# sub-panel?



## Cache (Sep 18, 2007)

Getting various answers on the topic, and I can usually get more helpful recommendations here. Background is in carpentry and engineering, so please excuse my electrical ignorance. Here goes....

4,000sf residence is converting a portion of attached garage to wood shop. Will be installing multiple 240V receptacles for large machines (table saw, jointer, compressor, etc). Space already has plenty of 120V receptacles.

Main panel is 200amp. 30/40 breaker box is completely full already. Even if it weren't, I have my doubts that the 200amp panel could service a busy wood shop and a few teenage kids at the same time.

Which would be the recommended course of action?



Swap out the panel for a 400A panel and exchange for a 40/60 breaker box?
Run a feeder to a 100A sub panel in the shop, dedicated only to powering the heavy load 240V machines. Lighting and existing 120V to remain on main panel?
I would think #2, but unsure of any obstacles that might be unknown to me. Thanks guys....:thumbup:


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## 480sparky (Feb 1, 2009)

Option #3: Perform a load calculation.


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## Cache (Sep 18, 2007)

I agree with your suggestion, but that wasn't really the question. I'm not wondering whether I need more amperage.

My question is better stated, "Assuming that a load calc indicates the need for an additional 100A, which of the two options is preferred. Or is there a better third option that other electricians haven't suggested yet?

Thanks


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## woodchuck2 (Feb 27, 2008)

X2 on load calculation. How many pieces of equipment are they going with? Tables saws, air compressors, miter saws, planers, radial arms saws, dust collector's, etc. all draw huge amounts of power no less good lighting. IMO to get around the fee's of a second meter if a second or larger service is required it may be better to install a new larger service "400amp" in the garage/workshop and use the home panel "200 amp" as a sub panel.


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## 480sparky (Feb 1, 2009)

Without doing a load calc, the question cannot be answered correctly.


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## Cache (Sep 18, 2007)

Ok, I better provide better information if I expect your help. :thumbup:

We aren't talking a full blown commercial wood shop here. We are talking about a well equipped hobbyist converting his garage.

Current Load Calc for the home:

Service Demand = 39,500VA
Demand Load = 165A
Min. Service Req. = 175A


Shop has adequate lighting and 120V power already. Only need additional 240V power. 

Shop equipment would increase that demand by 20,000VA

New Service Demand = 59,500VA
New Demand Load = 248A
New Min. Service Req. = 250A

So the additional shop equipment requires a minimum 75A @ 240V. So a 100A subpanel would suffice, provided there are enough breaker slots. Access to install the subpanel in the garage is easy. Swapping out the main panel for a 400A would be a bit more inconvenient, but not that difficult. There is open basement access to anything necessary.


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## 480sparky (Feb 1, 2009)

Cache said:


> ............ Only need additional 240V power.
> 
> Shop equipment would increase that demand by 20,000VA
> ............



So you're adding 83 amps of equipment?


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## Cache (Sep 18, 2007)

480sparky said:


> So you're adding 83 amps of equipment?


Correct. I think I overestimated a bit to be safe. And there is really no possible way to operate all at the same time. But just in case.


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## 480sparky (Feb 1, 2009)

Cache said:


> Correct. I think I overestimated a bit to be safe. And there is really no possible way to operate all at the same time. But just in case.



I generally take any equipment that is 'on' at all times, such as a dust collection & air compressor, and add that to the largest single tool load. There's no way you're going to be running a table saw, oscillating sander, planer, mortising jig, etc. all at once.

Heck, even Norm Abrams only uses one power tool at a time. The biggest load in his shop is the lighting required for the video crews.


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## Cache (Sep 18, 2007)

480sparky said:


> I generally take any equipment that is 'on' at all times, such as a dust collection & air compressor, and add that to the largest single tool load. There's no way you're going to be running a table saw, oscillating sander, planer, mortising jig, etc. all at once.
> 
> Heck, even Norm Abrams only uses one power tool at a time. The biggest load in his shop is the lighting required for the video crews.



By that standard we would be looking at an additional 58A. Planer + Dust + Compressor


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## 480sparky (Feb 1, 2009)

Cache said:


> By that standard we would be looking at an additional 58A. Planer + Dust + Compressor



Even so, if you've already calculated an existing load of 175 amps, and you're adding 58, you need to up the service.


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## Cache (Sep 18, 2007)

480sparky said:


> Even so, if you've already calculated an existing load of 175 amps, and you're adding 58, you need to up the service.


That's what I was unclear on, in my lacking knowledge. When the city installed the main line to the meter, they were sent over from a commercial build and had the heavier gauge handy, so they just used that for the main service. So that line is not undersized.

But the main service panel is 200A. I was wondering if it is a simple matter of running a feeder from the main panel to a 100A sub-panel, thus making the total 300A. Or is swapping out the main panel for a 400A a necessity?


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## 480sparky (Feb 1, 2009)

Cache said:


> That's what I was unclear on, in my lacking knowledge. When the city installed the main line to the meter, they were sent over from a commercial build and had the heavier gauge handy, so they just used that for the main service. So that line is not undersized.
> 
> But the main service panel is 200A. I was wondering if it is a simple matter of running a feeder from the main panel to a 100A sub-panel, thus making the total 300A. Or is swapping out the main panel for a 400A a necessity?


The service drop conductors are the utility's problem. Your responsibility starts where they attach to the house.

Adding a 100 breaker in the main panel will not increase the capacity of the main panel by 100amps, it will simply overload it. You need to replace your entire service.


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## Cache (Sep 18, 2007)

480sparky said:


> The service drop conductors are the utility's problem. Your responsibility starts where they attach to the house.
> 
> Adding a 100 breaker in the main panel will not increase the capacity of the main panel by 100amps, it will simply overload it. You need to replace your entire service.


That's what confuses me. I hear about and see people installing sub-panels all the time. If what you say is true (and it probably is) what is the point of installing a sub panel. If it doesn't increase the capacity of the building then it doesn't serve a purpose other than to provide a remote shutoff location.

Is there another benefit that I don't see?


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## Cache (Sep 18, 2007)

Well, thinking about it a bit more, I think I realized the benefit.

If the service is adequate and I wanted to add a bunch of new loads in a distant location, then a single line to a sub panel would be better than a bunch of individual circuits. 

I think I get it now. Just a bit dense sometimes. Thanks for the patience. Looks like I'll be telling the client that they need to swap in a 400A service. At least I caught this early, so I wasn't getting calls about breakers tripping every time the planer is turned on.


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## 480sparky (Feb 1, 2009)

Load is load, no matter how you break it up.

And adding too much of a load WILL overload your service.

It won't matter if you add 5 15amp circuits or a single 75a feeder..... it still adds 75 amps to the main panel.

Adding the load as a subpanel does not somehow make the added loads smaller. It is the same amperage draw on the service, which in this case is too much.


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## RH-Electric (Jan 16, 2010)

Cache said:


> If what you say is true (and it probably is) what is the point of installing a sub panel. If it doesn't increase the capacity of the building then it doesn't serve a purpose other than to provide a remote shutoff location.
> 
> Is there another benefit that I don't see?


Installing a sub panel is a way to distribute the load, it does not increase the capacity. It's an electrical design consideration, for example, if I'm wiring a large house with the service at one end and the kitchen at the other, I may choose to put a sub panel in the kitchen to service the load locally, it would reduce voltage drop and may be more economical than running many branch circuits a great distance to a main panel.

Also, having the breakers close to the load they serve is very handy, in a house this is not really a consideration since breakers almost never trip, but a wood shop like yours, you can expect breaker tripping to happen on occasion. 

I'd put an amp meter on your main panel and turn everything on in the house and see what your current (no pun intended) load really is. Then go from there.


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## Cache (Sep 18, 2007)

RH-Electric said:


> Installing a sub panel is a way to distribute the load, it does not increase the capacity. It's an electrical design consideration, for example, if I'm wiring a large house with the service at one end and the kitchen at the other, I may choose to put a sub panel in the kitchen to service the load locally, it would reduce voltage drop and may be more economical than running many branch circuits a great distance to a main panel.
> 
> Also, having the breakers close to the load they serve is very handy, in a house this is not really a consideration since breakers almost never trip, but a wood shop like yours, you can expect breaker tripping to happen on occasion.
> 
> I'd put an amp meter on your main panel and turn everything on in the house and see what your current (no pun intended) load really is. Then go from there.


Thanks for the info.

I like the suggestion of turning everything on, but this home is not completed to the point where I could do that. The client just informed me of the desire to use the garage as a wood shop. The house is currently under construction, but already has permanent power. I'm just trying to figure out the best way to make that happen.


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## Cache (Sep 18, 2007)

Thanks for the patience guys. I think my suggestion will be to still run a subpanel out to the shop, to prevent having to clean sawdust out of the carpet after a tripped breaker. 

By my calculations the ~90ft run would require a 100A sub-panel and 2 AWG THHN feeder.

What is the max # of breakers he could expect to get directly from a 100A sub panel?


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## RH-Electric (Jan 16, 2010)

Cache said:


> What is the max # of breakers he could expect to get directly from a 100A sub panel?


Square D Homeline MLO panels in the 125 Amp version go as high as 24 spaces. If need be, you can always sub another panel below this sub panel to add spaces.


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## Cache (Sep 18, 2007)

480sparky said:


> If you're coming from the main panel, you'll need four wires. If you're coming from the meter, you may need a breaker before you get too far into the building..... many local codes have rules as to how far into a structure you can go with unprotected wires.


Everyone around here used combo load centers, so the lines are breaker protected directly at the meter.


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## 480sparky (Feb 1, 2009)

Cache said:


> Everyone around here used combo load centers, so the lines are breaker protected directly at the meter.


Then you'll need 4-wire to feed the sub.


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## woodchuck2 (Feb 27, 2008)

As 480 stated local codes vary, here you can run only 15' without installing a dis-connect. Code may vary on if run 3-wire or 4-wire too, here you can run 3-wire as long as the sub-panel has an earth ground if not then 4-wire is required. If you are using the existing 200amp home panel as sub-panel then only 3-wire should be required assuming the 200amp panel is code compliant with its earth ground.


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## Cache (Sep 18, 2007)

woodchuck2 said:


> As 480 stated local codes vary, here you can run only 15' without installing a dis-connect. Code may vary on if run 3-wire or 4-wire too, here you can run 3-wire as long as the sub-panel has an earth ground if not then 4-wire is required. If you are using the existing 200amp home panel as sub-panel then only 3-wire should be required assuming the 200amp panel is code compliant with its earth ground.



The permits were pulled before July, so we would've been allowed to go with 3-wire. BUT.....The shop is in an attached garage. No 3-wire for an attached structure PERIOD!!! Thus says the inspector, who conveniently is the electrical instructor at the local college.


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