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Question DetailsAsked on 12/18/2017

Can I run an ADU off of a 125 amp Shop/garage service?

I already have a 200 amp service t the main house along with the 125 amp service to the garage. Can I have the 125 amp service do double duty to the ADU?

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1 Answer


Below is a link to a pretty similar question with answer in more detail, but the bottom line is that while this is sometimes done, at a minimum you need a load calculation by an electrician, because there are several factors potentially in play here:

1) if this is a separate ADU (rental or such) generally you are required to have a separate service to it. In-house inlaw apartment or such can sometimes be tapped off your house service, but basically if a separate living unit with separate entry door, especially if permitted/zoned as an ADU, it requires a separate service with its own service drop and meter to allow for separate electric usage billing.

2) additional to above - even if not separate unit now, consider possible future uses and, if it can legally be rated a separate living unit, whether having separate billing capability in the future would enhance resale vallue down the road.

3) some local codes prohibit this if the ADU is detached even if not a rental/lease unit, requiring the tap go back to the main service (at the meter base or adjacent disconnect/main breaker panel)

4) depends to a very significant extent on whether the garage 125A service is a sub-tap from the main breaker panel, or a parallel tap off the incoming service - if the 125A breaker panel is a sub-panel feeding off the main panel, adding a sub-sub tap to a new panel (as opposed to individual circuits) off that is prohibited in general

5) if the 125A garage panel is a parallel feed going back to the main service, plus the 200A panel, that likely takes up the available capacity of the incoming service drop and meter panel - adding another load on that might require a usually fairly expensive main service upgrade

6) depends on the service load in the garage, and what the ADU needs - many codes require a minimum 100 or 125A service for an ADU now, so on top of the existing garage loads that would likely be a disallowed overload on the garage panel

7) depends on whether ADU demand is high - if mostly gas heating/range/clothes dryer might get away with smaller (75A or 100A say) sub panel there and not overload the garage panel, but if all-electric ADU not likely to work

8) depends on the individual breaker capacity of the garage panel - many panel buses are limited to 50A maximum size breaker in the panel, so if that is the case you would have to run the feed off the main service (assuming it can handle the load). Also, some limit high amperage loads to the top few slots so if they are already in use with high amperage loads could not do it.

9) new ADU panel is almost certain to have to have a main breaker just for it - either integral in the panel (which I dislike because in the event of a panel overheating/ arcing/ catching fire you cannot safely shut off the power to it. I prefer a main breaker at the point where the feed to the panel comes off the main service, typically at a main breaker panel at the meter panel, protecting the breaker panel PLUS the feed wire to it - so in your case might possibly need 2 or 3 main breakers at the meter box, number depending on whether the garage panel sub-taps off the 200A one or not.

10) depending on what the garage loads are, especially if welder or large motors or air compressor are involved, subtapping off that panel might cause inadvertant tripping of the main breaker when combined with the ADU load if tapped off the garage panel - and even if it does not trip the main breakers, might cause objectionalbe flickering and browning-down of the ADU circuits when they are used.

As you can see - not so simple, and I suspect you got hte feeling I do not care for the sub-tap idea, as opposed to going back to the service drop to tap off there, or providing a totally new service drop nd meter for the ADU. Because of all these potential issues, you need a licensed electrical contractor to assess what you have and how it is wired, existing wire sizes and connector limitations, existing loads, and what your options are from there.

I am sure if Kestrel Electric chimes in here he can add a few more considerations too - and this sort of add-on is CLOSELY looked at by inspectors, so you want to get it right up front even if it adds a few hundred $ to the picture.

Answered 2 years ago by LCD

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