Service Size for Home and also Cottage

Status
Not open for further replies.
M

mkgrady

Senior Member
Location
Massachusetts
I know how to size a service for a single family home but what changes when the home's main panel also feeds a small cottage on the same property? I'm somewhat familiar with the main house which has a 200 amp service. The cottage is fed with a 40 amp breaker which I guess is not really relevant. I will not see it until tomorrow.

I'm a little stumped. Do I just add the square foot living space, add two SABCs, laundry, appliances, etc.? In other words do I just plug in all the added loads to the spreadsheet to see if the 200 amp is big enough? Something tells me it's more complicated.
 
Is the cottage a "dwelling unit" on its own right? Does it have provisions for cooking, sleeping, and sanitation? If so, then calculate it like the main house. Be advised that there is a minimum rating for the disconnect, regardless of the calculated load. I don't have time to look it up right now. But if the total load is below 40 amps, you can feed the cottage with a 40 amp breaker from the main house. But you will still need a main disconnecting means for the cottage itself, and I believe that has to be at least 60 amps.
 
charlie b said:
Is the cottage a "dwelling unit" on its own right? Does it have provisions for cooking, sleeping, and sanitation? If so, then calculate it like the main house. Be advised that there is a minimum rating for the disconnect, regardless of the calculated load. I don't have time to look it up right now. But if the total load is below 40 amps, you can feed the cottage with a 40 amp breaker from the main house. But you will still need a main disconnecting means for the cottage itself, and I believe that has to be at least 60 amps.
Click to expand...

I figure it must have all the things that make it a dwelling because a person lives there.

For now I'm assuming the cottage has the proper wiring and service size. My bigger concern is how to calculate the load as it relates to the main house service size. The customer is adding a hot tub. If it were not for the load of the cottage the main house service size would be ok. The cottage load could push them past the 200. Are you saying just add the loads to the calculation?
 
If you want to determine if the 200 amp main house service is still large enough you'll need to recalculate adding all of the new loads that you plan on connecting.
 
I misunderstood. I thought this was a new building project. If you are only adding one load, a hot tub, to an existing system that has the house loads and the cottage loads, then you have two choices. One is what Rob has already given. Start from scratch, calculate the two dwelling units separately, include the hot tub, and see what you get. The other option is to use the existing load method of 220.87. If the owner has a year's worth of utility bills, and if those bills show peak demand each month and not just total energy used, then you can use that as the basis for determining whether there is spare capacity to add the hot tub. Or you could measure the loads in 15 minute intervals for 30 days. My guess is that either method would tell you it is OK to add the hot tub.
 
I went to the site today to check out all the loads. Counting the house and the cottage I calculate 202.7 amps. That includes doubling the SABC's, the first 3000 watts of each dwelling at 100%, two laundry circuits, two electric dryers, both AC systems, all appliances, etc.

I'm going to suggest they change one of the two dryers to gas which will reduce the calc by 20 amps. Replacing the service will be expensive as it is underground, it will require a meter/main, and the service enters the home through a finished room that has a stone exterior wall.

Is it fair to say I can not use the optional service calculations found in 220.80? I'm thinking it does not apply because the service is feeding two dwellings.

I am not able to see what the past years usage has been because there is no demand meter, just total KW used each month.

I've never done the optional service calculation. Does it generally result in a smaller service size calculation? It won't matter if I can't use it but I'm curious
 
I think you treat any common supply conductors same as you would if you were feeding say a two family dwelling from a single supply. Total VA for all the square footage, at least 2 SABC's per dwelling unit, use demand factors for two ranges, two dryers (if that is what you have, but use demand factor of one range/dryer for the feeder that only supplies one unit).
 
Status
Not open for further replies.
Top