How different are actual and calculated loads? Case study.

B

brycenesbitt

Senior Member
Location
United States
I did a quick load study for a multifamily apartment. Here's the 2 bedroom unit load calculation, alternative method, based on the 2023 NEC:

NEC Section §220.82 Determining Existing Loads : Two Bedroom Units#xtotal
General light, power @3 volt-amperes75032250va
Kitchen appliance circuits (min 2 @ 1,500 va)150023000va
Laundry circuits (minimum 1,500 va)150000va
Dryer (NP rating)720000va
Electric range (12.1 kW table 220.55)800018000va
Wall mounted oven (NP rating)00va
Dishwasher (nameplate)144011440va
Disposal (nameplate)114011140va
Other720000va
first 10k of load10000va
40% after first 10k2332va
heating load4800419200va
single unit load31532va
convert to amps240131A

The building was built in 1971.
Anyone care to guess what the breaker size was, and what actual measurements showed ?
I have 15 minute data, and will later have 1 second data. San Francisco Bay Area. 14 total units, mix of bedroom sizes.
 
I like Winnie's guess a lot. Maybe slightly higher unit peak load, if you cherry pick for the unit where the people run the dryer at the same time of day that they use the oven.
 
A typical 2 bedroom apartment would have (1) 4' BB heater in each bedroom and (1) 8' heater for living area. This would be 4,000va of heat.

If using 220.82 the range should be calculated at nameplate.

If the heat source is multiple heaters, such a baseboard heaters a 65 or 40 percent demand factor can be applied.

Existing dwellings can also be calculated using 220.83
 
curt swartz said:
There is no way there is 19.2KW of heating load for a 750sqft apt in the SF Bay Area.
Click to expand...
Yeah, that struck me as odd also.
According to the owner each 2br has four baseboard heaters, each nameplated at 4800 Watts on 30A breakers.
That does seem high, I'd expect more like 3000 watts total.

===============================================================
Laundry is central, so no washer, no dryer.
Note on the load sheet quantity of dryers is zero.

Each 2BR unit has: dishwasher, disposal, 4 heaters, range, lights, receptacles and SABCs.
No owner supplied A/C and no window units were observed.

Here's an updated load sheet: thanks @curt swartz
Looks like 220.82 gives a 40% factor for four heaters, but 220.83 does not, if I'm reading it right:

§220.82 for Two Bedroom Unitsva#total
General light, power @3 volt-amperes750100%32250va
Kitchen appliance circuits (min 2 @ 1,500 va)1500100%23000va
Laundry circuits (minimum 1,500 va)1500100%00va
Dryer (NP rating)7200100%00va
Electric range (Using 220.55(C))12100TABLE18000va
Dishwasher (nameplate)1440100%11440va
Disposal (nameplate)1140100%11140va
Other7200100%00va
First load10000100%10000va
Remainder of load40%2332va
heating load [ Reduction per 220.82(C)(5) ]480040%47680va
single unit load20012va
convert to amps24083A
§220.83 for Two Bedroom Unitsva#total
General light, power @3 volt-amperes750100%32250va
Kitchen appliance circuits (min 2 @ 1,500 va)1500100%23000va
Laundry circuits (minimum 1,500 va)1500100%00va
Dryer (NP rating)7200100%00va
Electric range (Ignore 220.55(C))12100100%112100va
Dishwasher (nameplate)1440100%11440va
Disposal (nameplate)1140100%11140va
Other7200100%00va
First load8000100%8000va
Remainder of load40%4772va
heating load [ reduction per 220.83(B) if (B) applies ]4800100%419200va
single unit load31972va
convert to amps240133A


@winnie guesses are mostly low. Next contestant :) ?
 
220.55(C) does not apply when using the optional calculations (220.82 & 220.83). Nameplate needs to be used.

For the heat you need to verify what is really there. There is no such thing is a 4,800va electric baseboard heater. That would be 20' long.
If there are 4 heaters I'm guessing each bedroom has a (1) 4' and the living room has (1) 4' and (1) 6'. Could be slightly different but not by much.
 
@curt swartz that owner is out of town, and I did not get a nameplate picture.
So I'll insert some placeholder numbers for the baseboard heaters, as you suggest 250W per linear foot, 6 foot units:

§220.82 for Two Bedroom Unitsva#total
General light, power @3 volt-amperes750100%32250va
Kitchen appliance circuits (min 2 @ 1,500 va)1500100%23000va
Laundry circuits (minimum 1,500 va)1500100%00va
Dryer (NP rating)7200100%00va
Electric range (Do not use 220.55(C))12100100%112100va
Dishwasher (nameplate)1440100%11440va
Disposal (nameplate)1140100%11140va
Other7200100%00va
First load10000100%10000va
Remainder of load40%3972va
heating load [ Reduction per 220.82(C)(5) ]150040%42400va
single unit load16372va
convert to amps24068A
§220.83 for Two Bedroom Unitsva#total
General light, power @3 volt-amperes750100%32250va
Kitchen appliance circuits (min 2 @ 1,500 va)1500100%23000va
Laundry circuits (minimum 1,500 va)1500100%00va
Dryer (NP rating)7200100%00va
Electric range (Do not use 220.55(C))12100100%112100va
Dishwasher (nameplate)1440100%11440va
Disposal (nameplate)1140100%11140va
Other7200100%00va
First load8000100%8000va
Remainder of load40%4772va
heating load [ reduction per 220.83(B) *]1500100%46000va
single unit load18772va
convert to amps24078A
 
Last edited:
The measured data:

The building has 600A main and 90A breakers for all unit types. We focused on the 2BR because of anticipated higher load>
Water heating is central, and gas based. There's likely no insulation, and windows are single pane. No solar. No EVSE.

We got daily utility data for the all the meters aggregated, which is consistent with the electric baseboard heat and local climate:

energy-use-by-calendar-m (1).png
For a peak of 6,716 kwH for 12/31/2023 a day on which about 6% of the 600A main capacity was used on a 24 hour basis.



The unit load we plotted as a scatter plot:
chart (1).png
That's two years of data for 4 units one dot for every 15 minute segment.
You'll note the pink unit apparently goes on vacation at the same time each year in January.

The highest 15 minute segments are about 3.5 kWh used in 15 minutes.
Neither of these are worst case demand factors, but hint at peak demand.
Other data we have suggests that one minute demand is about 40% higher than 15 minute demand.
 
Last edited:
Ok, so 90A per unit, and a 15 minute peak of 60A on one unit.

The 6176kWh was for a month, not a day, correct? So a building average of about 35A over a month.

I'd still guess a 15 minute peak for the whole building of less than 200A.
 
That’s why we would put in a 4/0 UG service to some of the apartment complexes.
Also the reason we would put #2 to feed a 200 or even 320A panel.

Well, when I worked there we did it that way. This retirement life is awesome!
 
winnie said:
Ok, so 90A per unit, and a 15 minute peak of 60A on one unit.
The 6176kWh was for a month, not a day, correct? So a building average of about 35A over a month.
I'd still guess a 15 minute peak for the whole building of less than 200A.
Click to expand...
Correct: that's a month figure. Unfortunately the source I have does not give daily data, but you can see the trend ramp clearly:
@winnie

From To kWh $
5/1/2023 5/31/2023 2,710.57 837.26
6/1/2023 6/30/2023 2,308.06 775.02
7/1/2023 7/31/2023 2,300.65 830.01
8/1/2023 8/31/2023 2,201.05 794.36
9/1/2023 9/30/2023 2,218.23 776.31
10/1/2023 10/31/2023 3,007.45 1,004.68
11/1/2023 11/30/2023 5,430.6 1,923.71
12/1/2023 12/31/2023 6,715.75 2,506.33

6715 kWh/31 day = 224 kWh/day. 224 kWh/Day / 24 hours/day = 9.33 kWh/hour. 9.33 kWh/hour / 240V = ~ 39 amps.

The 58A peak at 15 minute data shows that we can readily use a little bit of load shifting and shaping, to lower peaks, and do additional electrification. Certainly swapping resistance heaters for heat pumps is possible. But adding loads can be done also, especially if they are agile. The mix between the range and wall heaters is not known, but the time lag on the wall heaters means they could be load managed likely. Or, insulate the attic and lower the heating load a ton.

I could probably project the 15 minute building load from the 15 minute unit data: the units are pretty consistent.
Your guess of under 200A is probably about right. I'll be able to instrument this building and then could get 1 second data.

Similar units in other buildings with 60A service do fine as well.
 
You must log in or register to reply here.
Top