Electrical load calculation

[Electriman]

Hello,

I was wondering when I do the load calculation for a house that has a pool and also EV charger, if I apply 40% to the pool and EV charger when I use optional method. The other way is to consider it 100% similar to HVAC equipment.

Thanks

 

[wwhitney]

Hello,

I was wondering when I do the load calculation for a house that has a pool and also EV charger, if I apply 40% to the pool and EV charger when I use optional method.

The text of the NEC allows that for the optional method.

The other way is to consider it 100% similar to HVAC equipment.

There is no requirement in the NEC to do that, although some jurisdictions may require it. I would say that if they haven't formally amended 220.81/220.82, they would be in error to require it.

For EVSEs, this may change in the 2026 NEC.

Cheers, Wayne

 

[Electriman]

The text of the NEC allows that for the optional method.

There is no requirement in the NEC to do that, although some jurisdictions may require it. I would say that if they haven't formally amended 220.81/220.82, they would be in error to require it.

For EVSEs, this may change in the 2026 NEC.

Cheers, Wayne

Thanks. Any other comments?

 

[Elect117]

I think from a design perspective not considering the EV as 100% is frivolous. Especially if it is a 50A charger on a 100A panel.

I also, do not interpret the code in the same way Wayne does for the optional method since it doesn't cover a majority of equipment that would elsewhere be considered at 100% if it wasn't a dwelling unit. The term "appliance" is being loosely applied to the EV chargers and other equipment not seen in 422. EVSE is considered to be capable of being a load and a source. It is a big "boo boo" by the code making panels. It deserved to be in the 400s but the manufacturers wanted to back feed into the house as a selling point. 90% or higher ignore the backfeed requirements since they do not always apply to the install. But since they can, the code puts it in special equipment. You will also find that the other 600s and 400s sections do not fit into the mold of the optional method. So in my opinion, you need to consider it as an EVSE and calculate it as a continuous load, based on 625.

Which appears to be the intent based on some of the revisions in the 2026.

 

[wwhitney]

I think from a design perspective not considering the EV as 100% is frivolous. Especially if it is a 50A charger on a 100A panel.

Frivolous? I can see saying it is non-conservative, but I don't see what's frivolous about following the rules as written.

As to non-conservative, the incremental effect is clearly non-conservative. Adding an EVSE can actually boost measured current on the service by the full amount of the EVSE rating.

However, the rest of 220.81/220.82 is in fact overconservative. It is surprising when the actual load on a residential service without EVSE is more than half of the calculated load. So in practice, for the typical case of adding a single EVSE, it will not tip the balance to the point where the actual measured current on the service exceeds the load calculation. There is no problem with the overall calculation in almost all cases.

Now, obviously we can construct examples where that is not true. Say a small house with a 125A service and a load calculation of 80A, which actually sees a peak current of say 40A. If we add (3) 32A EVSEs, that will increase the load calc from 80A to 118A, so still allowed on the 125A service. But if we are actually charging 3 EVs at 32A each while the rest of the service hits its peak load, that would be 136A on the service, too much. So this is a rare example where we should use our judgement and not do the minimum the code currently allows.

As to the 2026 First Draft, it recognizes that the rest of 220.81 was overconservative. The portion of the load figured at 100% was dropped from 10 kVA to 8 kVA, which is a reduction in the calculated load of 1.2kVA, or 5A. And the load per sq ft for general lighting and general use receptacles was dropped from 3 VA/ft^2 to 2 VA/ft^2. For a 2400 sq ft house, that's another reduction of 960VA (after the 40%), or 4A. Together that means that for a 2400 sq ft house with a single 32A EVSE, the calculated load will only be 10A higher under the 2026 First Draft.

Cheers, Wayne

 

[Elect117]

Frivolous? I can see saying it is non-conservative, but I don't see what's frivolous about following the rules as written.

Commeee onnnnnn, your own example with the 3 chargers shows that it is silly to not consider it, from a design perspective, at 100%.

I disagree with the take that NEC, in it's existing state (2020), allows for the demand factor to be applied to EVSE. That isn't how I read it. It isn't specific and I can see how people would make that decision.

I have never liked 220 as a way to calculate. With efficiency standards being what they are, and most people being on LEDs, it is also frivolous to assume such high values. But I will not budge that some loads need to be considered at 100%. Demand factors to mostly everything make sense except for EV chargers, washer, dryer, A/C unit / heater. Use 220.87 if you need the exact number to use. If not, then make due with standard or optional method but keep the EV charger, washer, dryer, AC unit / heater out of the reduction.

 

[wwhitney]

Commeee onnnnnn, your own example with the 3 chargers shows that it is silly to not consider it, from a design perspective, at 100%.

I reach the opposite conclusion, that example is the exception that makes the rule.

I disagree with the take that NEC, in it's existing state (2020), allows for the demand factor to be applied to EVSE. That isn't how I read it. It isn't specific and I can see how people would make that decision.

220.82(A) is very specific. It says "The calculated load shall be the result of adding the loads from 220.82(B) and (C)." So which subitem in 2020 NEC 220.82(B) or (C) says to include the EVSE load at a factor of 100%?

The only category the EVSE can fall into is 220.82(B), and all of those loads get a factor of 40%.

But I will not budge that some loads need to be considered at 100%. Demand factors to mostly everything make sense except for EV chargers, washer, dryer, A/C unit / heater. Use 220.87 if you need the exact number to use. If not, then make due with standard or optional method but keep the EV charger, washer, dryer, AC unit / heater out of the reduction.

220.82 explicitly says in 220.82(B)(3) that the 40% demand factor gets applies to clothes dryers. So your proposal clearly exceeds what the NEC currently requires.

Cheers, Wayne

 

[ramsy]

I also, do not interpret the code in the same way Wayne does for the optional method since it doesn't cover a majority of equipment that would elsewhere be considered at 100% if it wasn't a dwelling unit. The term "appliance" is being loosely applied to the EV chargers and other equipment not seen in 422.

How to calculate a new ESVE load is clear for existing buildings, regardless if not defined as an appliance.

220.16(A)(2)
Loads for new circuits or extended circuits in previously wired dwelling units shall be calculated in accordance with 220.14.

220.14(A) Specific Appliances or Loads.
An outlet for a specific appliance or other load not covered in 220.14(B) through (M) shall be calculated based on the ampere rating of the appliance or load served.

While 220.14(A) wont be limited to appliances, the 625.42 rating determines load calc for ESVE not covered in 220.14(B-M)

625.42 Rating
..Electric vehicle charging loads shall be considered to be continuous loads for the purposes of this article. Service and feeder shall be sized in accordance with the product ratings.

220.14(A) punts to 625.43 Rating of ESVE, which is 125% for branch, but only nameplate (product rating) for service / feeder.

220.14(A) and 625.43 can only define rated ESVE load, then punt that to 220.83(A) with 100% <= 8kVA => 40% demand factor.

Unless of course the "actual max demand" is used in 220.87 to bypass all load calcs.

 

[Elect117]

"
220.40 General.
The calculated load of a feeder or service shall not be less than the sum of the loads on the branch circuits supplied, as determined by Part II of this article, after any applicable demand factors permitted by Part III or IV or required by Part V have been applied.

220.82 (B) General Loads.
The general calculated load shall be not less than 100 percent of the first 10 kVA plus 40 percent of the remainder of the following loads:

"

Since it is not explicitly mentioned, it is not applicable. All demand factors are permitted and not required to be applied. Meaning, you don't have to use them and shouldn't if it is not explicitly mentioned.

The optional method is just that. Optional. You can use the standard method and the optional method, include the EVSE at 100% and make your call from that.

220.82 explicitly says in 220.82(B)(3) that the 40% demand factor gets applies to clothes dryers. So your proposal clearly exceeds what the NEC currently requires.

I know the optional method includes it. I think the standard method handles it better. But like I said, these are all just my opinions.

I think we have all said our piece in other posts on the same subject. The OP asked for other input and I wanted to provide the counter argument to including it. The OP and the AHJ can sort out their interpretations.

 

[wwhitney]

220.40 General . . . 220.82 General . . .

220.82 also says "It shall be permissible to calculate the feeder and service loads in accordance with this section instead of the method specified in
Part Ill of this article." So when choosing to exercise the option that 220.82 provides us, we may ignore all of Part III of Article 220; we don't even have to read it. 220.40 is in Part III, so is irrelevant in determining what 220.82 requires or allows.

There is no enforceable language in Article 220 that requires EVSEs to be factored at 100% when using the optional method in 220.82. You can reasonably argue that's an oversight in the NEC, and the CMP who made a change to the article for the 2026 First Draft apparently agrees with you. But until that 2026 NEC is published and adopted by a jurisdiction, or a jurisdiction otherwise amends Article 220, that is the current state of play.

Cheers, Wayne

P.S. Earlier in the thread I referred to 220.81/220.82 in error, I meant 220.82/220.83.

 

[brycenesbitt]

There is no enforceable language in Article 220 that requires EVSEs to be factored at 100% when using the optional method in 220.82. You can reasonably argue that's an oversight in the NEC, and the CMP who made a change to the article for the 2026 First Draft apparently agrees with you.

Agreed.
If there ever was a load for the 100% rule, it's an EVSE load.
Having measured various loads in operating buildings, the load calculations are way conservative.... except for EVSE.

 

[Birken Vogt]

Agreed.
If there ever was a load for the 100% rule, it's an EVSE load.
Having measured various loads in operating buildings, the load calculations are way conservative.... except for EVSE.

Well if the 100% EVSE is mixed in with all the other conservative calcs, the end result is the calc will be a bit closer to reality and not completely overblown as usual.

 

[wwhitney]

Well if the 100% EVSE is mixed in with all the other conservative calcs, the end result is the calc will be a bit closer to reality and not completely overblown as usual.

I assume you mean "if the EVSE factored at 40% is mixed in with all the other conservative calcs, . . ." In which case I agree for the typical case that the total EVSE load is not disproportionate to the rest of the load.

Cheers, Wayne

 

[don_resqcapt19]

I think from a design perspective not considering the EV as 100% is frivolous. Especially if it is a 50A charger on a 100A panel.

I also, do not interpret the code in the same way Wayne does for the optional method since it doesn't cover a majority of equipment that would elsewhere be considered at 100% if it wasn't a dwelling unit. The term "appliance" is being loosely applied to the EV chargers and other equipment not seen in 422. EVSE is considered to be capable of being a load and a source. It is a big "boo boo" by the code making panels. It deserved to be in the 400s but the manufacturers wanted to back feed into the house as a selling point. 90% or higher ignore the backfeed requirements since they do not always apply to the install. But since they can, the code puts it in special equipment. You will also find that the other 600s and 400s sections do not fit into the mold of the optional method. So in my opinion, you need to consider it as an EVSE and calculate it as a continuous load, based on 625.

Which appears to be the intent based on some of the revisions in the 2026.

It has always been in Chapter 6, starting with the 1996 code when Article 625 first appeared in the code, long before any electric vehicle was thought of as a possible source for the house or grid.

 

[Elect117]

It has always been in Chapter 6, starting with the 1996 code when Article 625 first appeared in the code, long before any electric vehicle was thought of as a possible source for the house or grid.

That is interesting. I was talking with someone about it at a conference and they mentioned that the back feeding was part of why it could never be considered an appliance or exist in the 400s. I just assumed that is how it ended up in the 600s.

 

[brycenesbitt]

That is interesting. I was talking with someone about it at a conference and they mentioned that the back feeding was part of why it could never be considered an appliance or exist in the 400s. I just assumed that is how it ended up in the 600s.

I've seen a ton of press in the last days, about EVs being used in the hurricane areas to operate neighborhood freezers and fridges, and generally patch over the massive grid outages. The press could accelerate vehicle to grid (V2G) or vehicle to home (V2H).

 

[don_resqcapt19]

I've seen a ton of press in the last days, about EVs being used in the hurricane areas to operate neighborhood freezers and fridges, and generally patch over the massive grid outages. The press could accelerate vehicle to grid (V2G) or vehicle to home (V2H).

I would love to know the percentage of EV chargers that are actually set up to operate bidirectionally..I would bet it is not very high.

 

[retirede]

I would love to know the percentage of EV chargers that are actually set up to operate bidirectionally..I would bet it is not very high.

Most of the examples I’ve seen are F150 Lightnings with the ProPower onboard generator. Bidirectional EVSE not required.

I saw where one guy was running his essentials and his neighbor’s. He has to drive 25 miles to a Tesla Supercharger every 3 days to recharge. He’s fortunate to have a charging station that close that actually has power.