I currently have a 100A main panel and 100A service from the utility for my house. I would like to add an EVSE. I looked at 220.83 (a) and (b) since this is an existing dwelling. It looks like if I'm not adding HVAC equipment I can use (a) and don't need to count HVAC equipment at a 100% (40% for everything over 8 kVA), but if I add a heat pump and EVSE at the same time, I have to use (b) where HVAC is at 100%, and "other loads" are 40% over 8 kVA. To be conservative I checked my loads using (b), and here is what I came up with:

2400 sqft @ 3 w/sqft=7,200
2 kitchen 20A appliance circuits: 3,000
Dishwasher NPR 7.1A: 852
Fridge NPR 9.5A: 1,140
Microwave NPR: 1,550
1 20A laundry circuit: 1,500
Pool Pump: NPR 10.6A @120V: 1,272
EVSE NPR 30A @240V: 7,200
Total: 23,714
First 8,000 VA at 100%: 8,000
Remaining 15,714 VA at 40%: 6,285.60

3-ton A/C NPR 20.1A: 4,824
Furnace blower (used for AC and Furnace) 8.4A : 1,008
1-ton heat pump and A/C minisplit NPR 7.4A: 1,776  
1/2-ton heat pump and A/C minisplit NPR 7.9A: 1,896
Cooling / Heating Total: 9,504  

Total: 23,789.6 VA - > 99.12 A @240V

Does that seem right? I think I have to add the furnace blower to the 3-ton A/C since that blower is used to move the cold air through the ducts and my cooling load is the dominate one. Do I have to multiply the main A/C by 1.25? I'm using 240V for all my calculations, so if the A/C says 20.1A on the name plate, I used 4,824 VA, is that correct?

I had earlier seen worksheets that suggested the first 10,000 W at 100% and the remaining loads at 40%, but when I looked in the 2017 NEC, I see 8 kVA.

Based on this am I OK to add a circuit for my 30A EVSE to the panel? It will be on a 40A breaker (125% of load). I am planning to get a service upgrade to 200A in the future.

Edit: Here are some pictures. The EVSE breaker could either go in the main panel or the 60A sub panel. There are some unused circuits in the main panel: the quad in the upper left was for an electric range+oven that were removed. The middle breakers on the quad below it were for an electric dryer that has also been removed. The 50A sub panel for the heat pump could be on a smaller breaker. 100A main panel

60A sub panel

50A sub panel for Heat Pump


Edit: Added description of all the loads on the various breakers. Main Panel Circuits description 60A Sub Panel Circuits 50A Sub Panel Circuits

  • 1
    Can you post photos of your panel please? Nov 21, 2022 at 2:55
  • 2
    I'd be seriously concerned about adding an extra 30A/40A and ending up that close to total capacity. But often related is the condition of the panel. 200A has been standard in most areas for a long time, and it is extremely common to find panels overloaded (too many tandem breakers and other issues). Unless you actually have a really big panel that just happens to be breakered to 100A to match service (in which case a heavy-up is relatively easy - replace feeder, replace main breaker, meter pan, possibly the actual meter) it may make a lot of sense to do the heavy-up first and instead Nov 21, 2022 at 3:05
  • 1
    run the EVSE for now off a 20A ordinary circuit and live with limited charging capacity until you do the heavy-up (which will cost a lot less than the electric car). So, as already asked by ThreePhaseEel, please upload a picture of the panel so we can see what is really going on. Nov 21, 2022 at 3:06
  • If you have a dedicated 20A circuit to the garage that only serves 1 or 2 receptacles, then install NEMA 6-20 receptacles and a 20A/240V breaker. Now you're EV charging at 3.8kW, which sufficient for almost any need. On that rare time you had a 200 mile day today and also have a 200 mile day tomorrow, simply stop at a DC fast charger on the way home for 5-10 minutes and get it within 100-120 miles of full. Then let it do the last 100-120 miles at home that night. Nov 21, 2022 at 20:59
  • 1
    @ThreePhaseEel 2400 sqft. I put it in the list of loads above: 2400 sqft @ 3 w/sqft=7,200 Nov 26, 2022 at 0:16

1 Answer 1


First, correct some math on the EV circuit

EVs are deemed continuous loads and need a 125% derate as a result. Thus you get assuming 240V:

  • 15A breaker = 12A actual = 2.9 kW = 75-90 miles in 10 hours
  • 20A breaker = 16A actual = 3.8 kW = 100-120 miles in 10 hours
  • 30A breaker = 24A actual = 5.7 kW = 150-180 miles in 10 hours
  • 40A breaker = 30A actual* = 7.2 kW = 190-225 miles - a very few EVSEs such as Siemens Versicharge are actual 30A, wasting a few amps on the 40A breaker. (Pointy Haired Boss?)
  • 40A breaker = 32A actual = 7.7kW = 200-250 miles in 10 hours (gosh!)

Speeds above this are just bonkers. Most people grossly overestimate how much power they really need, and/or get swept into "50-amp thinking" because of a certain misconception.

ClipperCreek does not make a "30A actual/40A breaker" EVSE. The HCS-40 is 32A.

Charging 200 miles a night doesn't make any sense

The typical lease is 12,000 miles a year. If you only drive 250 workdays a year, that is 48 miles a day. Needing 200-250 miles of charge in a day is rather silly, and paying a fortune for a service upgrade simply to achieve that is even sillier.

Your EVSE would be 2 amps larger than Alec's from the video, and review this again. Alec is in Chicago and has been driving EVs for 8 years. I didn't cue up this part, but elsewhere Alec talks about having a 50A outlet and being able to run a 50A EVSE but simply sees no point.

Let's fix the Load Calculation too.

You made one critical error in the Load Calc here. You took all equipment associated with "Heating, Ventilation and Air Conditioning" and dogpiled them into HVAC. That's not how it works. You must actually do THREE sub-calcs for HVAC load, as follows:

  • Load when the system is in A/C cooling mode.
  • Load when the system is in heating mode normally.
  • Load when in heating mode, heat pumps have failed due to temp below minimums, and Emergency Heat has come on. (better heat pumps do not require this).

You then select the one largest number of the three, and that's the one you carry onward into the Load Calculation.

Now typically, the EVSE's are advertised as the breaker value and then their true charge rate is 80% of that. ClipperCreek is no exception. They don't make a "30A actual, 40A breaker" EVSE. This is flat wrong for a Clipper Creek HCS-40.

  EVSE NPR 30A @240V: 7,200

I do note that your Load Calc does work with the fake 30A number and doesn't work with the true 32A number. This leads me to a certain skepticism.

Unfortunately, Clipper Creek units deny you any way to jumper to different ampacities, otherwise I'd say "jumper to 24A and don't worry about it". Not a fan of the brand because of that. Honestly given your load calc and the fact that you could get a 16A or 24A unit in there with room to spare, I would have recommended a smaller unit instead of a service upgrade. Doing unnecessary service upgrades is practically the #1 blunder made by novice EVers.

Honestly you may be better researching Federal, state and local incentives for wall-unit installations, and see if you aren't money-ahead selling the HCS-40 on Craigslist and then buying the right unit with pro installation with incentives.

I'm sorry you made a purchasing mistake but it seems like you're going to extremes to try to salvage it. Just write it off as tuition from the school of hard knocks: don't impulse-buy technical products.

Level 1 vs Level 2 charging

I hear you in comments hating on level 1 charging, but don't create a "false dichotomy" where the only other option is your already-bought 32A (7.7 kW) unit. This is more "trying to rationalize the purchase".

In the US, we assume all 240V units get the circuit to themselves, and run at the "continuous load" 80% limit. As such they are more powerful than European units sized to share the circuit, and are properly called "Level 2". Even the smallest 15A/240V circuit (2.9 kW actual) does alright, as the earlier chart shows.

You are correct that level 1 is hurt by winter due to the "parasitic load" of keeping the battery heated sufficient for charging. However this is a fixed cost and if you increase power, all additional power goes to the battery. Thus even a 15A level 2 circuit, at 2.9 kW, isn't bad. By 20A (3.8 kW), the portion lost to heating the pack is negligible.

The charger is on the car. The EVSE is simply a "smart GFCI" and a tone generator to tell the EV's onboard charger how many amps it can draw. As such, many EVSE's are field-programmable to set the amps.

Most cars come with a free small travel unit, intended for opportunity charging "on the road". They provide dongle plugs for sockets found "on the road" - the standard one and the huge RV socket found at RV parks. This is the misconception. That travel unit runs 32A with that RV dongle, because the socket is also used on 40A circuits. However, vendors will sell you OTHER dongle plugs and those dongle plugs will command the car to use an appropriate ampacity. The dongles have a microchip in the plug, which also senses "hot plug" (this is a UL requirement). The microchip, since it's signaling allowed ampacity anyway, is able to reduce ampacity when it detects hot plug.

What I would do

I don't like unnecessary service upgrades. It's obvious that your house is slowly transitioning away from inefficient A/C units and to highly efficient heat pumps one section at a time. I expect that to continue, your service needs to decline going forward, and upcoming smart-breaker tech will only make it easier to fit stuff on limited services. Yours is a good example of a house that may never need a service upgrade.

Mind you, I am not burdened with an emotional need to salvage an unfortunate purchase.

So my winter configuration for your house would be to disconnect the A/C entirely - pull the wires off the breaker. It is now off the Load Calculation. If I could get a wall unit EVSE that is jumperable for different ampacities immediately I'd just do that, otherwise I'd get a 30A dongle for your travel unit, and wire a circuit to power it.

"30A" (actual 24A) charge rate should ease your winter charging anxiety.

By spring, I'd reconnect the old A/C and jumper the EVSE down to 20A. Shouldn't be a problem, just keep an eye on it. If it is, DC fast charging can always cover the gap for those "but sometimes" moments. The next fall, I'd try leaving it at 20A and see if 20A winter charge is sufficient. I bet it is.

  • One issue with Level 1 charging is that there is a certain amount of power used by the car to be switched 'on' during charging. The longer you charge, the higher this penalty is. Some estimates are that level 1 charging is about 70-84% efficient, and level 2 ~90%. Tesla won't charge until the battery is at 50F. When charging in 32F weather outside, you waste more energy by charging longer. My cars can only charge at 32A max. The ClipperCreek I have has a 30A max charging current and should be on a 40A breaker. Since I already have it, I would like to use. I've charged using a 15A outlet before Nov 22, 2022 at 19:12
  • Sorry, I didn't realize you had edited your response to address the winter charging issue. I like the idea of disconnecting the A/C for winter, but I'm not sure what the inspector is going to think about that (he would probably worry that I would reconnect it and use it as the same time as the EVSE. We received the EVSE as a gift from friends who moved. We've had 2 EV's for 4+ years, so I'm familiar with charging speeds. I think I will start another post here about how to hard-wire the EVSE into my panel. Thanks Nov 27, 2022 at 1:33
  • One question about the HVAC part of the load calc. Since I have one central A/C Condenser at 20.1A/240V, the blower motor on the indoor air handler at 8.4A/120V and two mini splits (that can heat or cool), I think I have to add all of this together for "Load when the system is in A/C cooling mode." Or do I not need to count the mini splits because it is assumed that I don't run central A/C and the mini splits at the same time? I think the original 2013 HCS-40 is limited to 30A (see nameplate picture in my post). They might have made it 32A at a later date. Nov 27, 2022 at 1:45
  • @ChristianK if you have two EVs, you should really, really be using Power Sharing, because of the huge impact that has on your Load Calculation. Under Power Sharing, you run (whatever) power to each EVSE and then tell the networked EVSEs "do not use more than XX amps among all of you. That one number then goes into your Load Calculation. Running 50A to two EVSEs and telling them to share 50A is best of both worlds... you can get Beast Mode on either car simply by telling the other one to pause charge. (the system sees the one isn't drawing so gives its share to the other). Nov 27, 2022 at 5:59
  • The deciding factor on the mini-split/AC split is going to be how your thermostat is set up. If they can't run in A/C mode then they don't count. The 80% continuous load rule is not a recent development, EVSE builders knew about it from day one (but SAE didn't, their early documentation talks about charging being multiples of 6 amps, not 4 amps as it should be. Doesn't matter, it's analog anyway.) I think the HCS-40 label is a misprint, possibly from UL. But a clamp meter would say for sure. Nov 27, 2022 at 6:03

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