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 (http://www.diychatroom.com/#)(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 http://www.diychatroom.com/images/smilies/icon_wink.gif)

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 (http://www.paigewire.com/pumpWireCalc.aspx).
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 http://www.diychatroom.com/images/smilies/icon_wink.gif)

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.

http://www.diychatroom.com/attachments/f18/2416d1203439929-sub-panel-grounding-question-4-wire-subpanel-detached.jpg

[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.

http://www.diychatroom.com/attachments/f18/2416d1203439929-sub-panel-grounding-question-4-wire-subpanel-detached.jpg

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.

[Adam Trombello]

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.


I wonder what their thought on a ground going back to the main is. And how would you have problems with voltage spikes and weird things happening? Youre establishing the same thing by pounding rods out there. The neutral is going to carry any of the unbalance current back.

"....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."
Are you talking about the voltage on each phase? Because there will always be 120v give or take present on each phase. Correct me if I'm wrong.

[Lewis Anderson]

Google "grounding vs bonding" if you are truly interested. There is a lot of information already out there so no need to clutter up this thread further.

[Julie Moriarty]

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

Of all the different facets of working with electricity, I've found grounding to be the one that brings about the most debate. Most inspectors will require a single ground source for a single service. All sub panels that are fed off that service get their ground from that service. It's the way I was taught to do it and. based on what I've learned over the years, the only method that should be used. But as I mentioned above, there are some inspectors who require more than one ground source and even though there's evidence to prove that can cause problems, they insist that's the way to do it. If you want your work to be approved, you have to accept the "fact" the inspector is always right.

I think the reason grounding methods are so often debated is because it's not entirely contained within insulated materials and devices and therefore we lose control of it. At that point, the flow of electricity becomes as unpredictable as the path of lightning.

[Richard Kulesa]

Great information, perfect, thank you!
I'm going to go with #2 (and #6 for ground).


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.

http://www.diychatroom.com/attachments/f18/2416d1203439929-sub-panel-grounding-question-4-wire-subpanel-detached.jpg

[Adam Trombello]

While youre at it, you might want to consider running a 3-way for garage out lights or floods and a low-voltage wire for detach garage door opener in the house :)

[Tom M King]

http://www.esgroundingsolutions.com/blog/638/why-separate-the-ground-bar-from-the-neutral-bar-in-a-sub-panel

http://www.city-data.com/forum/house/1869832-electrical-120v-subpanel-suddenly-outputting-160v-3.html

[Adam Trombello]

http://www.esgroundingsolutions.com/blog/638/why-separate-the-ground-bar-from-the-neutral-bar-in-a-sub-panel

http://www.city-data.com/forum/house/1869832-electrical-120v-subpanel-suddenly-outputting-160v-3.html


Separating neutrals and grounds in subpanel - completely understandable, you don't want current to travel back on ground wire.
Running a ground wire back to the main ground source with a ground source at detached building - I don't get it.


Lewis Anderson, I will have to do a little research on grounding and bonding

[Keith Outten]

My shop has its own service and meter base. I would not have it any other way.
.

[Mike Heidrick]

Keith, Many place do not let you have a second meter base on your primary residence property at all and those that do many time require you to pay commercial rates for the second service.

[Tom M King]

This is a good one, with pictures for each example of being done improperly:

http://www3.ambest.com/DPSDirectorySearch/MMShow.ashx?mmid=12761

Be sure to read page 8.

[David L Morse]

This is a good one, with pictures for each example of being done improperly:

http://www3.ambest.com/DPSDirectorySearch/MMShow.ashx?mmid=12761

Be sure to read page 8.

Excellent reference! You're right, page 8 is the key, especially the first sentence of item 2. This is exactly the reason for the separate ground wire to an outbuilding. That sentence could be presented more in layman's terms by replacing "clear ground faults" with "cause the breaker to open on a circuit with a short from hot to ground".

[Keith Outten]

Keith, Many place do not let you have a second meter base on your primary residence property at all and those that do many time require you to pay commercial rates for the second service.

Mike,

I figured that a second service might not be an option in some localities but thankfully its not a problem in my area. It didn't cost me a cent to have my shop service installed and now that my shop is a commercial business it makes it easy to report the electrical bills on my business expenses and track them for taxes.

[Richard Yeskoo]

I have a service pole about 300 feet from house and want to build shed next to pole. Any suggestions for tapping off pole service and not running through house panel?

[Lewis Anderson]

I have a service pole about 300 feet from house and want to build shed next to pole. Any suggestions for tapping off pole service and not running through house panel?

Is there a disconnect at the pole or just a meter?

[Rollie Meyers]

It used to be OK to run a 3-wire feed to a outbuilding if there were no parallel paths like a metallic waterline & treat it like a 2nd service, drive a ground rod or use another allowable grounding electrode listed in art. 250, now a 4-wire feed is required and ground rods, a Ufer, & if metallic underground water piping is present at the building it has to be used.

In the early 1990's I used a 3-wire feed out to the building that my shop was attached to in 1993, in 1994 the service on the house was changed & the feeder cables were removed to relocate the conduit into the new panel & pulled back in, I still regret not going to a 4-wire feed & still may do so even though it was compliant at the time.

[Bob Kinsman]

House three way switch or low voltage goes along with what I would recommend. I also accomplished this same detached garage sub panel a bit over a year ago, using 3 #3thhn hots/neutral 1 #6thhn ground in conduit. I only feed the sub off a 60amp breaker but wanted to have the correct cable in the ground if I ever got the itch for 100amps.


Suggestion: Make sure you put two tubes of conduit in the ground while you have the trench open for the power!

'Cause someone's gonna think cat5e or coax goes mighty well with a well supplied shop!

[Jeff Caulk]

I'll leave the electrical code to the experts, but the schematic above is what I understand and how my shop is wired.

As for capacity, I have a 20 x 40 shop on a 100A sub panel fed with a 100A breaker.
Rememeber that's 100A per each 120V leg.
I have three 20A 240 circuits, dedicated table saw and dust collector circuits and a third for the band saw and jointer (only use one tool at a time).
There is a 20A 120V general lighting circuit, an exterior 20A security lighting circuit, five 20A receptacle circuits, a dedicated 20A compressor circuit and a 20A feeder to another outbuilding.
I have two double space couplers for my automated lighting system.
With all that I still have 6 spaces open.
100A is more than enough capacity for a one man shop.

I ran conduit for power between the primary building and the shop along with two additional conduits for LV wiring.

[Richard Yeskoo]

Is there a disconnect at the pole or just a meter?

Both meter and disconnect.

[Lewis Anderson]

Both meter and disconnect.

Per current code the ground should be bonded at the disconnect and you should have a 4 wire feeder going to the house. If that is the case than it would be easy and up to code to tap on after the disconnect to feed the outbuilding with another 4-wire feeder. If not, I would recommend, as mentioned above, speaking to your local inspector or electrical contractor to see what they will accept.