Load calculation for addition

A

Alan2025

Member
Location
East Bay San Francisco
Occupation
Electrician retired
I am building an addition to a cabin that I own in Idaho. I am adding a detached garage with a living unit above. The cabin is 860 square feet with a 300 square foot loft. The addition will be 630 sq. ft. garage (unheated) + 630 living space sq. ft.. Both cabin and garage/living unit will be ALL electric.
I want to make sure that I figure my loads correctly. Another set of eyes, please.
Cabin load -
Baseboard heaters 3-1500 w, 1-1000w, 1-500w, Hot Water Heater-3,800w, Refrigerator 20 cu ft. , LED lighting and 20 amp receptacles (per code)
Garage/living space addition load-
Baseboard heaters- 2-1500w, 1-1000w, 2-500w, Hot Water Heater-3,800w, Refrigerator 20 cu ft. , LED lighting and 20 amp receptacles (per code)
I have a 200 amp service panel that feeds a 200 amp sub-panel. I am thinking of replacing the 200 amp sub-panel main with a 100 amp breaker (I want to avoid coming off the sub panel due to access issues. It can be done but I would prefer coming off the service panel.
From the service I can feed the sub-panel in the garage with a 70 amp breaker or I can use a sub feed lug to a 100 amp sub-panel. Everything is Square D.
 
So a 200A service hits an existing dwelling, and now a feeder is to be installed to supply a new accessory structure off that service.
Both buildings will be supplied by the same service.
So is question then asking if the service is adequate for the new load and
what is the minimum required feeder to the new accessory structure?

We know the cabin is a single family dwelling, but
is the living space over the garage a 2nd dwelling unit or guest suite?
Dwelling Unit:
A single unit, providing complete and independent living
facilities for one or more persons, including permanent provisions
for living, sleeping, cooking, and sanitation.

Guest Suite:
An accommodation with two or more contiguous
rooms comprising a compartment, with or without doors between
such rooms, that provides living, sleeping, sanitary, and storage
facilities. (no permanent provisions for cooking)
 
It is a guest suite, not a separate dwelling unit.

"So is question then asking if the service is adequate for the new load and
what is the minimum required feeder to the new accessory structure?" YES
 
retirede said:
Why would you mess with the subpanel at all? As long as you can feed to new building from the service, you’re golden.
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retirede said:
My comment was based on this part:
“Replacing the 200 amp subpanel with….”
Click to expand...
"I am thinking of replacing the 200 amp sub-panel main breaker with a 100 amp breaker"
I am placing the 100 amp subpanel in the garage just under the living unit to make the home runs shorter and more convenient for a guest to reset any tripped breakers. Thanks for the help
 
Alan2025 said:
It is a guest suite, not a separate dwelling unit.

"So is question then asking if the service is adequate for the new load and
what is the minimum required feeder to the new accessory structure?" YES
Click to expand...
With the info given 100A feeder off a 200A service seems fine.
The reason I ask about the guest suite is since it is a separate structure it may be considered separate from the 'dwelling unit' and certain optional load calcs may then not be available, but your load is so low it does not matter.
Even if you had electric cooking or washer / dryer in both units it would probably be fine.
 
retirede said:
That’s my point…..WHY?
Click to expand...
Exactly, Why?
No need to mess with the existing setup if the OP can feed from the service panel to a Sub-panel in the new added garage. The existing 200A Sub in the main bldg. if feed with properly sized conductors for 200A can remain untouched. If loads combined with the new don't exceed the 200A service the wouldn't be a problem with even the 100A Sub to the garage. Worst that would happen as long as all the wire sizes are correct is a tripping service panel main breaker (unlikely given loads being suggested). But that is why you do the load calculations in the first place.

While many like to "future proof" an installation there is no requirement to do so, and around here the POCO won't even allow a calculation that has some sort of indefinite future proofing.
 
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