Residential Service Calculation

[VENgineer]

I have a question. I'm calculating the electrical load for a house of approximately 3,500 sq. ft. To summarize the major loads, there are 2 dryers, 1 range+oven, 1 steam shower (15,000 kW), 12,000 kW in AC equipment (2 units in total), a pool water pump, and an electric car charger, 1 outdoor kitchen on the pool area with diswasher and g.disposal. The water heater is gas-powered.

Using the Standard method, I get a total load of 64,500 VA - 267 Amps, while with the Optional method, I get a result of 43,000 VA - 179 Amps.

My question is, due to the significant difference between both results, which method should I lean towards when calculating the service?

I'm more inclined towards the Standard method in this case because the main difference lies in the shower steamer that the customer wants to install. However, I'm unsure how common a 300A service is for a residence.

What do you advise in this case?

Thank you in advance.

 

[VENgineer]

 

[suemarkp]

Where is the car charger on the Optional calculation? Also, 7000 KVA doesn't seem right for a charger unless you're setting it to 24A and treating it as a continuous load.

Residential 320/400 services are somewhat common. They are easy and relatively cheap to do using a 320/400 meter and two 200A main panels (or a 200 + 100 or other mix).

 

[retirede]

Where is the car charger on the Optional calculation? Also, 7000 KVA doesn't seem right for a charger unless you're setting it to 24A and treating it as a continuous load.

Agree. If you’re providing a 14-50R for the EVSE, it should be 9600.

 

[electrofelon]

One option when you need a service just over 200 is to use a 225 or 250A main panel and a standard meter socket. I believe most (all?) 200A meter sockets are rated 250A max/noncontinuous. 225A load centers are of course pretty easy to get, but sounds like you may need more than 40 spaces. If you wanted 250 you would probably have to go with a true panel board, which might be preferred for this type of place anyway. You could get something like a Siemens P1 with 66 spaces, and a QR main which is a 250A frame, that's what I have at my house. It's a real nice setup. Of course most go with a 320 socket and two load centers for something like this.

 

[wwhitney]

Agree. If you’re providing a 14-50R for the EVSE, it should be 9600.

Seems to me it depends on what EVSE is intended to be installed. You can get 30A, 32A, and 40A EVSEs, all of which, if cord and plug connected, would require a 6-50R or 14-50R. But the 30A EVSE would only require 7200 VA.

Cheers, Wayne

 

[retirede]

Seems to me it depends on what EVSE is intended to be installed. You can get 30A, 32A, and 40A EVSEs, all of which, if cord and plug connected, would require a 6-50R or 14-50R. But the 30A EVSE would only require 7200 VA.

Cheers, Wayne

Taking to the extreme, there’s nothing in the load calculation that says you need to account for installing a random 14-50 in a garage.

 

[Ravenvalor]

View attachment 2567818 View attachment 2567819

If there was a detached garage on the property as well would you add that calculated load to the "OTHER LOADS" column?

Thank you,

 

[nizak]

One option when you need a service just over 200 is to use a 225 or 250A main panel and a standard meter socket. I believe most (all?) 200A meter sockets are rated 250A max/noncontinuous. 225A load centers are of course pretty easy to get, but sounds like you may need more than 40 spaces. If you wanted 250 you would probably have to go with a true panel board, which might be preferred for this type of place anyway. You could get something like a Siemens P1 with 66 spaces, and a QR main which is a 250A frame, that's what I have at my house. It's a real nice setup. Of course most go with a 320 socket and two load centers for something like this.

Is the 250A rating for meter sockets listed right inside the socket?

Hadn’t heard of that until now.

 

[electrofelon]

Is the 250A rating for meter sockets listed right inside the socket?

Hadn’t heard of that until now.

You would thinky so, but sometimes that information is rather hard to find. For example I just looked at the milbank website and their single position sockets say "200 amperes continuous" but they do NOT provide a non continuous rating . I don't recall if the label on the actual units say anything more. Not sure if this is standardized in ul414, the standard for meter sockets....

 

[ptonsparky]

You would thinky so, but sometimes that information is rather hard to find. For example I just looked at the milbank website and their single position sockets say "200 amperes continuous" but they do NOT provide a non continuous rating . I don't recall if the label on the actual units say anything more. Not sure if this is standardized in ul414, the standard for meter sockets....

What would be a non-continues load? Something that runs less than 3 hrs, correct?
Motor loads can easily exceed 200 amps for a brief period of time. Not something I worried about.
The same meter plugs into either base IIRC.

 

[electrofelon]

What would be a non-continues load? Something that runs less than 3 hrs, correct?
Motor loads can easily exceed 200 amps for a brief period of time. Not something I worried about.
The same meter plugs into either base IIRC.

For residential, they're typically wouldn't be much continuous load.

Strange how with class 320 sockets they practically but up billboard signs stating the continuous and non-continuous ratings, but they keep it a secret with 200's.

 

[electrofelon]

I just looked at the cooper B-line catalog. Sorry I'm having trouble posting the link. Many, but not all are rated "250A max, 200A continuous". The ones that are not show 200 in the max column, and nothing in the continuous column.

 

[wwhitney]

Not sure if this is standardized in ul414, the standard for meter sockets....

From UL 414 for single meter sockets:

27.3.3 The continuous ampere rating of a meter socket shall be marked as follows: "____ Amp Continuous."
. . .
27.3.5 If a meter socket is marked with a maximum ampere rating, the complete marking shall appear as follows: "____ Amp (____ Amp Continuous)." The maximum ampere rating shall be 125 percent or less of the continuous ampere rating.

Cheers, Wayne

 

[jaggedben]

I have a question. I'm calculating the electrical load for a house of approximately 3,500 sq. ft. To summarize the major loads, there are 2 dryers, 1 range+oven, 1 steam shower (15,000 kW), 12,000 kW in AC equipment (2 units in total), a pool water pump, and an electric car charger, 1 outdoor kitchen on the pool area with diswasher and g.disposal. The water heater is gas-powered.

Using the Standard method, I get a total load of 64,500 VA - 267 Amps, while with the Optional method, I get a result of 43,000 VA - 179 Amps.

My question is, due to the significant difference between both results, which method should I lean towards when calculating the service?

I'm more inclined towards the Standard method in this case because the main difference lies in the shower steamer that the customer wants to install. However, I'm unsure how common a 300A service is for a residence.

What do you advise in this case?

Thank you in advance.

Based on my experience of never seeing actual max draw over a 15min interval exceed 50% of the optional load calc, I would personally never hesitate to use the optional load calc.

Consider that you're already being conservative because most of your small appliance figures are way too high, and if the house is using any meaningful proportion of modern lighting fixtures (i.e. LED) then the lighting load is also an insane overestimate.

 

[electrofelon]

From UL 414 for single meter sockets:

27.3.3 The continuous ampere rating of a meter socket shall be marked as follows: "____ Amp Continuous."
. . .
27.3.5 If a meter socket is marked with a maximum ampere rating, the complete marking shall appear as follows: "____ Amp (____ Amp Continuous)." The maximum ampere rating shall be 125 percent or less of the continuous ampere rating.

Cheers, Wayne

Great thanks. Kinda interesting, so it seems a meter socket can have ONLY have a continuous rating with that ALSO being the max rating. But once it has a max rating, the 125% applies .

 

[Mr. Serious]

Someone already said this, but I believe your numbers are too high for many of the appliances. I think this is making your standard method calculation come out too high by about 50 amps.

Comments on individual lines:
range hood - not sure if you even have to count it, but I use 180 VA for a regular fan/light range hood. Even if it's a more industrial type, I doubt it would be even half of the figure you used.
bathroom fans - I'm sure I saw some note somewhere saying you don't have to count them, but (if they don't have heaters) they're really only 30 to 50 VA each, not 120. So they don't make much difference whether or not you count them.
dishwashers - I never see a residential dishwasher above 10 amps. I use 1200 VA, not 1800.
disposers - I assume they're 3/4 HP motors and use 690 VA, but they probably really run less than that.
You have a 1000W water heater. I kept that the same. I assume it's a small undercounter unit.
Same with ice machine - I used your number in my calculation, 1000 VA. I worked on a house with an ice machine once, though, and that unit lists an MCA rating of 3.6A at 115V. If we use 3.6A at 120V, that's only 432 VA.
built-in microwave - your number is probably too low - I use 1560.
Air conditioning - your numbers are probably too high, unless the units are oversized or very inefficient. On a similar size house with two older units of not the most efficient design, I had a total of 8880 VA. For your outdoor units, you use the MCA number. For the indoor blowers, I use 6 amps for a normal size unit or 5 amps for a smaller one. It's hard to find an actual amp rating on the indoor unit, because it may change depending on accessories hooked up - UV filter, humidifier, etc.
Pool pump - it's a continuous load, so you can't list it in with the other appliances for the 75% demand factor. Instead, you have to use article 430 methods, 125% of the largest motor, plus 100% of other loads (I usually plug in 750 VA for pool lighting unless you know the number and it's different). However, 125% of largest motor is already taken into account in a separate line of the standard load calculation, so you don't have to consider the pool motor at 125%, unless using this number to size a feeder for the pool panel or something like that.

Question - you seem to list a cooktop of 8 kW and a range of 5 kW? Is that correct? They are both electric cooking appliances, not gas?
If so, then your demand from table 220.55 is 8.45 kW.

 

[Mr. Serious]

If there was a detached garage on the property as well would you add that calculated load to the "OTHER LOADS" column?

I would. Even if you know there's only one person living there and he's not going to use his welder in the garage at the same time as his cooking appliances in the house, that could always change if the house gets sold.

 

[nizak]

I recently went back to a residence that I wired new 2 years ago.

Out of curiosity I read the smart meter to see what type of load had registered.

At the new build I was mulling over what size service was needed . My load calculation came out to around 170 amps and thought I’d install a 320 meter base for future expansion.

At the time the Utility provider told me they could not provide the pad mount transformer for the 320 service so we installed a 200 amp service.

The highest recorded usage on the meter was 19.7 KW. I’m not sure when it was recorded and it probably resets after a period of time.

The house was as follows when I wired it, there may be some additional loads that have been added but couldn’t imagine what.
- detached outbuilding with various power equipment ( including 30A wire feed welder receptacle)
- 40 amp hot tub
-Electric dryer
- 8KW wall oven
- 5 ton total A/C ( 2 units)
- 4300 Sq ft finished space
- (1) Garage heater 7.2 KW
- dishwasher
-disposal
- 1HP pool pump
- 3 garage door openers
- (1) 2 KW under counter hot water heater

Over the years I’ve found that residential consumption is usually always considerably less than actual calculated load.

 

[Mr. Serious]

Yes, if you think about it, utility bills would be astronomical if we used more than a few 10s of amps regularly. If someone used 100 amps for 4 hours per day, plus 20 amps for the remainder of each day, that would be 5760 kWh per month. That's about 5 times my highest usage during the summer when I'm running the A/C all the time.

As electricians, we like to install a service that will meet every need without ever tripping a breaker, but in doing so, we waste a lot of resources installing oversize services. I admit I use the standard method for a new service build. Thinking about the future, it makes my job easier if I come back years or decades later and they want to add a load, and there's still plenty of capacity without upgrading. Over the long term, is that more wasteful than installing a smaller service and then upgrading it decades later when they add a new load? There are a lot of variables to consider when you start asking such questions, and it's easier to just build a new service to meet the standard load calculation result. It's guaranteed to work in maybe more than 99.9% of cases, but that's because it's overkill in more than 90% of them.