Electrical Question - New Detached Workshop, 100Amp Subpanel

[Richard Kulesa]

Hello,
I have few questions but will start with this.
I have 200Amp service in my home. I'm building a 28' x 42' workshop that is 150' away and was planning to have a 100Amp subpanel there to feed a variety of items:

• Power Tools
• Lights
• Mini Split AC
• Some 240V machines (Tablesaw, jointer, planer) will run only 1 at a time

While I have a lot of tools, I'm only running one at a time plus a dust collector (probably also 240V).

I was advised to run #3 THHN plus a #8 for ground to the new building. 100 Main Lug panels don't have many spaces (i like Square D so I plan on using a Load Center Convertable Main (Lugs) type....is that still ok? It has 24 slots and is rated for 125A.

Other question - since the building is 150' away, is #3 THHN still ok for 100amps? In the back of my head I wonder if it's more prudent to run #2....also leaves capacity if need to up to 125A down the road (just in case...I worry about these things )

I'm in NJ if matters.
Thanks so much!!

(In addition, I'm aware of need to have 2 ground rods and ensure a ground bar is added to separate from the neutral)

[Chris Padilla]

Don't forget the compressor!

[James Tibbetts]

There is a calculator here.
Looks like it should be 1/0.

[Rollie Meyers]

If new construction, forget the ground rods & use a concrete incased electrode, more commonly known as a "Ufer", if there is rebar in the foundation your required to use it.

[Matt Meiser]

Use a big panel. 220 circuits will eat up your space quick.

[Steve Meliza]

Hello,
I was advised to run #3 THHN plus a #8 for ground to the new building. 100 Main Lug panels don't have many spaces (i like Square D so I plan on using a Load Center Convertable Main (Lugs) type....is that still ok? It has 24 slots and is rated for 125A.

Other question - since the building is 150' away, is #3 THHN still ok for 100amps? In the back of my head I wonder if it's more prudent to run #2....also leaves capacity if need to up to 125A down the road (just in case...I worry about these things )

I don't think you are allowed to use a main lug panel in a detached application nor would you want to even it allowed to. Go with main breaker type and feel free to get a 200A panel just like you have in your house. It is the feeder breaker in your house panel that must be 100A, the 200A breaker in the sub panel becomes an extra disconnect and not over current protection.

I'm not very familiar with sizing of feeders for sub panels, but when I did mine I just went off of NEC table 310.15(B)(16). I wanted a 60A sub panel so I used NM-B 6/3 which is rated for 55A and stuck a 60A breaker on it as allowed by the NEC. Passed inspection with flying colors. Based on that I'd say that #3 THWN is probably enough for your 100A sub panel, but I'd look into local codes and double check with the inspector before buying anything.

Edit: I wanted to add that if it were me and I was looking at a 150' feeder like this that is buried in conduit I'd not only go as generous with the wire size as the budget would allow but I'd use at least 1" conduit if not larger. In fact, I would run two separate conduits and cap one off. You never know when that other conduit might come in handy or why. It could be used for telephone, ethernet, alarm wiring, etc. Dig and bury once I say.

[Richard Kulesa]

Would the 200A panel be configured to keep the neutral and ground completely separated/isolated as required for a subpanel?

I don't think you are allowed to use a main lug panel in a detached application nor would you want to even it allowed to. Go with main breaker type and feel free to get a 200A panel just like you have in your house. It is the feeder breaker in your house panel that must be 100A, the 200A breaker in the sub panel becomes an extra disconnect and not over current protection.

I'm not very familiar with sizing of feeders for sub panels, but when I did mine I just went off of NEC table 310.15(B)(16). I wanted a 60A sub panel so I used NM-B 6/3 which is rated for 55A and stuck a 60A breaker on it as allowed by the NEC. Passed inspection with flying colors. Based on that I'd say that #3 THWN is probably enough for your 100A sub panel, but I'd look into local codes and double check with the inspector before buying anything.

Edit: I wanted to add that if it were me and I was looking at a 150' feeder like this that is buried in conduit I'd not only go as generous with the wire size as the budget would allow but I'd use at least 1" conduit if not larger. In fact, I would run two separate conduits and cap one off. You never know when that other conduit might come in handy or why. It could be used for telephone, ethernet, alarm wiring, etc. Dig and bury once I say.

[Richard Kulesa]

Answered my own question. The Square D QO Load Center I'm looking at is convertible to both types. I'll use the main breaker as an additional disconnect and buy the separate grounding bar kit to separate from the neutral bus.

Would the 200A panel be configured to keep the neutral and ground completely separated/isolated as required for a subpanel?

[Don Roberson II]

150 feet with #3 might give you a little voltage drop on start ups. I would go with #2 if possible.

[Adam Trombello]

100A will be more than enough for what you plan on using with plenty of room for future use! If it's getting inspected, forget the #8 ground back to the main. Youll have to do a separate grounding system. Ground rods or concrete encased electrodes(if new construction). Still a subpanel, so keep in mind that grounds and neutrals will have to be separated.

I believe you will need a main breaker for the subpanel in the garage, so there is a disconnect. and I would just get a 200A main breaker panel so you have plenty of spaces.

x100 on running two conduits back to the house!! Any low voltage tv,tel,internet would have to be in a different chase.

[Lewis Anderson]

If it's getting inspected, forget the #8 ground back to the main. Youll have to do a separate grounding system. Ground rods or concrete encased electrodes(if new construction). Still a subpanel, so keep in mind that grounds and neutrals will have to be separated.

This is incorrect. You need both ground rods and a ground wire back to the main. With the neutral and ground not bonded(which is correct) at the sub-panel you basically have no short circuit protection. Thus you need a ground path back to the main where the neutral and ground are bonded. Here is an illustration with code references.

[Tom M King]

You absolutely do want a ground wire connecting both panels, even with ground rods at the sub. Not only is it required, but all sorts of weird things will start to happen with separate grounds-like all the voltage jumping over to one bush bar so that you have nothing on one side, and 220 on the other in the subpanel.

[Julie Moriarty]

When I was working on cell site installations (about 5 years total) I had to meet with probably half of the electrical inspectors in the 4 county area in and around Chicago. Cell site installations are very heavy on grounding requirements. Many inspectors had no experience with these at the time so I had to explain what we were doing and why. When it came to setting up the service, there was no consistency with the inspectors regarding establishing a ground source. I took many of these situations to the code and grounding gurus in my local and returned many of their arguments to the inspectors. It didn't matter. The inspectors all held firm with their version of how the ground source should be established.

When it comes to any installation that will be inspected, I always go with talking to the inspector first. You will get the right answer and a chance to establish good will at the same time.

[Chris Padilla]

Good grief, how many different ways can you "ground" (provide an earth return path for current) a panel?!

[Adam Trombello]

This is incorrect. You need both ground rods and a ground wire back to the main. With the neutral and ground not bonded(which is correct) at the sub-panel you basically have no short circuit protection. Thus you need a ground path back to the main where the neutral and ground are bonded. Here is an illustration with code references.

Hey thanks for the correction! That's news to me, I would have thought a separate ground source out there would do it. But then again I'm no electrician lol.