Finished Basement New Sub Panel + Hot Tub, EV Charger Load Calculation [closed]

Question

I have 200 amp main service with a existing 100a sub-panel and a photovoltaic system (for what it's worth). Both the main and existing sub-panel are full (thanks commercial builder!).

Now finishing the basement (1,200 sq ft) with a bed, bath, general living area and kitchenette/bar (dishwasher, disposal, small fridge, microwave) and would also like to add additional capacity for a near future hot tub, an EV Charger in the garage, and an additional 20A line to the garage for general use. It is these elements that can push things over the edge. I was thinking I could add a new 100a sub-panel off the existing 100a sub panel (fyi, it's just 6ft away) just because it has 2 small load circuits that I can move to the new sub vs the main panel is all large loads (the 100a sub panel, AC, furnace, double wall oven).

Questions

1.) In NEC I don't see that you need to account for the 20a bath circuit in addition to the general load calc (3 VA / sq ft) is that correct?

2.) My standard NEC calc, if correct comes to 161a, which is very high (most due to EV and Hot tub), did I calculate this correct? (I included the hot tub and EV and new garage circuit at 100% and add 1.25 safety multiplier, is that correct?)

3.) NEC Alternate method (200.80) comes in significantly lower at 95a. Is this method allowed for my case? Did I calc this correct? If so, with such a difference, even though it may be allowed, perhaps it is not feasible as it will cause frequent tripping?

4.) I'm currently not including the 2 circuits I need to move from the existing sub to the new sub. They are partial 2nd story lighting circuits...how do I add those loads to my calcs?

(I am looking for an electrician to help with this, but just trying to learn here)

Answer 1

1.) In NEC I don't see that you need to account for the 20a bath circuit in addition to the general load calc (3 VA / sq ft) is that correct?

That's correct. For kitchen and laundry, all the 20A circuits dedicated to that room get allocated 1500 VA. The dedicated 20A circuits to bathroom and garage do not require a 1500 VA allocation, and are simply counted in the "3 VA per square foot of habitable space" catch-all.

So if Joe decides to install four kitchen and two bathroom receptacle circuits, exceeding the slumlord bare minimums... then Joe counts 6000 VA (four kitchen circuits x 1500 VA and none for the bathroom). I.E. the number of actual circuits is what counts.

Also note the 20A circuit for garage and laundry are not exclusive the way kitchen and bathroom countertops are. As long as laundry and garage have the one statutory 20A circuit, any random circuits* can also serve those spaces. Code is fine if the washer is served by a bedroom circuit and dryer is on lighting circuits, with the mandatory dedicated 20A circuit serving only a Febreze lol. That's relevant to garage; my strong reco is to have the mandatory 20A circuit serve only one receptacle - positioned to be viable for EV charging. That way it can be trivially converted to 240V for 3.8 kW charging, which will take care of almost all needs for almost everyone (for the exceptions, DC fast charging).

2.) My standard NEC calc, if correct comes to 161a, which is very high (most due to EV and Hot tub), did I calculate this correct? (I included the hot tub and EV and new garage circuit at 100% and add 1.25 safety multiplier, is that correct?)

You have to be careful with the multiplier there not to apply it twice. That has to already be accounted in the size of the circuit breaker. So a hot tub or EVSE on a 60A breaker can only draw 48A actual. Thus a 60A/14400W circuit is providing 11.5kW EV charging, which is the hard max for most EVSEs, and more than most cars can even use*. If you wonder how the car has a say in it, see this.

Everyone's expecting me to give a big lecture on 60A EV charging not being necessary; I'll let Technology Connections do that. But hey -- if you're building, and have a blank-sheet number where you can write any number with no interlocking consequences, why not? It's bonkers for one EV, but get 3 or 4 (or 2 and a time-of-day tariff) and suddenly 60A on Power Sharing just makes a world of sense. Power sharing lets a group of EVSEs share a single current allocation, so if only 1 car is charging it gets the max, otherwise it is split. This calculates on the fly because of EV charging magic.

By "magic" I mean that this is SAE's 4th major attempt at an EV charging standard, and it wasn't designed to sell service upgrades. It was designed to sell cars :)

And yes, it's a standard. Tesla uses it with a different shape connector, and Europe does the same (the shape accommodating two more phase wires, but same protocol).

Note also that "Grid Limiting" is an EVSE feature that allows you to ignore the Load Calculation. It puts a clamp ammeter on the service or feeder wires and tell the EVSE "don't let the feeder amps exceed 80" and it will throttle EV charging accordingly.

When commissioning an EVSE you generally set the breaker size via DIP switches or equivalent, and it automatically figures out the 125% for you (e.g. on a 25A breaker it tells the car to take 20A actual).

* You might think "Oh, as the EV market matures, AC charging will get faster" - nope, they're getting slower. a) EVs aren't luxury anymore, and large chargers make the cars costlier and heavier. b) field data is showing high power AC chargers rarely make a difference. And c) DC fast charging is proliferating, with prices falling - expect home models for "I want the Fastest Charge Possible" types. This is the best of all worlds, and what most people thought they were getting all along.

3.) NEC Alternate method (200.80) comes in significantly lower at 95a. Is this method allowed for my case? Did I calc this correct? If so, with such a difference, even though it may be allowed, perhaps it is not feasible as it will cause frequent tripping?

The deal there is, there's only one NEC. So the standard method has to accommodate everything from laundromats to nuclear power plants. The alternate method is simplified for residential dwellings.

4.) I'm currently not including the 2 circuits I need to move from the existing sub to the new sub. They are partial 2nd story lighting circuits...how do I add those loads to my calcs?

If they serve general lighting or appliance loads that aren't assigned as part of an appliance's calculation, then they fall in the "3 VA per square foot" part of the calc. So the square footage served needs to be allocated to the subpanel.

Answer 2

You need to do a load calculation at each feed:

• Utility feed - 200A - this includes everything in your house or connected to it that is running off of your meter. 200A is a typical feed size (standard in my area). Larger feeds are available but that gets complicated for a bunch of technical and practical (breakers much larger than 200A get very expensive and the feed wires are huge). If there is any capacity remaining then you can look at adding new stuff. If there isn't much capacity remaining then there still may be some solutions, but it can get a little complicated.
• Existing 100A subpanel. Note, this includes any subpanels fed from it.
• New 100A subpanel.

If the existing subpanel only has 50A to spare then it can't very well handle a 60A EVSE plus other stuff.

One specific load is both simple and complicated at the same time: EVSE. EV charging can be done anywhere from 15A @ 120V to 60A @ 240V. The sweet spot is 20A - 30A @ 240V. Why? That's large enough to make a real difference (30A @ 240V is 4x 15A @ 120V) but small enough to not require super-expensive wire (30A uses 10 AWG wire, same as a standard clothes dryer), small enough to often be within the remaining capacity of your utility feed (but only a load calculation will tell you for sure), and also enough to charge a typical EV in a reasonable time. For example, Tesla translates charge rates into "miles of range added per hour of charging". Obviously that's based on normal conditions - if your driving include pulling a trailer or very cold weather then your mileage per kWh will go down and the "miles of range added" will go down as well. But from the Tesla chart, with a Model 3 or Model Y you can typically add 15 miles of range each hour on a 20A 240V circuit and 22 miles on a 30A 240V circuit. If your car charges for 8 hours a night that's 120 - 176 miles of range added each night. If you travel more than 150 miles a day on a regular basis then things are a little different. But most people drive well under 100 miles on a typical day, so even 20A is plenty. Really.

Where EVSE gets complicated, in a good way, is that it can be shared. Harper has written about that, so I won't duplicate it here. But suffice it to say that shared 60A is far more useful than 2 x 30A. But in reality, for most people, shared 40A would actually be enough for 2, and possibly even less than that.

Load calculation is complicated. But don't try to cheat it. It actually accounts for a lot of things that you might want to arbitrarily compensate for. It actually includes some serious limits on cooking appliances, presumably because they discovered that hardly anyone actually runs broil and bake and all the cooktop burners at the same time. Similarly, there are percentage reductions above certain levels of usage. That being said, certain big loads must be included at their full usage level (note, that's actual usage, e.g., 80% of a continuous load, not circuit breaker size). 60A EVSE will add 48A - that's a lot - and it is half of a 100A panel, and if this 100A panel is a subpanel of another subpanel, it is half of both of them.