I’m adding some new wiring to an existing garage. The walls are presently open (exposed studs) so I’m running 1/2 inch flex conduit with 12 AWG wire to supply 120Vac to several outlets and overhead lights. The walls will be insulated and drywalled. Could have used romex, but what the hell, flex seems safer.
I’ve added a new 100A square-D subpanel in the garage which will feed the garage lights and wall outlets. (I realize the 100A capacity is overkill, but the panel was only $20). It’s a 6-12 model. I’d like to put my 120Vac 5.5hp, 25 gallon air compressor on a single outlet and feed it with a 30A breaker ( when the compressor is about 1/2 full it pulls about 17A, which is close to poping the breaker.) The total run is about 25 ft from the subpanel to the single outlet. The other 6 outlets will have a single 20A breaker feeding them. By the way, the subpanel will be fed from a new 200A main panel. I’ve run 4 8AWG wires plus a single 10AWG ground from the main in 3/4 inch flex, the distance is about 60 feet.
My questions:
1. What is the maximum size breaker that I can use to supply 12 AWG wire going to a single wall receptacle ? Is a 30A breaker safe ?
2. How many 12AWG conductors can I run in 1/2 flex conduit, the Ugly’s reference book at Home Depot says 12 in 1/2 inch and 18 in 3/4 inch flex, but that sounds like way to many.
3. Is there anything else you would recommend about adding a new subpanel in general ? I’ve pulled 5 spare 1/2 flex lines out of the subpanel, 2 will go below my house and 3 go into the attic for future use, anything else you would do ? The wall sockets will be fed with a GFCI breaker, any other suggestions ?
4. Does anyone know any websites that give info (max. # of conductors in flex, etc) like this, I wasn’t too sucessful in my searches ?
thanks !
Replies
" I've run 4 8AWG wires plus a single 10AWG ground from the main in 3/4 inch flex, the distance is about 60 feet."
You only need THREE wires PLUS a ground. And while you can downsize the ground wire for larger the wires sizes #10 is too small. For #2 to $8 supply wires, #8 copper is the smalles ground that you can use.
"1. What is the maximum size breaker that I can use to supply 12 AWG wire going to a single wall receptacle ? Is a 30A breaker safe ?"
Normally 20 amps, but there are special rules for dedicated motor circuits that will allow you to go up in breaker size. I don't have the link but go the Sq D site and they have either a calculator or downloadable chart. A google search will probably find it or similar.
"2. How many 12AWG conductors can I run in 1/2 flex conduit, the Ugly's reference book at Home Depot says 12 in 1/2 inch and 18 in 3/4 inch flex, but that sounds like way to many."
It depends on how what kind of "flex conduit" and what kind of wires.
For Flexible Metal Conduit and THNN/THWN the number is 9 & 16.
BUT! there is also a derate for multiple wires in one conduit. For 4-6 CURRENT CARRYING wires you need to derate to 80%. But #12 has an ampacity rating of 25 so that with the derating it can still handle 20 amps. Any more then you need to go with a larger size wire.
"3. Is there anything else you would recommend about adding a new subpanel in general ? I've pulled 5 spare 1/2 flex lines out of the subpanel, 2 will go below my house and 3 go into the attic for future use, anything else you would do ? The wall sockets will be fed with a GFCI breaker, any other suggestions ?"
I for future use I would probably go up a little in supply wire, Maybe #6 - 70 amp.
Bill-thanks for the info.
Ok, for the supply wires I mispoke, I did pull 3 wires plus a ground, not 4 wires plus a ground. I used three #8 and one #10 based on inputs from an electrician friend. Now, what you said " For #2 to $8 supply wires, #8 copper is the smalles ground that you can use.", I'm wondering where you got this info ? If there is some code that says this please let me know, because it differs from what the electrician told me. It really all depends on the amount of current to be pulled by the subpanel. Even though I installed a 100A sub, I'm a typical DIYer, will not be pulling tons of current in the garage, typical things like powered hand tools, drill press, 10 inch (120Vac) table saw, bench grinder, etc. Most importantly, these devices are powered one at a time.
Also, from other posts, I see there is a limit on how much one can put in a junction box. Is there some code that says how much (wire nuts ?). My boxes are pretty full, maybe I need to get bigger boxes. Any ideas ? Is this info in Ugly's ?
I see that I was wrong.
There are two different requirements, Grounding Electrode Conductor (GEC) and Equipment Grounding Conductor (EGC).
The GEC is the conductor going from the main pannel to the "grounding rods", water pipes or other grounding electrodes. It is sized by what the size of the supply wires as I gave.
The EGC is what is run to ground equpment back to the main pannel.
The is EGC is sized based on the size rating of the over current device supplying that equipment. You have a EGC and #10 is good for upto 60 amps and #8 upto 100.
As to your other question boxes are rated by cubic inches. For plastic ones they are molded in the box.
Each wire that ENTERS the box is counted except grounds as a unit. Only the first ground wire is counted. And each yoke (device) is counted as 2 units. For #12 wire each unit needs 1.25 cu in. Pigtails, jumpers, and wirenuts are not counted.
I suggest that you get a book such as Cauldwell's "Wirng a House" or Black and Deckers "Complete Home Wiring".
Also "Code Check Electrical" has all of the these tables and numbers, but you have to know what to look for first.
For the other poster you can use any size wire to run to the sub-pannel as long it is within the range of size wires that are allowed for the terminals on each end.
You can not use a supply breaker in the main pannel any larger than appropriate for the size of the wire or the sub-pannel rating which ever is lessor.
You DO NOT NEED TO RUN 100 amp wiring.
I thought he was using a 100 amp main breaker. What I was saying when I stated that "if you are using a smaller breaker for the main never mind, meaning forget what I said if you are using a smaller breaker for the main, Sorry if it was confusing, but I must have missed it where he stated he was using a smaller breaker then the 100.
Edited 12/10/2003 4:03:47 PM ET by rjgogo
Edited 12/10/2003 4:04:32 PM ET by rjgogo
I went back and looked.
He never said specifically what size the he is using for a feed breaker.
So we may both be wrong <G>.
The 3 things that are clear is that he is using 100 amp sub, a new 200 amp main pannel and #8 feeder to the sub.
Based on that he can't use over a 50 amp breake in the main pannel to supply the sub-pannel.
Note that the sub-pannel will probably have a 100 amp "main" breaker in it to add some confustion.
We should all be more precise when referring to conductor size by specifying copper or aluminum. To feed the panel with a 100 amp breaker, I think he needs #4 AWG CU or #2 AWG AL conductors. As previously mentioned, bigger would hold down voltage drop if the run is long.
Also, the max number of #4 AWG conductors that can be pulled in 3/4 inch FMC is 2 (from NEC Table C3). So, if you do decide to wire it as a full 100 amp subpanel, you'll have to go up to 1 inch FMC.
A question for one of our pro's: Suppose he leaves the feed conductors and the subpanel as is, and goes with a 50 amp breaker in the main feeding the 100 amp breaker in the subpanel. Confusing, yes, but I don't think it is an NEC violation. Is that correct ?
Edited 12/10/2003 9:24:03 PM ET by r
You are right about the wire type statement, I think everyone assumes copper and overlooks alum. For long runs for sub panels aluminum can be very cost effective. Many seem to have a bad feeling about aluminum because of sins of the past and improper small gauge usage. With proper components and procedures there is nothing wrong with it, most drops to houses are aluminum.
With 50 amp protection at feed the 100 amp breaker at sub becomes only a disconnect. I think I would still use 50 amp at sub. But I am not a real sparky.
ok guys, thanks for the inputs. Sorry about my original posting causing any confusion. I'd really like to keep with using the #8 copper feeds from the main, since I already ran it under my house in flex.
I realize having a 100A sub means that it should be able to handle 100A from the main, but I (and my electrician) sized it to only #8 since I'll never pull anywhere near 100A in the garage. (I'm a typical DIYer, not a production facility). But, a good point was brought up:
If I have a 100A sub but it's not supplied with the correct wire (#4 ?) from the main, is this a violation of the NEC ? Does anyone know for sure ? If so, I'll tear the sub out and put in a smaller ampacity (40A or so) which will be ok with my #8 wiring. I initially went with the 100A knowing that I'll never need that much ampacity and the boxes are cheap and have nice covers.
Isn't having two 40A breakers at the main that feeds the sub a safe way of limiting the current to the sub, so the #8 wire is ok and safe ? From what I've read at http://www.faq.org/faqs/electrical-wiring/part1 #8 is safe to 40A (but crap, says to go up a size (#6) when the wire is in conduit ??).
Help !
As I said the controlly factor is the size of the breaker in the main pannel feeding the sub-pannel.
The 100 amp rating on the sub-pannel is the MAXIMUM. Just because a light fixture is rated for a maximum of 100 watt bulb does not mean that you can't use a 60 watt bulb in it.
#8 cu is rated at 40 amps at 50 degrees C and 50 amps at 75.
Almost all wiring and circuit breakers are rated for 75 (or higher) to day.
For a 100 amp sub panel I think #8 is to small, I ran #4 for a 60 amp sub, that may be a little overkill for a 60 but I am guessing it is a little short for a 100, Even if you are not going to use the 100 you need to wire for it. I would rather err on the side of caution. The book I am looking at says the max for #3 is 110 amps for 3 wires in a cable depending on the wire type, but I would go #2at the least to compensate for any voltage drop over that distance. This is information from "Pocket Ref" by Thomas Glover, not an electrical book per say but a good book with a lot of useful information. If you are putting a smaller main in there then never mind.
On another note, Why not run it in pipe. You can surface mount it in a garage and it makes it pretty easy to add on or change if you want to add a 220 for a welder or something like that. A little more work as you have to bend a bit but give a nice clean look over plywood walls.