which Level2 EVSE to add to this subpanel

At our vacation house, I installed a ductless minisplit heal-pump (to partially replace existing electric-resistance heat). Thinking I might want to add 1 or 2 additional minisplits in the future, I put in a 40 amp subpanel (fed with 8/3 NM-B and protected by a 40 amp breaker in the main panel). I've since had second thoughts about the additional minisplits - even with some rental, the house is nowhere close to continuously-occupied, especially in the winter - and this changes the economics.

Meanwhile, I'm interested in providing Level2 EV charging capability there. In another thread it was discussed that I cannot plug the Leaf factory EVSE into a 30amp dryer outlet, that I need a lower-amperage EVSE if I'm not going to install a dedicated circuit. So I'm thinking about installing a lower-amperage hard-wired EVSE, connected to this subpanel. I'm wondering what size is appropriate.

Currently the 40amp subpanel has the minisplit, protected by the recommended 20amp breaker, plus a 20amp 120vac "convenience" outlet. I know that the sum of breakers in a panel is allowed to exceed the breaker feeding that panel. So I believe it's ok for the EVSE to be larger than 20 amps (40amp subpanel minus 20amp minisplit). I'm thinking a 24amp EVSE is appropriate. I'd protect it with a 30amp breaker in the subpanel, and wire it using 10awg THWN (though I think maybe 12awg would suffice, if it's THWN-2) in PVC conduit.

Add'l info about the minisplit load: "minimum circuit ampacity is 16.5amps, and "maximum operating curren"t is 13.4 amps.

Does this sound right ? How do I compute what size EVSE it's reasonable/safe/code-compliant to add to my subpanel ?

Extra-credit problem: Assuming a 24amp EVSE is acceptable, Clipper Creek lists two diferent hard-wared 24amp units, the LCS-30 and the HCS-30; what is the difference between these two, other than price ?

RustyShackleford said:

I'd protect it with a 30amp breaker in the subpanel, and wire it using 10awg THWN (though I think maybe 12awg would suffice, if it's THWN-2) in PVC conduit.

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No, #12 AWG Cu is limited to a maximum 20A OCPD, with only a few exceptions. HVAC equipment and motors are the most common exceptions, and EVSEs are not an exception.

RustyShackleford said:

Add'l info about the minisplit load: "minimum circuit ampacity is 16.5amps

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So that means that you have 40A - 16.5A = 23.5A of remaining capacity, and so the maximum EVSE size is 0.8 * 23.5A = 18.8A. In practice a 16A EVSE on a 20A circuit. That doesn't include any allowance for the receptacle; I'm not sure what load is required to be allocated to it, but I think 180VA may be right. The 3.5A remaining if you use a 20A EVSE circuit would suffice for that.

Cheers, Wayne

wwhitney said:

RustyShackleford said:

I'd protect it with a 30amp breaker in the subpanel, and wire it using 10awg THWN (though I think maybe 12awg would suffice, if it's THWN-2) in PVC conduit.

Click to expand...

No, #12 AWG Cu is limited to a maximum 20A OCPD, with only a few exceptions. HVAC equipment and motors are the most common exceptions, and EVSEs are not an exception.

Click to expand...

I was referring to Table 310.15(B)(16) aka. 310.16, the third column for THWN-2 rated at 90-degreesC. But it's picking nits. Probably want the 10awg for voltage drop anyhow.

https://media.distributordatasolutions.com/ThomasAndBetts/v2/part2/files/File_7437_emAlbumalbumsOcal20(USA)oc_1_g_nec31016pdfClickHerea.pdf

RustyShackleford said:

Add'l info about the minisplit load: "minimum circuit ampacity is 16.5amps

Click to expand...

So that means that you have 40A - 16.5A = 23.5A of remaining capacity, and so the maximum EVSE size is 0.8 * 23.5A = 18.8A. In practice a 16A EVSE on a 20A circuit. That doesn't include any allowance for the receptacle; I'm not sure what load is required to be allocated to it, but I think 180VA may be right. The 3.5A remaining if you use a 20A EVSE circuit would suffice for that.

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Isn't that saying the sum of the MCAs can't exceed the subpanel's rating ? Surely that's not true is it, for example, all the breakers in a typical house's 200amp main load center sum up to a number far greater than 200. Most or all of them could be driving a load whose MCA is equal or near to the breaker size. But I guess you described a load calculation for the whole panel.

wwhitney said:

I'm not sure what load is required to be allocated to it, but I think 180VA may be right.

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I also found 180 VA per yoke, but it was in the context of commercial bldg receptacles where it is presumed that most receptacles will not be used concurrently. A single convenience receptacle in a home is a different matter. I don't know of any NEC rule, but it would be reasonable to at least allow for a vacuum cleaner use. Those are in the 1,000 - 1,200 watt range, so perhaps 1,500 VA.

The EVSE has to be calculated for continuous usage. I'm not sure about the mini-split, but let't say yes to cover worst case:
40A * 240V = 9,600 VA
Excluding the receptacle, and accounting for continuous duty for the mini-split and EVSE, (9,600 - 1,500)*0.8 = 6,480 VA can be split between the mini-split and EVSE.

The mini-split start up current 16.5*240 = 3.960 VA, so 6,480 - 3.960 = 2,520 VA remain for the EVSE. That is 10.5 Amps.

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NEC is NEC, and NEC is a minimum standard. You also have to consider the location and use common sense. A bldg that has dozens if not hundreds of devices is well served by a load calc that presumes that the majority of the devices are not in concurrent use, but as that presumption becomes less true, the load derating allowed by NEC makes less sense.

RustyShackleford said:

I was referring to Table 310.15(B)(16) aka. 310.16, the third column for THWN-2 rated at 90-degreesC.

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Right, there's at least 2 obstacles to being able to actually use that for #12 Cu. First 240.4(D) has limits on OCPD size for #14, #12, and #10 AWG Cu that do not apply for larger sizes. Second equipment terminations are limited to 75C at most, so any conductor segment terminating at a breaker or a piece of equipment is going to be limited to the 75C column. [A third possible limit is that some wiring methods are limited to the 60C column overall, e.g. NM, or LFNC in a wet environment.]

So you could in theory have a piece of HVAC equipment (an exception to 240.4(D)) with an MCA of 30A and an MOCPD of say 50A, and use a 50A breaker with a run of #10 NM, say, to a junction box to #12 THWN-2 in metallic conduit to another junction box to #10 to the equipment. As long as you use 90C rated wire nuts or connectors you get to use the 30A 90C ampacity of the #12 Cu that way.

RustyShackleford said:

Isn't that saying the sum of the MCAs can't exceed the subpanel's rating ?

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Not quite, as the MCA of a piece of equipment is basically 125% of the largest motor rating in the equipment plus all the other loads in the equipment, and with multiple pieces of equipment, you don't need to add all the MCAs, you just need to include the extra 25% of the largest motor. [I think, unless the rules for HVAC equipment don't allow you that flexibility. Certainly the rules for feeders for other motor driven equipment work that way.]

But basically, yes. The rules for the load calculation for a feeder that just supplies two or three pieces of equipment basically say just to add up the loads of those equipment. Diversity factors and other reductions only come into play for larger numbers of equipment. And circuits supplying general lighting and receptacles have an aggregate load based on the area served, not the number of branch circuits you use. You could supply every general purpose receptacle in your house with a separate 20A branch circuit, and they'd still only count as 3 VA/sq ft of area.

Cheers, Wayne

SageBrush said:

I also found 180 VA per yoke, but it was in the context of commercial bldg receptacles

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The receptacle in the OP is almost certainly the service receptacle required to be installed within 50 ft (?) of the HVAC equipment. So it would not be likely to see a large load while the HVAC equipment is also running at full load.

I don't see any NEC problem with installing a 20A circuit for a 16A EVSE. But if you do plug a 1800W space heater into that receptacle while the HVAC is running at full load and an EV is charging at 16A, you'll have some risk of tripping the 40A breaker.

Cheers, Wayne

wwhitney said:

SageBrush said:

I also found 180 VA per yoke, but it was in the context of commercial bldg receptacles

Click to expand...

The receptacle in the OP is almost certainly the service receptacle required to be installed within 50 ft (?) of the HVAC equipment. So it would not be likely to see a large load while the HVAC equipment is also running at full load.

Click to expand...

Yes, that is correct, for running the vacuum pump during install primarily. So, I'm taking from this that I'd be ok with a 16amp EVSE, wired on a 20amp double-pole breaker in the subpanel using 12awg wire (be it romex or THWN in conduit).

However, is it not true that the issue is nuisance tripping of the 40amp breaker serving the subpanel, and not safety, since all wiring is protected by an OCPD of appropriate size ? Thus, since the "rated" current of the minisplit is only 5.2 amps (as mentioned above, the 13.4amp figure is probably startup current and thus not continuous), I'm tempted to think I'd be ok with a 24amp EVSE. The continuous current in the subpanel would be 24 + 5.2 amps, well below 80% of 40 amps. Peak current would be 24 + 13.4 amps, less than 40 amps.

P.S. Here is the technical manual for the minisplit: https://portal.fujitsugeneral.com/files/catalog/files/(D&T)%20ASUG09-15LZAS.pdf. The electrical requirements are on page 2 of the manual (which is page 6 of the pdf).

RustyShackleford said:

However, is it not true that the issue is nuisance tripping of the 40amp breaker serving the subpanel, and not safety, since all wiring is protected by an OCPD of appropriate size ?

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That's mostly true. But nuisance tripping becomes a safety issue when users ignore it and repeatedly reset the breaker rather than downsizing some of the equipment or changing their operating behavior. Or when they upsize the breaker to keep it from tripping. Since you mentioned this is a rental, I would suggest that you need to play it by the book. On your personal home, you could make a choice to exercise your discretion.

RustyShackleford said:

Thus, since the "rated" current of the minisplit is only 5.2 amps (as mentioned above, the 13.4amp figure is probably startup current and thus not continuous)

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Per the footnotes to the table you referred me to, the "rated" specs are for a particular design condition, e.g. for heating 70F indoor dry bulb, 59F indoor wet bulb, outdoor 47F dry bulb, outdoor 43F wet bulb. So if it never gets below 47F there, you may never see more than 5.2A in heating mode. But the 13.4A figure is "maximum value when operated within the operation range." So you need to use 13.4A for this discussion.

Cheers, Wayne

wwhitney said:

Since you mentioned this is a rental, I would suggest that you need to play it by the book. On your personal home, you could make a choice to exercise your discretion.

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Ok, thanks Wayne. Then a 16 amp unit it is, on 20amp circuit.

wwhitney said:

But the 13.4A figure is "maximum value when operated within the operation range." So you need to use 13.4A for this discussion.
Cheers, Wayne

Click to expand...

Why is the 13.4A used instead of a higher value that takes into account continuous duty and/or start-up current ?

I'm mostly imagining EVSE operating when mini-split is cycled to high

SageBrush said:

wwhitney said:

But the 13.4A figure is "maximum value when operated within the operation range." So you need to use 13.4A for this discussion.
Cheers, Wayne

Click to expand...

Why is the 13.4A used instead of a higher value that takes into account continuous duty and/or start-up current ?

I'm mostly imagining EVSE operating when mini-split is cycled to high

Click to expand...

The 16.5 amp figure is not startup current, it is minimum circuit ampacity (MCA). It's roughly similar to 13.4 * 125%.

SageBrush said:

Why is the 13.4A used instead of a higher value that takes into account continuous duty and/or start-up current ?

Click to expand...

Yes, sorry, I misspoke, the 16.5A MCA is used.

Although with an inverter driven compressor, I expected it would be reasonable to use the 13.4A figure, since there should be no startup surge like with a motor started "across the line" (full AC voltage switched on immediately). But there's no allowance in the NEC for that, you go by MCA for a single piece of motor driven equipment.

Cheers, Wayne

A related question: if I run THWN in conduit for the EVSE, I need EGC and two hots. Must the two hots be black and red, or can both be black ?

RustyShackleford said:

A related question: if I run THWN in conduit for the EVSE, I need EGC and two hots. Must the two hots be black and red, or can both be black ?

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For a dwelling unit, where there's only one voltage system present, there's no need to use any particular colors (edit: for the hots). Each ungrounded (hot) can be any color except green, white, or "natural grey".

But conventionally you'd distinguish one of the ungrounded conductors from the other by putting a ring of red electrical tape around the insulation at each end at the termination. No need for a continuous red sheath.

Cheers, Wayne

Rusty, the convention is to use Red and Black (+,-) for low voltage DC, Black & White (Hot, Neutral) for 120 volt AC circuits, and Red, Black & White (Hot, Hot, Neutral) for 240 volt AC systems with two 120 volt legs being combined. I have no idea how they do it in the EU where most stuff is 220 (230?) volts. AFAIK, all of them use Green for Ground.

To elaborate on what Wayne said, my understanding in the US is that for any non-cable wires (eg THWN in conduit) EGC must be green, neutral must be white or grey and any other color is 'hot'. The converse is also true, any green wire MUST be the EGC, any white/grey wire MUST be neutral. The EGC wire can also be bare but I don't think you can run bare wires in conduit so in this case the EGC must be green.

If you have a properly bonded metallic conduit, it can serve as the EGC so in that case you wouldn't need a EGC wire.