Electrical is one of the major trades or sub-contractors involved in almost any project.
We recently clarified one pesky nuance about the building code in California as it relates to what size electrical panel and service you need for an accessory dwelling. You can build an accessory dwelling with a 100 amp panel but a smaller service within that panel or 60 amps. Doing this can save thousands.
This is a brief summary and the actual electrical needs of any project will surely vary based on your home design, the demand for electricity and the always changing discretion of the local inspector.
There are three common ways we’ve seen electrical service play out:
1) No major change to the main house and a well designed smaller panel in the office or accessory structure you are building. This can run $2,000 or less.
If your main home’s panel looks like this then you may be able to keep it and just add another wire from this box back to your accessory structure. You can count the space for 14 breakers or 14 circuits and that is typically adequate for a main home and second home. Each white sticker is one breaker.
The new smaller interior panel in your office or accessory structure will probably look like one of these smaller “40 amp” or “60 amp” panels. You can see there are eight breakers or circuits for the electrical in the new accessory structure or dwelling.
This is a great solution if you know that your new accessory structure won’t have a large demand of a larger house. However this does not meet code for a “DWELLING”. So even if you have a small dwelling with trivial electrical demand, these smaller panels are not allowed. There’s a trivial savings to these smaller interior panels though so we always recommend that you get a bigger one so there is room to grow.
2) A major overhaul to your main home electrical panel If you’ve designed a new 100 amp panel in the accessory dwelling you will most likely need to increase your main home panel to 200 amps so that there is enough power for 100 amps to and possibly install a new electrical wire from the street to the main house. This can easily cost $3,000 or more for the new panel on the main home, $2,000 for a wire from the street and $2,000 for the accessory dwelling electric subpanel.
If your main home panel looks something like this then you are in for a lot of new work no matter what. You’ll have to replace the main home panel, possibly the wire from the street and then still add the wire to the accessory dwelling and the new panel for that home.
3) No major change may be needed for the main house if it already has a 100 amp service or more but you can still add an accessory dwelling. This can spare you the cost of $3,000-$5,000 in work to get your main home up to current standards. You’ll have to pay $2,000 or more for the accessory dwelling.
In this scenario you are lucky to have a big enough 100 amp panel on your main home to pull a 60 amp service off of it. You’re typically allowed to have 60 amps for the main home and another 60 amps for the accessory dwelling. 60+60=120 which is over 100 but that’s ok because both homes aren’t expected max out their full 60 amp demand at the same time.
But, and this is a big but, don’t add a 40 of 60 amp panel (the actual size of the gray box) to your accessory dwelling because code requires that you have a 100 amp panel in your accessory dwelling. The key insight we’ve found is that you don’t need a 100 amp wire or “100 amp service” as an electrician or contractor would say. You just need a box that is big enough to hold 100 amp service. Those smaller boxes in option 1 above can’t fit that size service in the future so they are ruled out.
The 60 amp service inside a 100 amp box in your accessory dwelling is a great solution because it doesn’t force you to upgrade your main home to a 200 amp service. That’s a big savings.