Electrical Questions

[Mike Ruggeri]

I moved about a year ago and the "new to me" house has limited electric in the attached garage. The garage is at the same level as the basement as the house is somewhat built into a hill. Now I would like to add a sub-panel in the garage for the workshop. I have read a bunch of old posts on sub-panels but I still have some questions in an effort to avoid doing something wrong. I did a bit of wiring in my old house and will have an electricial friend do the connections at the main panel. The main electric panel is on the opposite side of the basement from where I want the sub-panel. Probably a run of around 50-60 feet. I will have to run the wire from the main panel to the sub-panel via the basement ceiling/1st floor joists. The basement has a dropped ceiling and the rafters overhead run perpendicular to the direction of the cable (i.e. the rafters run front to back and I need the electric line to go side to side). I will most likely use a 100 amp panel for the shop sub-panel to "give me room" and feed it with a smaller sized breaker in the mail panel.

Questions:

1. I think a 60 amp sub-panel should be fine. The most I can ever see running at the same time will be a 3hp machine (20 amp circuit) and the 1 1/2hp dust collector (15amp circuit). The lights will be tied into the main panel and not run through the sub-panel. Do you think a 60 amp will do? Even if I get a 5hp machine (30amp) in the future, do you think 60 amps is OK?

2. My next question is related to wire size & type. I would like to use 6/3 with ground non-metalic (i.e. Romex) as it will be the easiest to run (conduit would be a real pain). I have read on-line that it is OK to run 6/3 attached directly to the bottom of the basement's ceiling joints and then will run it in conduit when it goes into the garage as the garage walls are block wall to the sub-panel. Is 6/3 Romex and my plan of hanging it to bottom of the ceiling joints OK? Is their any concern that there is a dropped ceiling covering this (any given tile in the dropped ceiling is removable in seconds and the dropped ceiling is a few inches below the bottom of the joists)?

3. Do you see any other problems with my plan?

Thanks in advance.

Mike

[Rob Russell]

1. I think a 60 amp sub-panel should be fine. The most I can ever see running at the same time will be a 3hp machine (20 amp circuit) and the 1 1/2hp dust collector (15amp circuit). The lights will be tied into the main panel and not run through the sub-panel. Do you think a 60 amp will do? Even if I get a 5hp machine (30amp) in the future, do you think 60 amps is OK?

IMO, 60 amps @ 240v should be fine. The only thing you might ever run into is if you have a 240v compressor wired on its own circuit so it can cycle on as needed and it kicks on while you have a 5hp table saw and dust collector running. Even then you'd probably be OK.



2. My next question is related to wire size & type. I would like to use 6/3 with ground non-metalic (i.e. Romex) as it will be the easiest to run (conduit would be a real pain). I have read on-line that it is OK to run 6/3 attached directly to the bottom of the basement's ceiling joints and then will run it in conduit when it goes into the garage as the garage walls are block wall to the sub-panel. Is 6/3 Romex and my plan of hanging it to bottom of the ceiling joints OK? Is their any concern that there is a dropped ceiling covering this (any given tile in the dropped ceiling is removable in seconds and the dropped ceiling is a few inches below the bottom of the joists)?

This one could get a bit stickier.

The easy answer is on the installation - you can attach the NM 6-3 to the bottom of the floor joists (per 334.15(C)].

The not so easy answer is whether your local AHJ (Authority Having Jurisdiction) will allow the use of 6-3 for a 60 amp subpanel. My guess is yes, but you should check with them because the code as adopted by the AHJs is not always the vanilla NEC.

Table 310.15 lists the ampacities of various types of conductors. The ampacities are specified according to a temperature rating - 60, 75 and 90 degrees Centigrade. The higher the temperature rating, the higher the ampacity. There is a specific rule about NM-B (334.80) which says that the ampacity for NM-B "shall be in accordance with the 60 degree Centigrade conductor temperature rating". That means NM 6-3 with copper conductors has a rated ampacity of 55 amps.

There is another rule, 240.4(B), which allows the use of the next highest standard overcurrent device provided some conditions are met. The next highest standard overcurrent device is 60 amps. So, if your AHJ allows 240.4(B) and considers your subpanel as meeting the conditions of 240.4(B), then you can use NM 6-3 copper for your 60 amp subpanel.

There is another option. Run 2-2-2-4 aluminum SER. It has an ampacity of 75 amps and is probably cheaper. NM 6-3 copper at our local big box is $2.62/foot. 2-2-2-4 aluminum SER is $1.92/foot. The 2-2-2-4 will be a bit stiffer to work with, but you'll save yourself $40 at my area's prices.


3. Do you see any other problems with my plan?

Nope.


Rob
Addy protocol: unlicensed, homeowner electrician

[Ben Cadotte]

I agree on the aluminium cable. Even though larger, it will probably be cheaper. Just make sure you use the anti-corrosion fluid on the ends. Also make sure you seperate your neutral, and ground in your sub panel. May already know this may not.

Now for panel size. I would go with a 20 space 100amp panel. They are only a couple dollars more than a 60 amp panel (at the borgs). Why not have the space available for future use. Its litterally just a couple dollars. Maybe a hot tub or something for the wifey is in the future needs. May or may not be convient for your situation.

[John Hixon]

There is another option. Run 2-2-2-4 aluminum SER. It has an ampacity of 75 amps and is probably cheaper. NM 6-3 copper at our local big box is $2.62/foot. 2-2-2-4 aluminum SER is $1.92/foot. The 2-2-2-4 will be a bit stiffer to work with, but you'll save yourself $40 at my area's prices.


Look at that aluminum wire and consider what it will take to pull through your conduit. (Just a thought.)

[Rob Russell]

Look at that aluminum wire and consider what it will take to pull through your conduit. (Just a thought.)

Yeah, use a large enough conduit and it shouldn't be too bad. The SER is pretty stiff, but with one person pushing/feeding and one person pulling, it should go through without a major problem.

Mike,

One thing you need to do is find out what the diameter of the 2-2-2-4 aluminum SER cable is. You'll need that to determine what size conduit to run.

Rob
Addy protocol: unlicensed, homeowner electrician

[Rick Gifford]

The cost of the copper verses aluminum here is just crazy. So I ran the aluminum.

Our code allows the SER not to be in conduit as it has its own sheath. If I had ran individual wires then it had to be in conduit. While the added cost of the conduit isnt a big concern, getting it through the basement and then pulling those large wires gave me a headache just planning it. It would have been a major pain.

So the SER is what goes through the basement. Secured to the joists. Fully inspected and 100% in code. Don't know where you are at, but I certainly would check your local code first.

[Kewan Floyd]

I don’t mean to thread jack here but I’m in the planning process of wiring my unfinished basement and will be doing it very similar to Mike’s plans. Hopefully my questions will benefit the discussion. I also plan on running the wire/outlets myself and having an electrician hook up the main panel and check my work.

I was planning on a 100amp sub panel (for space reasons) connected to the main panel via a 60amp circuit breaker. My max amps are very similar to Mike’s so I think I will be ok with a 60amp breaker. I was leaning towards running 2-2-2-4 aluminum SER for the reasons mentioned. Since most of my basement is finished I will have to run the wire outside along the back of the house for about 70 feet.

My two questions are:

1) What type of conduit is best for running the 2-2-2-4 outside?
2) Is there any issue using a 60amp circuit in the main panel to feed a 100amp sub panel? Is it okay since my needs will be under 60amps and the amps would be limited by the 60amp breaker at the main panel? Is this ok with code?

Thanks in advance for the help.

[Rob Russell]

The cost of the copper verses aluminum here is just crazy. So I ran the aluminum.

Our code allows the SER not to be in conduit as it has its own sheath. If I had ran individual wires then it had to be in conduit. While the added cost of the conduit isnt a big concern, getting it through the basement and then pulling those large wires gave me a headache just planning it. It would have been a major pain.

So the SER is what goes through the basement. Secured to the joists. Fully inspected and 100% in code. Don't know where you are at, but I certainly would check your local code first.

Rick,

The SER needs to be sleeved or otherwise protected where it's exposed to damage. An example of that would be running down a concrete wall if the calbe would in a position so it's subject to damage.

Mike isn't planning to sleeve the entire run of the cable - just the part where it's on the cinder block wall.

Mike - as a thought - check with your local AHJ, but if you attach some boards to your cinder blocks at the top of your wall, you might be able to run the SER along those instead of in conduit. You might need to sleeve it from the ceiling down to your subpanel, but not necessarily. If your AHJ considers the wall space above your subpanel NOT subject to damage, then some boards to attach the SER to would suffice there also.

I'd ask your AHJ what they consider to be an acceptable installation given your specific circumstances.

Rob
Addy protocol: unlicensed, homeowner electrician

[Rob Russell]

... I was leaning towards running 2-2-2-4 aluminum SER for the reasons mentioned. Since most of my basement is finished I will have to run the wire outside along the back of the house for about 70 feet.

My two questions are:

1) What type of conduit is best for running the 2-2-2-4 outside?
2) Is there any issue using a 60amp circuit in the main panel to feed a 100amp sub panel? Is it okay since my needs will be under 60amps and the amps would be limited by the 60amp breaker at the main panel? Is this ok with code?

Thanks in advance for the help.

Kewan,

#2 first - there is no problem feeding a panelboard rated for 100 amps with conductors and overcurrent protection rated for 60 amps.

#1 - you can't run 2-2-2-4 SER in conduit outside, because it's not a wet location rated cable. You could run USER if it's available (Underground SER), but I don't know if that's available in 2-2-2-4. That you'd actually want to bury with Schedule 80 PVC to sleeve it where it runs vertically between the ground and the bend into the house. If you're running conduit outside, you could also run conduit inside and pull individual conductors.

Rob
Addy protocol: unlicensed, homeowner electrician

[John Hixon]

Hey Rob, what's an AHJ?

[Steven Wilson]

Authority Having Jurisdiction

[Rob Russell]

Authority Having Jurisdiction

Yep. Sorry about that - I try to spell it out the fist time I use it in a post.

The NEC definition of the AHJ is "The organization, office, or individual responsible for approving equipment, materials, an installation, or a procedure." Generally speaking, that means the local inspector who is approving your installation and applying the electrical and other codes (like plumbing, building) as adopted in your municipality. I live in CT and the electrical code is adopted by a state agency, enforced by town inspectors.