I've built a small detached mother-in-law in my back yard (in Seattle, which has adopted the 2014 NEC). The unit is a studio layout with kitchen and 3/4 bath. All major appliances are gas powered. I am running service from the main panel in the primary residence underground to a sub panel at the unit.

I have two city inspectors disagreeing about a couple of issues with my project. Inspector #1 is in my favor, and #2 is asking for changes that would be a big pain, but is being open-minded that she might be wrong, so I want to make my case by citing the code. That's what I'd like your help with.

Here are the issues:

1) Inspector #1 says that minimum service amperage is based on load calcs. #2 says that there is a minimum 100a requirement. My load capacity calcs come out to 30a, so 100a service would be ridiculous. However, 230.42(A) seems pretty clear that minimum service is based on load, favoring my position. Anything I'm missing on this one?

2) Inspector #2 says I'm required to pull 220v (two hot conductors) out to the sub panel, despite the fact that there is nothing that will consume 220, and there is no requirement for me to install a 200v outlet. Inspector #1 says I can just pull one hot conductor and Y-splice it to the two hot lugs in the sub panel, as long as I put a sticker in the panel saying "no double pole breakers." On this one, I'm having trouble finding anything in the NEC to cite. Would love some help..

If she makes me run 100a 220v service, I have to scrap over 200' of #4 wire (plus my conduit), and then buy 300' of #1 wire.

Thanks in advance!

 

Nominal voltage is 240 not 220. Minimum amps for a house is 100 amps. That may be what one inspector is basing his opinion on. 120 volt breaker panels are specialty items. A 240 panel is what you will almost certainly be using. By using 120/240v instead of 120v you double the available amperage for 120 volt circuits using the same size wire. Or to put another way you could get the same amperage for 120 volt circuits on smaller wire.It would be 120/240 volt not 220v.You can do that or just use half the panel.

Just for comparison a 120 volt 100 amp panel can supply 100 amps at 120 volts on #3 (@1oo feet). A 120/240 100 amp panel can supply 200 amps at 120 volts on the same #3 wires. Another comparison. A 120/240v panel using only #6 wire on a 60 amp breaker can supply 120 amps for 120 volt circuits (@ 100 feet). A 120 volt panel would only supply 60 amps on a 60 amp breaker.

How big was the conduit you ran? What is the one way distance?

 

I think the thing here is going to be whether the inlaw is considered a dwelling or accessory building. A dwelling calls for a minimum 100 amp service .

 

First - you are not running a service but a feeder so section 230 does not apply so should be section 225. Wording is the same, though.

Note that requirement is for disconnect. This has been argued before that it is disconnect size, not feeder/service/OCPD size but most lose argument.

 

How did you describe the outbuilding on the permit app.?

 

It doesn't have to be a service, feeders also have the same 100A disconnect requirement for One Family Dwellings. Inspector #2 is going on NEC 225.39 (C). I feel bad for you because you posted earlier about what you were doing? Someone on this forum including myself should have thought to bring that aspect up about the minimum feed size. Astuff has a good point about disconnect size vs. actual ampcapacity served may be less because of the wording of the code. A lot of inspectors interpret the minimum disconnect size as setting the minimum feed size.

 

It does seem strange to require at least a 60 amp disconnect for a 30 amp feeder to an outbuilding. It certainly should not require a 60 amp feeder to a shed either.

 

Thanks guys. It sounds like this fundamentally comes down to whether this is a single-family dwelling or not. So in Seattle, the city issues a specific permit for what they call an Accessory Dwelling Unit (ADU), or in my case, a Detached ADU (DADU). It does not have its own address, nor does it have any separately-metered utilities. Does the NEC define a single-family dwelling?

 

Another data point: my lot is only zoned for single-family, and the main house is a single-family dwelling, so then it seems like the DADU could not also be a single-family dwelling, given that the the city issued the permit even though my lot is not zoned for multi-family. Just trying to come up with every possible argument here.

 

You're going to have to get a definitive definition established between the town and the inspector(s).

As of right now..... your Detached Accessory Dwelling Unit.... is still a dwelling.

 

This is going to be up to your local authorities. It really doesn't matter what a message board thinks.

 

Thanks all for your help, and apologies for the long delays in my responses. I spoke with inspector #2 this morning and she confirmed basically everything I was arguing for, i.e.:
- I don't have to pull two hot conductors
- I don't need 100a service to the DADU
- I can splice the single hot conductor to both hot lugs in the panel to get use of all breaker spaces

The only real concession she demanded was exactly what pattenp and Astuff pointed out, which is that the disconnect needs to be rated at 100a (even if service is half that).

Thanks again for your help and feedback on this! So glad to have an inspector who is reasonable and open-minded.

 

That's a bizarre requirement on her part, but hey it's a cheap way out for you. Glad to hear they ruled mostly in your favor.

Generally the NEC does not define terms like "dwelling" specifically to allow for local authorities to apply the local zoning standard or state/local laws to the model template code provided at the national level.

 

Well, I celebrated too soon. The district supervisor called me this morning to tell me that I do, in fact, have to pull two hot conductors (though I don't need 100 amps). This is after I called into the inspectors office to explicitly ask this exact question before doing the work, then reconfirming with a different inspector during my cover inspection, also before doing the work. Absolutely infuriating.

Apparently my project generated a lot of controversy and discussion at the inspectors office, which is ridiculous because hundreds of DADUs have been built in Seattle in the last few years. They should have their sh*t together by now.

Anyway, just updating you all.

 

Since you have #4 wire it is probably usable. You just need to add a fourth wire assuming the conduit is at least 1". Are all your current wires black and #4? If so you just need to add a #8 green and use a 40a or 60 amp breaker at the house. What are you sing for a sub panel?

 

Yeah that was my plan--reuse the two #4 wires as my hots, and get another length of #8 for the neutral. I have a 50a breaker for the house. I bought a 125a 8-space lug center I was going to use as my sub panel. But all the 100a outdoor disconnects I'm finding are pretty expensive. I think it will be cheaper (and cleaner all-around) to just get a main breaker panel in place of the lug panel.

Thanks again, all. I'm going to be really good at fishing wire by the time I'm done with this project.

 

You will also need a ground wire (EGC) from the house in addition to the GEC from the ground rod(s) at the subpanel. Neutral should be #6 or #4 depending on distance. Ground (EGC) can be #10 or #8 depending on distance.

 

I haven't commented yet, but think I now will. I believe the supervisor made the correct call. From time to time I see threads on this forum where someone wants to install a 120 volt subpanel and is offered various suggestions on how to power up both sides of the panel with one leg, there are a few ways to do it. The bottom line is that having just one leg to power up both sides of the panel flies in the face of the manufacturer's installation instructions. I have never seen instructions from a manufacturer that detailed how to install just 120 volt power into a 120/240 volt loadcenter and I have also never seen any publication or brochure that detailed the acceptable rated voltages as 120/240 or 120. Single phase loadcenters are always rated at 120/240 or 120/208 and never just at 120. Of course, this is just my opinion.

 

I don't see any listing or code issues with only pulling 120 to a panel. I think the rating is just showing the voltages that are typically used. It does seem to be a less efficient way to power the panel, even if a 240 circuit is not needed.