Sizing sub panel power supply conduit

[Grant Morris]

I need to run a 60-100 amp sub panel for my garage workshop. The existing main 125 amp panel sits flush on the outside garage wall and I can get to the back of it from the inside of the garage once I open the drywall up. It has two slots left for a breaker to power the sub panel.

The sub panel doesn't need to be any further away from the main panel than the next set of studs so I will have a very short run for the power supply to it from the main panel. I want to flush mount the sub panel and keep the power supply conduit inside the walls. I don't want to compromise the structural integrity of the studs by drilling too large a hole in them so my questions are:

1. What is the minimum size conduit I need to run four #6 cables inside?
2. Is it Ok to split up the sub panel power supply runs into smaller two conduits (2 cables in each one etc) rather than larger one?

I will be running 2 220v tools at the same time, no more. The dust cleaner is 12 amp and the table saw is 18 amp. I may add more powerful tools later so I will be running 10 awg cable for the 220 circuits. There will also be a 110v receptacle circuit and a floursecent light fixture circuit.

Thanks a lot in advance.

[Jim Becker]

I cannot comment on the sizing question, but I suggest you try to get your lighting off of the existing service if there is capacity. That way, you could put in a cut-off switch and shut down all the machine circuits without killing the lights.

[Duncan Horner]

2. Is it Ok to split up the sub panel power supply runs into smaller two conduits (2 cables in each one etc) rather than larger one?


I got part of this, kinda

No, you can't split the wires into different conduits. You run into magnetic/induction/grounding issues.

You *may* have duplicate runs (All 4 wires in each conduit), terminating in the same places, only for a certain wire size and above, and you must have lugs marked for more that one wire termination, you may not terminate 2 or more wires on lugs designed for one wire only. Contact your local code office or the NEC to find the wire sizes this is permitted for, but I'm pretty sure it's well above what you'll need.

[Rollie Meyers]

#8 THHN is rated for 50 amperes, 3 #8 and 1 #10 will fit in a 3/4" conduit just fine BTW since your neutral is smaller then # 4 you must use a white colored conductor, tape is not acceptable see 200.6(A) 2005 NEC.

6-#8 or 4-#6 w/ THHN insulation will fit into a 3/4" conduit.

[Roger Frazee]

If your going access your panel back plate knockouts to run your feeder to the new sub-panel then conduit is not necessary. Your staying inside walls and their cavities so a cable such as NM-b or SER would serve just as well. Center drill the stud just enough to let the cable pass through. The ampacity of #6 awg NM-b or SER is a table 310.16 60C selection and would allow you 55 amps. You would put this on a 60 amp breaker. You will need a 4 wire cable H-H-N-Grd. You can use conduit if you like but since the outside panel is flush mounted it seems overkill to me. You might have meant surface mounted but I would still use cable and a short nipple or EMT stub to go from the outside panel to the inside wall cavity.
One important thing to remember in your situation is to keep the equipment ground and neutral separate in the sub-panel. You very likely will have to purchase a ground bar kit to do this. This simply means that the main bonding jumper will not be installed in the sub-panel, many times this is a green screw supplied with the panel in a paper envelope....do not install it.
I've attached a diagram of mine for you to use as a guide. However you should verify with a qualified individual anything you get over the internet when it comes to electrical... no offense will be taken....:)
http://media4.dropshots.com/photos/440526/20080120/074224.jpg

[Paul Gatti]

No conduit needed if you keep the wiring inside the walls. I have the nearly same setup as you do. My main service panel is on the outside of the garage and I surface mounted my subpanel right behind my main panel on the inside of the garage. I ran #4 SER (Service Entry Cable) to my subpanel.

[Anthony Watson]

I need to run a 60-100 amp sub panel for my garage workshop.

The sub panel doesn't need to be any further away from the main panel than the next set of studs

I don't want to compromise the structural integrity of the studs by drilling too large a hole


For a 60 amp subpanel, you could use 6/3 cable with ground, commonly used to power electric ranges. You could run this directly in the wall, no need for conduit. Just add a metal protection plate over any holes you drill through the studs.

For a 100 amp subpanel, you would need to use four individual #2 copper wires. You'll need two black wires for the hots, one white for the neutral, and one green or bare for the ground. (most home centers will cut wire by the foot). You will need at least 1-1/4" conduit to protect the four #2 wires, but larger conduit will make pulling the wires easier.

As for the hole sizes, you shouldn't drill a hole larger than 40% of the stud depth (max 1-3/8" hole in a 2x4 wall, max 2-3/16" hole in a 2x6 wall) in a load bearing wall. The hole should be centered in the stud, of course.

If the stud wall is NOT load bearing, you can drill a hole up to 60% of the stud depth (2" hole in a 2x4, 3-1/4" hole in a 2x6).

Remember the outside of the conduit is larger than the rated size.

Anthony

[Grant Morris]

As usual you guys are a great source of knowledge. Thanks to everyone that offered feedback.

[Chris Padilla]

The ampacity of #6 awg NM-b or SER is a table 310.16 60C selection and would allow you 55 amps. You would put this on a 60 amp breaker.

I ran #6 NM-b put I protect it with a 50 Amp breaker. Why can you use a 60 Amp breaker?

[Brad Sperr]

I ran #6 NM-b put I protect it with a 50 Amp breaker. Why can you use a 60 Amp breaker?

I'm also wondering about this, because I'm planning on running 6/3 to my subpanel. According to the table, it looks like it should be allowed to carry up to 75amps, since the NM-B is rated at 90 degrees. I was planning to use a 60amp breaker just to be safe. Anyone know if this would be a problem.

[Rollie Meyers]

I'm also wondering about this, because I'm planning on running 6/3 to my subpanel. According to the table, it looks like it should be allowed to carry up to 75amps, since the NM-B is rated at 90 degrees. I was planning to use a 60amp breaker just to be safe. Anyone know if this would be a problem.

You cannot use the 90°column for sizing conductors,that is the advice that the dimwits at Home Depot pass out because there is no terminals rated to accept 90° conductors. NM cables must use the 60° column to determine allowable ampacity per NEC article 334.80 2005 edition. BTW, as long as the load does not exceed 55A you can use a 60 A breaker.

[Roger Frazee]

I ran #6 NM-b put I protect it with a 50 Amp breaker. Why can you use a 60 Amp breaker?

Rollie has pretty much summed it up. To directly address your question the cable is rated for 55 amps for the reasons already mentioned. You are allowed to protect it with a 60 amp breaker because there is no 55 amp breaker. The electrical code allows you to use the next size up standard breaker in situations like this....which is a 60 amp breaker.

In general (there are several variances) and a bit of risk of oversimplification, the ampere rating of a fuse or circuit breaker is the maximum amount of current that it can safely carry without opening over time. Fuses and circuit breakers have a range of ampere ratings. NEC 240.6 lists the standard ampere ratings for fuses and inverse time circuit breakers. Standard amperage sizes of the Code are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000 and 6000.

If you notice at 50 amps the next size breaker is 60 amps and then the list continues after that. So in explanation if the ampacity of the conductor falls on a standard breaker size you cannot move to the next breaker up. So if the ampacity is 50 amps for the conductor you would use a 50 amp maximum breaker or any one standard size less than 50 amps. In the case of a NM-b cable #6 using the required 60C column of 310.16 the ampacity is 55 amps. This falls inbetween standard size breakers and you are allowed to use the next size up. As mentioned there are exceptions so it is important to understand what those are. For those with access to the code book read 240.4(B). In the case of this thread and a sub-panel feeder you are allowed to move to the next size breaker above 55 amps.
Without some degree of experience and formal training opening the NEC code book and reading a chart would place you at risk of error. As an example if I walked up to your lathe I would be at risk of error.....:) Hope this helps guys and gals.

[Chris Padilla]

Thanks a bunch for the explanation, Roger. I wasn't going to quibble too much about 10 A and since I've had this set up for 2 years and it has worked fine, I don't see a need to bump up my breaker but it is nice to know I CAN do it should the need arise. :D

[Rod Sheridan]

Boy, you guys are going to laugh, my shop is fed from a 30 ampere feeder.

I don't have lighting on the shop panel, however I have a 1.5HP cyclone (8.5 amperes), and a 3HP Cabinet saw and Shaper as the largest motors (12.5 amperes).

So the largest possible load is 21 amperes, however only the cyclone is loaded to maximum on a continuous basis.

I guess you guys must run air conditioning/heating and perhaps larger machinery than me, must be nice to have large shops, I'm jealous.

Regards, Rod

[Brad Sperr]

Thank you for clearing that up Rollie and Roger. I wanted to make sure everything in my shop was being done by the book, NEC that is, and having the code references definitely helps.

[Roger Frazee]

Hello Brad

I think at this point one of the important exceptions to the 'next size up rule' should be brought to attention. NEC 240.4(D). This subsection makes exception to small conductors such as found in typical nm-b (romex) cables that we commonly use to supply lighting and receptacles. Most common is 14,12, and 10 awg copper conductors. You are not allowed to protect these at more than 15 amps, 20 amps and 30 amps respectively and you will notice you could easily misunderstand if reading 310.16 which allows higher ampacities. You will notice that for these conductors astericks are next to them referring you to the footnote to see 240.4(D). Exceptions to this are found in the listings of table 240.4 G. Air Conditioning branch circuits are a good example where a motor compressor is operating. In these cases you are allowed to exceed the required ocpd for small conductors due to starting amperage inrush to get the compressor going in accordance with the nameplate specifications of the equipment. It would not be unusual to be allowed a 35 amp breaker on 12 awg conductors to allow the compressor to start without tripping out the breaker and then the unit will return to normal running amps which will always be within the ampacity of the supply conductors of the branch circuit. Many hardwired woodworking power tools such as 5 hp cabinet saws are treated the same.

EDIT:

The biggest key here is small conductors that supply multiple receptacles (outlets) for cord and plug equipment cannot use the next size up rule. A duplex receptacle is a multiple outlet. I know this gets confusing but I did not want someone thinking they could put a 25 amp breaker on 12 awg romex serving multiple general purpose receptacles even though table 310.16 allows 12 awg romex to carry 25 amps.
I apologize for getting a little off base but this stuff is not easily or simply explained...guess thats why we have professional trades for woodcraftsmen and electricians, plumbers etc....