Wiring Questions for New Detached Garage

[egertk]

I'm finally getting ready to tear down the 1977 pole barn on my property and build a 1056 sq ft detached garage/workshop for my tools and toys. I'm planning to do at least the majority of the electrical myself and am looking for some advice on the wiring. I have a somewhat unique situation as my house has 400 amp service from the days when electricity was cheap (it was built in 1968) and it had two electric furnaces to keep the house heated. The "smaller electric furnace" as it's called out on the main panel was run from a 70amp breaker on the main panel while the main electric furnace was powered from a 100 amp breaker in its own separate disconnect panel that splits off from a CT box where the 400 amp service enters the house. Since that 100 amp disconnect is no longer utilized I'm hoping to use that instead of tying into my main breaker panel. The run from the 100 amp disconnect to the is approximately 75-85 feet, I plan to run conduit along the back side of my house and then go underground to the new sub-panel on the garage. Since I'm also planning to add a 7-9 kw solar panel setup on my garage roof the solar guy advised that I have a down-rated main breaker on my panel to meet backfeed requirements, so the panel will be a 200 amp 42 space panel with only a 100 amp main breaker. So my question is, for the run out to the sub-panel, can I run x3 3 AWG CU THHN for my hot and neutral legs in conduit or do I need to go with a thicker gauge? I'm finding conflicting info on the amperage capacity for what I'm proposing. Also what should I run for the ground wire?
Thanks!

 

[wyliesdiesels]

If u have the dough $$$$ for #3 cu, then that will work for a 100a feed to the garage voltage drop not being an issue. If u go with #3 THWN then u can use #8 cu THWN for the ground. It will need to be green insulated.

Now Im not sure what your solar guy is refering to in regards to needing a smaller main breaker in your subpanel.

Ive done a few garages with solar panels on them many years ago and never needed to derate the main disconnect breaker. Im not seeing what this will do or accomplish. Perhaps this is a new code Im not aware of. Need to grab my code book and look.

Dont forget grounding rods (2x) and u may need a separate ground bar kit. Also make sure the neutral bar is isolated from the enclosure and the ground bus.

BTW is there a breaker before the #2 AL?

Regardless, #2 AL is too small for 100a. It may have been code back in 68 for the furnace but seeing as your repurposing this circuit, it really should be replaced. Use either #1 AL or #3 CU.

 

[captaindiode]

When you back feed a panel, you lock two breakers together so when one is on the other must be off. That way, when the solar is feeding the panel (or a generator) it cannot feed back to the main panel and on to the grid and kill an unsuspecting lineman. When the solar is on, the feed from the main is off and when the main is on, the solar panels are switched out.

Be sure the panel you get can accept lockout kit. Also, check the price of the kit.

Google "generator interlock" to see examples.

 

[wyliesdiesels]

When you back feed a panel, you lock two breakers together so when one is on the other must be off. That way, when the solar is feeding the panel (or a generator) it cannot feed back to the main panel and on to the grid and kill an unsuspecting lineman. When the solar is on, the feed from the main is off and when the main is on, the solar panels are switched out.

Be sure the panel you get can accept lockout kit. Also, check the price of the kit.

Google "generator interlock" to see examples.

Huh?

Where does the OP say he is using the solar as a stand alone generator setup? No where.

The OP is doing grid tie solar NOT a stand alone solar generator.

An interlock is not needed in the OPs case.

If the utility power feed goes off the inverter shuts off.

 

[captaindiode]

Sorry, I misinterpreted the backfeed part.

 

[walrus]

Now Im not sure what your solar guy is refering to in regards to needing a smaller main breaker in your subpanel..

If you are feeding the subpanel with the solar it has to be derated and I forget the wording buts it s new code in 2014. Its based on the buss rating of the panel and the breaker feeding it. I think you end up derating the feed breaker not the main breaker in subpanel?

 

[bjcouche]

The panel derating thing sort of makes sense to me, but there are other issues as well. First, do you have to reduce the garage panel main breaker from 200A to 100A, or is it OK because the 200A breaker is really only acting as a disconnect, and the 100A breaker feeding the 200A breaker is essentially derating the 200A panel??? If you must remove the 200A breaker and install a 100A breaker in it's place, that may not be possible because the panels that I'm familiar with, the 100A and 200A main breakers are different physically so that you can't swap them. Instead you'd have to remove the 200A main, and install a 100A branch breaker and back feed it.
Also, I'd go with aluminum instead of copper. Most local electrical stores sell aluminum THHN single conductors, and some come prelubed

Brian

 

[wyliesdiesels]

If you are feeding the subpanel with the solar it has to be derated and I forget the wording buts it s new code in 2014. Its based on the buss rating of the panel and the breaker feeding it. I think you end up derating the feed breaker not the main breaker in subpanel?

Ok do u know the code article number?

I dont have 2014 code book so i will look it up online...

 

[justsam]

If you are pouring a new slab with rebar you may want to consider Ufer ground instead of ground rods. Check with AHJ.

 

[walrus]

Ok do u know the code article number?

I dont have 2014 code book so i will look it up online...

This may help
http://www.jadelearning.com/thejunctionbox/2014-nec-705-12d2-understanding-pv-interconnections/

 

[egertk]

The #2 alum is just from the CT box to that 100amp disconnect, what's#2 alum good for per today's code? If I need to replace that or the disconnect breaker I'll need the power company to shut off power, something I was hoping to avoid.
Brian, thanks for letting me know that the 100amp and 200amp main breakers aren't interchangeable, that's something I wasn't aware of. I was intending to use a 200 amp square D QO panel.
You recommended alum conductors over copper, was that just due to price? I've heard of your not a pro don't try to work with aluminum since you've got to be so careful with regards to oxidation.

 

[gtae07]

I was able to use 2-2-2-4 aluminum MHF for 90 amps. Unless you really need those last 10 amps or your distances preclude that (I had about a 100ft run) just go with a 90amp breaker in your main panel.

 

[mm08822]

Egertk, In your case you can use the 200a panel with 200a main as is because you are protecting it with a 100a breaker upstream. The solar guy is correct in what he is saying. Refer to articles 408.36 and 705.12(D).
For homes that have an inverter backfeeding into an existing main panel, I am sure most will not meet these requirements (like mine), but the chance of bus overload is very unlikely especially when the inverter breaker is put on the opposite end of the buss from the "main".

 

[mm08822]

BTW, these requirements were in the 2011 code and are virtually unchanged in 2014. Can anyone check 2008 book - not sure when 705.12(D) originated?

 

[theoldwizard1]

I assume you solar is going to be sold back to the grid and not stored in batteries for off-line use.

If so, the solar output will have to go through its own meter and be connected to the grid close to the other meter. I do not believe there is anyway of achieving this with one set of wires between the house and the garage.

Off line use means just that. Disconnecting from the grid before turning on the inverter.

 

[mm08822]

the solar output will have to go through its own meter and be connected to the grid close to the other meter. I do not believe there is anyway of achieving this with one set of wires between the house and the garage.

If his existing meter is replaced with a Net meter, he will be able to track the net energy put back on the grid from the solar generation. Depending on when and how much his consumption is relative to instantaneous solar generation, it is possible to just reduce the amount bought from POCO.

If there are energy credits for solar, then a customer supplied (most likely) revenue grade meter will be needed between the inverter and garage panel. This meter will record all solar MW's generated from system. Owner may have to read this and record it on a tracking website. It all depends on how the state sets up incentives/metering requirements and who is permitted to record the meters. (In NJ, the owner reads and records the "SREC meter" values into a website tracking system. POCO only reads their net meter.)