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

4
  • 2
    You are, essentially, doing a load calculation (or similar) for the subpanel. But you also should look at the total load - i.e., how much excess capacity do you have in the main panel, counting all subpanel loads as if they were in the main panel (i.e., the individual loads - the 40A breaker being irrelevant in this aspect). Commented Feb 16, 2023 at 17:47
  • What is the type of supply cable? Maybe there are some tricks we can do. Commented Feb 16, 2023 at 18:25
  • @Harper-ReinstateMonica I'm not sure what you mean by "supply cable". There's 8-3 (plus EGC) NM-B going from the 40-amp breaker in the main panel to the main-lug subpanel. Commented Feb 16, 2023 at 21:13
  • That's exactly what I meant, yeah, and 40A is what that is. Thanks, 334.80! Commented Feb 16, 2023 at 23:14

3 Answers 3

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).

36
  • 1
    @nobody because there's a huge difference between EPA economy/performance figures, and the ampacity which you must provision. Commented Feb 16, 2023 at 23:09
  • 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
    Commented Feb 17, 2023 at 3:02
  • 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. Commented Feb 17, 2023 at 4:37
  • 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. Commented Feb 22, 2023 at 22:47
  • 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. Commented Mar 25, 2023 at 20:00
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.

5
  • 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. Commented Feb 16, 2023 at 20:43
  • The two EVs we have can easily handle a 20-amp EVSE (6+kW chargers at least). Commented Feb 16, 2023 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. Commented Feb 16, 2023 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 ? Commented Feb 16, 2023 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
    Commented Feb 16, 2023 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.

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.