and woodworking at night, but there is nothing to stop you from doing that.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.