My house is 1940 square feet. Located in Sunnyvale CA. Sunnyvale has this electrical load calculator: https://ecityhall.sunnyvale.ca.gov/cd/images/New%20Electrical%20Panel%20Calc-Fillable.pdf

My calculations based on Sunnyvale's form are:

Description of Load Minimum Nameplate Rating Watts
Total Square footage of building times 3 5,820
Kitchen Small Appliance Branch Circuits (min. 2) 3,000
Laundry Circuit (min. 1) 1,500
Microwave 1,400 1,700 1,700
Dishwasher 1,500 1,200 1,500
Disposal 1,000 972 1,000
Electric Oven 2,000 3,600 3,600
Electric Range 5,000 6,400 6,400
Electric Clothes Dryer 4,000 4,000 4,000
Electric Water Heater 4,000 4,000 4,000
Subtotal 32,520
Subtotal (A) 22,520
x 0.40
Subtotal (B) 9,008
Subtotal (C) 19,008
Heat pump without supplemental heating (100% NP Rating) 4,000
Total Current Demand (Volt-Amps) = 23,008
Divided by 240 Volts = 95.9

I didn't include my clothes washer 1200 W since that's included in the required 1500 W laundry circuit right? I'm also not sure of the nameplate rating of my heat pump HVAC. I went with the common value of a 3T heat pump. I have a separate question on that here.

My service is 100A, my main panel is 125A, my subpabel is I think 100A.

Per ThreePhaseEel's recommendation here I plan on consolidating the range and oven to make space in the panel for a 30A water heater breaker.

Can I install a 30A 240V heat pump water heater? (I know there are new 120V ones without the backup but I think we really need the backup for a family of 6. It also seems they are impossible to purchase for self install). I've included it in my above calculations and it seems I'm under 100A.

Unfortunately I don't seem to have enough load for a dedicated EVSE, only maybe at 10A. Can you double check my calculations?

Based on previous answers I got here, I think my options for EV charging are:

  1. Continue using level 1 charging.
  2. Load shed the EV charging with products like DCC-10, EVEMS, SimpleSwitch 240M.
  3. Extend the dryer circuit and add a separate receptacle closer to the driveway running the cable in a conduit. I'll have to add a GFCI breaker. And I'll have to make sure I don't run dryer and EV charging at the same time.
  4. Plug a splitter switch (e.g. NeoCharge) on the dryer outlet that automatically switches between dryer and EV. I'll have to run an extension cord along the long edge of the garage to be close to the driveway where I park the car. Since this extension cord will be "permanently" plugged in, can I put it in cable clips on the ceiling edge of the garage?

electrical panel electrical panel label electrical panel open subpanel subpanel label subpanel open

  • There are EVSEs that put current sensors on your main feed and dynamically tell the car how much it can take without exceeding your service size, so they basically don’t count towards the load calculation. Might be an option.
    – nobody
    May 25, 2023 at 11:06
  • Yes I'm already mentioning DCC-10, EVEMS, SimpleSwitch 240M in my post as option 2.
    – user162793
    May 25, 2023 at 11:18
  • Is there a reason you're stuck on the 30A/240V hybrid units? I was thinking a pure-play split-system HPWH or reverse cycle chiller would be a superior option (the Sanden/ECO2 units are 15A/240V, for instance) May 25, 2023 at 11:44
  • @ThreePhaseEel yes I did consider Sanden/ECO2 but it will be harder to install. It will have to run to the other side of the garage and our front yard is small so we want to avoid placing any outdoor unit there. I just noticed Rheem has 15A/240V hybrid water heaters but they are hard to get. That might work. If my calculations are right I would have 25A/240V for EVSE.
    – user162793
    May 25, 2023 at 12:48
  • 1
    @manassehkatz many utilities are facing supply constraints on neighborhood delivery (not because of EVs necessarily), there seems to be a world transformer shortage for some reason... also note a place like Sunnyvale most stuff will be undergrounded and that makes everything more complicated. However this is the 21st century for Pete's sake. There is no earthly reason anyone needs a service upgrade anymore; Energy Management Systems exist (not that faff OP is mentioning; it's much better.) May 25, 2023 at 21:06

1 Answer 1


We are at the cusp of service upgrades being obsolete. Technology Connections has a whole video on the topic, but you don't need a SPAN panel to do it. More tech is coming up fast.

Are you completely married-for-life to the idea of your EVSE having a receptacle and not being hardwired? Because the receptacle forces you into sad old tech.

An EVSE is what people call a charger. It's much less, but much more.

EVEMS is already solved

From the first two comments it seems there's a knowledge gap. Let's fill that in so you can think properly about products. That thing on the wall is NOT a charger. Really.

It is just a gateway. It passes AC power straight through onto the L1 and L2 pins on the J1772 or Tesla connector. Its job is to place a 1000 Hz square wave on the "CP" control pin. The duty cycle tells the car the safe amperage it can draw. The actual AC/DC conversion happens onboard the car in a literal battery charger the size of a suitcase and water-cooled. It obeys the CP signal dynamically on the fly.

Gee, that'd be awfully handy if we're trying to do energy management!

Yeah. That was the entire point. SAE is the auto manufacturers' standards-setting organization. They knew "needing a service upgrade" would be a sales killer, so they built J1772 on purpose to make things like this easy (and coordinated with UL). Not their first rodeo; this is their fourth from-scratch attempt at an EV standard.

So yes. They intended from day one for EVSE's to be able to talk to CTs and use the square wave for EMS. Or let Multiple EVSEs dynamically share a fixed block of power. Or both at once. They designed it to make this easy.

Tesla uses J1772 in a different connector shape (an adapter is made only of copper and plastic), and Mennekes does the same with 2 more phase pins added (again, an adapter of copper and plastic will let an American J1772 or Tesla can charge at a Mennekes station, and all that square wave stuff will work correctly.)

That clunky old "EMS" tech is for water heaters.

I use that term laughably, because really, they're old Load Shed devices with a new sticker slapped on them. They're for a water heater, hot tub or our fathers' forklift battery charger. Selling them for EVs is almost deceptive since they totally ignore EV tech and do it "the hard way".

And worse, they're needlessly costly because they need the CT clamps and a huge contactor that is rated to interrupt under full load... because they just don't have a smarter way to do it.

So yeah. Forget those old things, at least until you get a hot tub.

Readily available models of modern EVEMS

In the American market, avoid the Emporia EVSE + VUE energy monitor, at least for now. They implemented it foolishly on the software side (because they were chasing the wrong feature-set), so it doesn't meet UL standards.

Another product I've heard good things is the Wallbox e.g. Pulsar Plus. This one supposedly does this with multiple EVs at once, so you can have three EVs plugged in, all grid-limiting.

Elmac and SimpleSwitch seem to have solutions. C'mon Tesla!*

Over in Europe they have the Myenergi Zappi doing the same thing, wired or wireless. The Zappi also solves the PEN fault problem they have over there.

And all these units can do "Solar capture". A lot of people don't have net metering, they get paid a penny a KWH for their solar yet must pay 15 cents a KWH to charge their EV at night. These products can sense when you're overproducing solar and divert exactly that much into your car's battery. And the Emporia is fine for that since it's not a safety system.

One simple trick - having the actual battery charger be on the car and obey a square-wave signal on CP - enabled so much smart tech it isn't even funny. The EVSEs aren't particularly complicated - you could build one with an Apple II. It's easy to understand. It makes the on-the-car battery charger complicated to engineer, but not much to build, so the smart design is practically free. Genius... go SAE!

And proof is that Europe didn't really find a way to improve on it. They just added 2 phase wires and left the CP/PP protocols alone.

Onto your Load Calculation

So, Sunnyvale's worksheet is not a genuine, valid NEC Article 220 Load Calculation. It has a lot of "stand-in" numbers that are "fanciful". For instance, the electric tankless water heaters come in anywhere from 7kW to 22kW; you can't use 15kW as a "stand-in" LOL. Likewise we just placed EVSE at 0kW, and they can range as high as 19kW. Sometimes you just can't use a unit like Wallbox or Empori

So that dryer number isn't right; if your laundry room has a dryer vent to outside (dis-preferred in newer tight homes), you need to allocate 5600W there because that is the power of almost every old-school electric "shove the wet air outside" dryer. Likewise tanked resistive water heaters are mostly 4500W with a few 3840W units for trailer homes.

So I don't know what to do with that sheet; obviously Sunnyvale can make it legal, but it feels like it didn't really receive the engineering scrutiny that the official NEC Article 220 methods receive. I would prefer the alternate method in 220.82 for dwellings which is similar to Sacramento's. (the standard method in Article 220 is a catch-all for everything from Nike Ajax launch sites to auto assembly plants; it's not wrong for dwellings, just unnecessarily complicated). The alternate method usually produces more favorable numbers, too. Most municipal worksheets (the ones which follow NEC, that is) use the alternate method.

Splitting the dryer circuit

I know automatic dryer splitters are popular; however, while they have a certain "gadget-buying appeal", they aren't actually required. There are two workarounds depending on your discipline and your panel.

First, there's a thing most people call a "generator interlock". The manufacturers don't call it that - quite on purpose! It's to interlock any two breakers you don't want on at the same time. Not every panel manufacturer makes side by side interlocks, but those who do sell them for around $30. You have to flip it to use one or the other. This is good when you don't have good family coordination.

Second, if you can self-manage not drying and charging at once, here's a little-known fact about NEC. You know general-purpose 15-20A circuits can have multiple receptacles - so can general-purpose 30A circuits! Dryers are not required to be dedicated circuits, and you can defend a 30A socket in garage or parking spot as "general use" - the spouse is thinking of buying a kiln. So the only gotchas here are #1 you can't extend a 3-wire dryer circuit with NEMA 10-30, and #2 a new outdoor/garage socket in a NEC 2020 state needs GFCI, so that's a $100 breaker you'll be popping for. But it beats an automatic switchie-doo, as well as the very hokey cable routing we typically see with switchie-doo's.

* Tesla has Power Sharing since Rev 2. Now that Power Sharing is wireless on the Rev 3, all you need is a frob that sits on the panel with two CTs and acts as the Power Sharing "master". And that would just work with existing Wall Connector Rev 3, including the J1772 model. And ClipperCreek, you put a very clunky but very public protocol for your Power Sharing scheme; same deal - a frob in the panel with CTs that acts as master. Anyone could build that box; we don't need your permission since you opened the spec. There's a small isolation issue, but solvable.

  • You are right, the dryer is actually 6240W (label says 110V/240V 26A) and the water heater I'm looking at purchasing is 4500W so that pushes me over 100A at 100.4A. Using Sacramento's form I'm at 99.9A. But I'm afraid the 4000W common value I used for a 3T heat pump might be low for my heat pump per diy.stackexchange.com/questions/273002 Installing a 240V/15A water heater might be my only option.
    – user162793
    May 26, 2023 at 19:09
  • You should edit your answer and not recommend Emporia EVSE. It cannot be used for load management, see reddit.com/r/evcharging/wiki/load_management and reddit.com/r/evcharging/comments/11jphw1/comment/jci3ecl
    – user162793
    May 26, 2023 at 19:11
  • The alternate NEC 220.83(B) is quite different than Sacramento's. It says "First 8 kVA of all other loads" when Sacramento's method is first 10 kVA. The alternate method gives me 94.9A (or 2000/240*0.6=5A less than Sacramento's) so I should be good for a HPWH. I'll ask the AHJ. For EVSE I think I'll go with a splitter switch on the dryer circuit.
    – user162793
    May 27, 2023 at 1:01
  • Apparently the Emporia unit can't be used for local load shedding as per this Reddit comment that was apparently posted by Emporia support (mostly because it involves cloud silliness instead of simply letting you dial in your service size and go without the need for a bunch of backend gobbledygook) May 27, 2023 at 1:39
  • @user162793 if you're sharing the dryer circuit, see my edit. May 27, 2023 at 4:04

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