Threshold for increasing electrical service
Looking for expert advice geared more towards code compliance (IRC or NEC) as well as best practice. We are renovating our house and I want to take the opportunity to address future electricity needs at this time. The question is how to determine when my electrical service needs to be upgraded.
We currently have a 200A service, but we are moving to more electrical demand. Currently we have the following 240V large electrical loads (in addition to the general receptacle and lighting circuits):
50A – GSHP
30A – Backup resistive heating
50A – Backup resistive heating
50A – EV Car charger
30A – Whirlpool tub
30A – Dryer
30A – Solar Array
30A – Outlet for my business shop in basement
with a handful of 20A 120V outlets for other appliances
Fridge
Dishwasher
Disposal
Garage outlet
Workshop outlets
Shed
Attic vent fan (will likely be eliminating)
Moving forward we might be adding the following:
Electric oven (maybe double)
Inductive cooktop
ERV
Larger EV charger
Possibly larger PV array
Looking at electric point-of-use tankless hot water (but not really expecting to do this)
Electric radiant floor heat in one bathroom
The current 200A service will be sufficient to meet the load in all but the most unlikely cases of overlapping use. However I want to make sure that we are compliant with any requirements and that we are doing the “smart” thing regardless of requirements. I am not sure if the next service size is 400A or something in between.
Everything I can find online is very simplistic and just shows how to add up your loads. The more sophisticated information I have seen shows how to include a usage rating factor for each load to make a more accurate total. Most of this seems to be focused on moving from a 60 or 100A service to a 200A service.
Can anyone chime in on what I should be looking at or what I should be considering? Do the wire count/size and enclosure volume ratios apply to a load center like the do to a switch box?
Thanks
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Replies
You have to do a demand calculation based on continuous loads. For example, even though you have a 50A breaker for your GSHP, I doubt it ever draws much above 3kW to 5kW running full tilt. You can search on-line on how to do this.
Since solar feeds into the house or grid, it does not add to your load but you have to be careful with sizing of the solar circuit. Most panels busbars are limited to 120% of rated capacity, so if you have a 200A panelr, you can only have a 40A PV feed with no more than 80% of that as solar feeding in (so 7.6kW). If you want more PV, the best setup is to upgrade your panel with one of the PV ready ones that have a dedicated busbar for PV connection which avoids this issue. In either case, larger PV (within reason) is usually a panel/breaker size issue not a service size.
Having 65000BTU of backup electric heat seems on the excessive side, I would check you actually need this. Electric heaters count 100% towards your demand, you only want to have as much as you actually need, in most cases you can reduce the number of elements connected to get a lower capacity.
Electric tankless is probably the one that can tip you over into oversized feeds, unless there is a good reason for this, I would stick to tank type units. You can even get small plug in point of use tanks fed by the larger main water tank if you want instant hot water.
Akos, yes you are correct about the GSHP. 6kW is about the most I have ever seen it pull in second stage heating towards the end of the season when water loop temps are at their worst.
The backup is is never used unless the GSHP is not working for some reason. So when it's used, it's used to heat the whole house until the heat pump is fixed. But your point remains, it could be reduced in size and run for longer when needed.
As for the PV, I'm not sure I understand. Actually, I'm sure I don't understand.
We currently have a tankless natural gas HW and a whole house electric version is totally unrealistic so, like you said, I am looking at point-of-use units to either replace the NG unit and simply boost the tank from the desuperheater or keep the NG unit and boost with POU or use an electric tank with POU units. Problem is, I don't have a good estimate of HW production from the GSHP in the winter. Summer production is good.
If you don't run the electric heat the GSHP at the same time, you use the heat source with the higher load for the demand calculation. For the GSHP you use the rated max power of the unit, which will most likely be higher than the 6kW you see.
For the solar limit, read here:
https://www.altestore.com/blog/2018/11/grid-tied-solar-breaker-box-120-percent-rule
Summary, the simple way to get more PV onto a standard 200A panel is to change the main to 175A. This does mean that your total demand must be bellow the new breaker size.
For hot water, instead of the tankless you can put either a standard resistance tank or a HPWH in series with the de-superheater. This way if you have plenty of de-superheat, the electric tank will not barely run, just enough to bring the water temperature above pasteurization levels. Not much point for point of use units unless you have a very sprawling layout with very long runs, even than recirc is probably simpler.
It looks like you're using breaker ratings rather than actual load ratings. Definitely not how it's done in Canada, doubt it's how it's done in the US either. For example, if the heaters you have hooked up to the 50A breaker are 10kW, that is a 41.7A load. Things like water heaters are considered on a duty cycle basis. I think here you count it as 50% of its rated power. It shouldn't be too hard to find a residential load calculation worksheet that lays it all out.
Yes, I'm simply listing the breaker ratings. The appliances themselves don't use anywhere near the breaker ratings.
Going through all the loads I can come up with a pretty good estimate of what our actual usage is in terms of max load, or average load. It's really unlikely that our house would be using more than 200A even with some of the additions I mentioned.
My question is more about what are the governing regulations? Who determines required service size? Is it the utility? Is it code enforcement? And depending on the answer, what are the criteria?
The sizing is done by the electrician installing the service. They need to do a demand calculation based on house size and installed equipment to come up with service size.
This is for Canadian code, but I'm sure you can find similar one for your local code:
https://douglashelmer.com/calculators/electrical/residential-service/index.htm
It should be covered in the NEC, but I'm just extrapolating from my experience in Canada. The codes are very similar. We have a simplified code book you can buy for a reasonable price that walks you through determining the load. I'd be surprised if you didn't have something similar there.
There is a very usable calculation checklist in "Code Check Electrical," sold in Home Depot and elsewhere. These flip books are designed for inspectors, but are a great general resource for all things code related. Everything is simple to understand, with charts and pictures. Also has references to the NEC so that you can cross-reference if necessary.
Seem to me upgrading you service is a must do given your want to do list.
You will likely end up with 320 amp service feeding two 200 amp panels.
If Solar and or a backup generator are in your future now is the time prepare for them.
Walta
I would not call the service upgrade a "must". Service upgrades are very expensive and unless you actually need the power there is really not point for it. Do the demand calculation and see where you are at.
Demand calculations are pretty conservative, I've done demand calcs for all electric places with heat pump space heat that came in around 75A with actual real world peak from the power meter was around 45A. So even if your demand calculation with the new loads is close to your service size, I wouldn't worry about it.
We have a 200 amp service fed to a pedestal with meter, 200 amp breaker, and then a breaker that feeds the pool panel. We have a 200 amp panel in the house with a 90 amp sub panel. We just converted our kitchen to all electric and it's very tight in the main 200 amp panel as far as the number of wires. We also have two ev chargers, and three heat pumps. We haven't had any load issues but we do have one of the cars set to charge on the middle of the night. When we redid the electric service 10 years ago, I wished we had put in another subpanel just to keep the wiring clean. At this point we'll just live with it as the construction is finished and everything is working fine.
As others have said, you don’t calculate loads from breaker ratings, you use “nameplate” ratings instead, which are the loads printed on the nameplates for connected devices. A typical electric resistance hot water heater, for example, will be 5,000 watts at 240 volts, but will be fed from a 30A breaker. A 30A 240v circuit is 7,200 watts if fully loaded, but the water heater will never draw that much.
There are some other things that come into play too. You probably won’t be able to use demand factors though, since you don’t have a large enough number of devices for that. Your 20A branch circuits will typically count for very little overall load. You biggest loads are likely to be your heat pump, car charger, and resistive heating. Keep in mind that your heat pump and resistance heat are unlikely to be running at the same time here.
My guess is you’ll be fine with a 200A service — almost everyone is. The easiest was to put in some future proofing if you want to keep your options open down the road is to run suitably sized conduit for larger conductors in the future. I would NOT consider an electric resistance hot water heater — they don’t play nice with the electric system; and they will actually increase your emissions in almost all areas if you are trying to reduce your overall emissions. Stick with natural gas for this particular device.
The next step up from a 200A service is 400A, but that’s usually limited to 320A by the meter can. Utilities usually feed these services from either a 25 or 50 KVA transformer, which is only good for 104A or 208A anyway.
Bill
Use a spreadsheet for residential load calculations based on code. Here's an example:
https://www.mikeholt.com/documents/calculations/formulas/ResidentialLoadCalculations.xls