4

I have an existing 40-amp subpanel: a 125-amp main-lug load center, wired to a 40-amp breaker in the main panel with 8-3 NM-B.

The subpanel has one existing load - a minisplit heat pump. I wish to add a Level 2 EV charger to it. I'm unclear as to what size charger is acceptable.

Here is the technical manual for the existing load (minisplit), mine being the largest '15' size: https://portal.fujitsugeneral.com/files/catalog/files/%28D&T%29%20ASUG09-15LZAS.pdf

Page 2 of the manual (page 6 of the pdf) shows the "maximum operating current" as 13.4 amps, and the "rated" current as 5.2 amps. On page 68 (page 72 of the pdf) the "minimum circuit ampacity" is specfied as 16.5 amps.

The two EVSE under consideration are rated at 16amps and 20amps, respectively (calling for a circuit 125% as large).

Clearly the 16-amp model, protected by a 20amp breaker in the subpanel, is acceptable. But what about the 20amp model ? There seem to be two issues.

I know a branch circuit supplying a continuous load must be rated at 125% of the load. But I don't believe that's the case with a panel, that the panel's rating must be 125% the sum of all the loads of all the branch circuits served by the panel. So I don't believe I'm restricted to 32 amps (80% of 40-amps).

I'm also unsure which figure to use for the minisplit: 5.2 amps, 13.4 amps, or 16.5 amps.

Will the 20-amp EVSE fail to be code-compliant, or is the only issue nuisance tripping, which seems unlikely given the typical operating currents of the minisplit ?

For reference, here is the nameplate for the minispit (on the outdoor unit, which powers the indoor unit as usual with minisplit) ...enter image description here

RustyShackleford
  • 2,186
  • 3
  • 26
  • 41

3 Answers3

5

The starting current isn't useful. It's an inverter drive/VFD, so starting current is a minimum case not a maximum. It's telling you essentially "don't worry about Locked Rotor Amperage (LRA) because this unit does not hard-start its motors".

I gather the indoor head takes power from the outdoor unit? The system has to figure for that voltage a sell. So per 440.6(B) it appears this unit gets 16.5 amps x 100% for breaker sizing.

Now, how it counts into the Load Calculation is a more complex matter. It appears to me based on 440.33 and .34, the figure used for the Load Calculation is the Rated-Load Current as defined at 440.2. This matches up to the 13.4A figure in my opinion.

That leaves 26.6 amps to allocate to all other loads.

EVSEs require a 125% derate off the actual charge rate so for example level 2 charging at 16A requires computing the breaker, wire and Load Calculation based on 20A. Don't accidentally apply this twice. Most EVSEs, when you commission them, will ask the circuit breaker size (that's not quite right, they mean the circuit size you want to use). If you say "20A" here, the EVSE will authorize the car's onboard charger to draw 16A actual. But don't add 25% more to that.

I think you can 25A from a panel that has 26.6A remaining, so it appears it can be a 25A breaker / 20A actual charge rate.

Speaking of that, you know about this thing, right? I know it can select 25A. Best bargain in town for the feature set, which includes firmware-configurable charge rate (so you don't need to buy a different EVSE if you upgrade ampacity), and Power Sharing across similar units (including the normal Tesla unit).

Harper - Reinstate Monica
  • 276,940
  • 24
  • 257
  • 671
  • ClipperCreek definitely makes a 20 amp (25 amp breaker) EVSE; and Siemens has a 25-amp double-pole breaker. – RustyShackleford Feb 16 '23 at 20:59
  • But you're saying, even so, you don't think I can use a 20amp EVSE ? Even though it seems pretty clear that the minsiplit cannot draw more than 12amps for an extended period ? – RustyShackleford Feb 16 '23 at 21:00
  • How is it that the heat pump needs to be provisioned at 16.5A at all times? The manual says it provides ~4kW (effective) of heating/cooling with a COP of ~4, seemingly consuming around 1kW of input power (~5A at 240V). – nobody Feb 16 '23 at 23:02
  • 1
    @nobody because there's a huge difference between EPA economy/performance figures, and the ampacity which you must provision. – Harper - Reinstate Monica Feb 16 '23 at 23:09
  • Also the Tesla J1772 Wall Connector is not really a bargain. It's about $150 more than comparable competing (legitimate, UL-listed) J1772 EVSEs. It's basically the same price as a Tesla Wall Connector plus Tesla->J1772 adapter, without the ability to charge a Tesla. Needing to share a circuit with another Tesla Wall Connector seems like the main reason to choose it. – nobody Feb 16 '23 at 23:09
  • Ah so the COP of 4 is best-case but it may actually go as low as 1 in worse-case conditions? – nobody Feb 16 '23 at 23:13
  • @nobody 4 isn't best case, 4 is average case based on whatever formula EPA or whoever uses. It should be doing better than 4 as often as it's doing worse than 4, I would think. They don't really compare; COP comes out of an entirely different universe of thought than the provisioning number. Imagine a 6.5A heat pump has a 10A defrost heater. It *rarely, rarely* uses it, so it's barely a factor in the COP calculation. But as far as provisioning goes, the circuit/feeder/service must be able to deliver 16.5A or whatever. – Harper - Reinstate Monica Feb 16 '23 at 23:18
  • @Rusty because one uses NEC Article 220 for the method for doing a Load Calculation. Page 68 of the PDF, you said it in your question. I can't find a basis in 220 or in 440.6 for using the lower number. But If you wanna use the 13.4A figure have fun. But here's the problem. If your inspector red-flags the 25A-breaker 20A-actual, and says you need the 20A/16A unit, there you own a 25a CrippleCreek unit. *Ask ClipperCreek where the DIP switch is to detune the 25/20A unit to 20/16A*. (there ain't one.) – Harper - Reinstate Monica Feb 16 '23 at 23:24
  • @nobody. Really, could you list several competing EVSEs that support field-settable ampacity and also Share2 style Power Sharing out of the box, in the sub-$400 range? This I want to hear! – Harper - Reinstate Monica Feb 16 '23 at 23:26
  • Emporia makes an EVSE that's field-settable for circuits from 15A to 60A for $399. UL listed. – nobody Feb 16 '23 at 23:29
  • @Harper-ReinstateMonica at least for me, the Tesla J1772 Wall Connector is listed at $550 – Jacob Krall Feb 17 '23 at 00:11
  • @nobody fair enough you can get cheaper units if you sacrifice the full feature set. – Harper - Reinstate Monica Feb 17 '23 at 01:58
  • @Harper-ReinstateMonica I'm not that concerned with the inspector. I respect the collective wisdom here as much as any random building inspector (especially in a jurisdiction where multiple improperly hung ceiling fans have been found - and I do mean improperly - another story). I feel like a 20-amp EVSE is ok, given the unlikelihood of the minisplit drawing more than 12 amps for any duration. But I defer to you, and unless I misunderstand, you're saying you don't believe it's code-compliant). – RustyShackleford Feb 17 '23 at 02:42
  • @nobody. The field-selectable amperage seems like the way to go. The Emporia only seems to support 40 and 48 amps though. – RustyShackleford Feb 17 '23 at 02:51
  • 2
    @RustyShackleford Read the detailed manual. It supports circuits 15A-60A. I have one and it's set to 15A. The 40A/48A is the upper limit depending on how it's connected (plug or hard-wired) but lower settings are always available. – nobody Feb 17 '23 at 03:02
  • @RustyShackleford Yeah, I'm just not getting any warm fuzzies about the 13.4A number. It's in the wrong section and I don't like the *2 weasel language on page 3. The contents of page 68 jibe better with what I expect to see. On the other hand they spec 14 AWG wire even on the 20A breaker (not unusual with motors). Is there a physical nameplate that could be a tiebreaker? UL won't let them put marketing faff on the nameplate. – Harper - Reinstate Monica Feb 17 '23 at 03:03
  • @Harper-ReinstateMonica Added pic of nameplate to OP. I don't think it helps us. I think that Emporia adjustable unit looks good. I'd wire it for 25 amps (12 awg THWN in conduit) and try a 25-amp breaker. If I get nuisance trips, replace with a 20-amp breaker and set the unit for 16 amps. – RustyShackleford Feb 17 '23 at 03:43
  • 2
    @RustyShackleford Aha. So there's the 13.4A. Yeah, I dug deep into Article 440 and it appears the split number is for sizing the breaker (16.5A) and doing the Load Calculation (13.4A). So I think you are OK at 25A for the EVSE. SMH motors, especially combination devices. – Harper - Reinstate Monica Feb 17 '23 at 04:37
  • Oh ok, great. I think I might still go with the adjustable Emporia unit, simply because it's cheaper (than the 20-amp ClipperCreek). Wire it for 25-amp circuit (12 awg THWN in conduit to 25-amp breaker). That way, if I decide to add a 2nd minisplit to the subpanel (which is why I put a subpanel there in the first place) and need to re-wire the EVSE, I could make it beefier and be able to simply adjust the unit. – RustyShackleford Feb 17 '23 at 17:01
  • Decided not to put 2nd minisplit, because it'd be for MBR, and as a vacation place, occupancy rates are very low, so the economics just don't make sense, even with a DIY install of the minisplit. – RustyShackleford Feb 17 '23 at 17:03
  • Probably should add another question, but would the usual 5% voltage drop rule apply here ? The round-trip resistance of the 8awg feeder to the subpanel is about 0.06 ohms, and if I use 12awg to connect the EVSE to the subpanel, that's about 0.25 ohms. So figure 32*0.06 + 20*0.25 = 7 volts. Assuming I can use the 75-degree column of Table 310.15(B)(16) and use 12awg with 25amp OCPD ... – RustyShackleford Feb 22 '23 at 06:37
  • 1
    @Rusty NEC 240.4(D) stops you from putting a 25A breaker on #12 wire and forces you onto the 60C column for #18 through #10 wires. With EV charging I advise figuring out how much $ you'll be spending charging, and figuring out whether the % of that money lost on voltage drop justifies larger wires. – Harper - Reinstate Monica Feb 22 '23 at 22:47
  • Thanks for the NEC reference, wasn't aware of that one. Maybe I should use the OCPD current when computing voltage drop, and using #10 wire, I get 40*0.06 + 25*0.15 = 6 volts. Less than 3%. – RustyShackleford Feb 23 '23 at 04:49
  • But for the economic calculation you suggest, I should probaby use typical currents. For the minsiplit, that's probably more like 5 amps; still 20 amps for the car. So 25*0.06 + 20*0.15 = 4.5 volts. If I use #8, 25*0.06 + 20*0.095 = 3.4 volts. I'm not sure how that translates to % savings with the #8. Maybe use V^2/R, except maybe the current isn't reduced proportionally to the voltage. – RustyShackleford Feb 23 '23 at 04:57
  • Looking like about $160 for the #10 and $250 for the #8 (three 75ft pieces of black, red, green). – RustyShackleford Feb 23 '23 at 05:07
  • LIke I said, it's a vacation place, so let's say 20 trips a year (that's a lot) at 400 miles round-trip. Call it 100 kwh per trip, at about $0.10/kwh, $10 per trip or $200 on the year. So regardless of if that 1 volt diference in voltage drop reduces the charging by 0.4% or 0.8%, it's certainly not worth $90 more on wire, even amortized over many years. – RustyShackleford Feb 23 '23 at 05:25
  • @Rusty Sorry, I'm used to expressing voltage drop as a *percentage*, which translates much more directly into savings. To get voltage drop from volts, divide by 240V, but I also advise using Southwire's voltage drop calc, as it's easy to make a mistake. Also note the 2 hot wires can be the same color. I generally consider that a good idea, as distinguising the poles *from each other* is useless, but telling circuits from each other is required. – Harper - Reinstate Monica Feb 23 '23 at 07:01
  • I was wondering if I need to square it, that is, looks at (Vactual/240)^2, or just Vactual/240. But as I showed above, we're still talking a percent or less, and with the low usage due to it being a vacation place, I think the #8 makes no sense. – RustyShackleford Feb 23 '23 at 17:48
  • Coming back to this, we've established it's ok to attach an EVSE drawing a 20-amp continuous load, to my subpanel. What if I want to install an outlet instead (and use an EVSE cord with that) ? Is it ok for the outlet to be a 14-30, and if so, should I protect with a 25- or with a 30-amp breaker ? – RustyShackleford Mar 25 '23 at 04:45
  • 1
    @Rusty Yes if you'll be pulling 20A-honest (so 4800W), you could breaker that at 25A but not if you want a receptacle. They don't make 25A receptacles so 30A... and then the breaker must also be 30A per 210.21. There's no safety issue there since both the EVSE and the car's onboard charger are safety rated to >30A. The reason you sometimes need 25A breakers on things like A/Cs is the NEC 430 motor rules, which don't apply here. – Harper - Reinstate Monica Mar 25 '23 at 20:00
  • The EVSE cord in question is a Tesla "mobile connector" and evidently, if you attach any of the many NEMA adapters (https://shop.tesla.com/product/gen-2-nema-adapters) it signals its identity to the logic in the cord. So a 14-30 adapter will tell the thing to pull 30 amps, which will overload the panel. Which leaves me using the 6-20 adapter, which is far less standard. Which is no good, because it's a rental house and I want to advertise it as a perk. So scratch the outlet idea. Hardwired it is. – RustyShackleford Mar 26 '23 at 03:53
  • @Rusty We're past the time where you can assume all EVs are Teslas. If you're providing an outlet for people to use their own travel EVSE your only practical choices are 5-15 or 14-50 as many automakers offer only those. So yeah, hardwired is the way to go there. Achshually... since our conversation began, I learned about the Emporia EVSE, which handshakes with the Emporia VUE home energy monitor, and will slow EV charging to keep whole-house load within the service limits of the panel. With that, you get to disregard the Load Calculation altogether. – Harper - Reinstate Monica Mar 26 '23 at 18:45
  • Yeah, I haven't looked into that fancy energy-monitoring stuff yet. It was actually in THIS thread that I learned about the Emporia. – RustyShackleford Mar 27 '23 at 00:13
3

Panels typically contain enough branch circuits that when their breaker ratings are added up equal way more than either the panel rating or the main breaker rating. The thing is, you aren't fully loading every branch at all times so it isn't a problem. If you were to load enough branches to add up to the main breaker rating, then it steps in to stop you, provided you do it for long enough. Small spikes (such as starting a motor) don't factor in because they pass quickly enough to avoid any dangerous heating.

So oversubscribing the panel by branch circuit ratings isn't a problem. Keep in mind that if your 40A branch breaker feeding the subpanel is fully loaded, does that oversubscribe your main panel?

As for your EVSE, there should be no code problem going with a 16A EVSE on a 20A branch circuit. The issue is, if you are loading the EVSE at full capacity at the same time the A/C comes on, you could temporarily spike above the main breaker rating. Short spikes aren't concerning.

Be mindful of the car you are connecting as well. For example, I currently own a 40A EVSE on a 50A branch circuit, but the charging unit onboard the car is only capable of 3.5 kW, so the most it will ever draw is ~16A at 240V. The next logical step for onboard AC chargers is 7.6 kW and there may be a step or two higher now, but for power levels much higher than that the car expects to draw direct DC from a public charger that's set up to deliver that. Don't bother provisioning for huge parking-lot power levels at home.

Chris O
  • 6,084
  • 1
  • 6
  • 42
  • Even if I go with the larger 20-amp EVSE (which is the purpose of my question), I don't think that's a very significant load in the scheme of things. The house is all electric baseboard, some of which I disconnected when I put in the subpanel and the minisplit. – RustyShackleford Feb 16 '23 at 20:43
  • The two EVs we have can easily handle a 20-amp EVSE (6+kW chargers at least). – RustyShackleford Feb 16 '23 at 20:44
  • Yes, I'm sure there will be time when the minisplit compressor starts up while a car is charging; but as you say, those episodes will be brief. But, in fact, that table on page 68/72 of the technical doc says even the startup current is only 5.2 amps (might must be a typo). If you go down to heating performance on page 12/16, in the worst case (-5 outside, 75 inside) it's only drawing 2.85 kW (12 amps or so); and that, plus 20 amps from the larger EVSE, doesn't exceed 80% of the panel rating. – RustyShackleford Feb 16 '23 at 20:51
  • Yes, no question the 16-amp EVSE is fine. The breakers protecting it, and protecting the minisplit, would both be 20 amps, summing up to no more than the panel's rating (the breaker connecting it to the main panel). The question is, is the 20-amp EVSE ok ? – RustyShackleford Feb 16 '23 at 20:52
  • Harper's answer explains better than mine that for an inverter compressor, startup current is actually low vs. something like a big inrush for a capacitor-start motor like I was assuming. His load calc puts the 20A EVSE a little bit of a bridge too far but the 16A within range. – Chris O Feb 16 '23 at 20:53
2

If you don't mind shopping around a bit more for a EVSE then you will be able to find smart units that have current sensing probes that will go around the feed wires of your panel.

That way you can put a 40A circuit to the EVSE and tell it that the panel is limited to 40 A (or 32 A continuous). Using that it can sense the other loads coming on and will throttle the current the car is allowed to pull to ensure the total draw of the panel remains within bounds.

ratchet freak
  • 10,047
  • 1
  • 20
  • 29