Building a new accessory dwelling or detached office? Adding safe electrical service can get expensive. Here’s the most cost-effective way to make sure your electrical is up to code.
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 run 60 amps of service within that panel. Doing this can save thousands.
This is a brief summary of the options for one accessory-dwelling project. Actual electrical needs vary based on home design, 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) Make no major change to the main house and add a well -designed smaller panel in the office or accessory structure. 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 a wire from this box to your accessory structure. In the picture below, each white sticker marks one breaker. You can count the space for 14 breakers or 14 circuits, and that is typically adequate for a main home and second home.
The new interior panel in your office or accessory structure will probably look like one of these smaller “4- 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 high demand. 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’s electrical panel. If you’re placing a new 100-amp panel in the accessory dwelling, you will most likely need to increase your main home’s panel to 200 amps so that there is enough power. You may also need to 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’s 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, and you may need to replace the wire from the street. You’ll also need to 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 100-amp service or more. 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 electricity panel in the accessory dwelling.
In this scenario, you are lucky to have a big enough 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 the two homes aren’t expected to max out their full 60-amp demand at the same time.
But, and this is a big but, don’t add a 40- or 60-amp panel (the actual size of the gray box) to your accessory dwelling. 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, 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.
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