View Full Version : Garage Electric Subpanel

11-15-2014, 05:25 PM
I’m considering a sub panel for my detached garage.

Reading responses to my 3-Phase-vs-Single-Phase-240-vs-120 post (http://bbs.homeshopmachinist.net/threads/65009-3-Phase-vs-Single-Phase-240-vs-120), it looks like 240 in the workshop is really nice if possible. A 240v in 1P to 3P VFD is about $50 cheaper than the 120v n 240 v out VFD. Someone also mentioned having breakers accessible in the garage which is a great idea. Current service to the garage is a buried conduit with 2 runs of 14-2 1 run of a 14-3 for outside lights on a 3 way switch. The run is about 25’ Main breaker pane to garage.

All licensed electricians and those who follow codes to the letter may wish to exit here. Back to Forum General (http://bbs.homeshopmachinist.net/forums/3-General)

I’m considering re-purposing both 14-2 cables to feed 240 to a sub panel. That panel wired like a small house main panel, could provide one or two 240 circuit and two 120 circuits to the garage.

By most codes, 14-2 cable is limited to 15A. By size it will carry 20A.
Running two 14-2’s in parallel with good connections should carry double the above.

The continuous loads on the garage are fairly small. Several LED lights, a clock radio, and a trickle charger.

Feeding with a 40A circuit breaker out of the main panel will allow for intermittent loads of motor starting or future welding. No plans for electric heat, a stove, or electric kiln out there so the continuous load should never approach 30A.

< EDIT- Note: Current cables are buried and its snowing this weekend. Next summer there is 90% chance of excavation next to the garage which will facilitate a new wiring and possibly a gs line.>

Does this sound feasible?

11-15-2014, 05:56 PM
JMO, but it sounds like youre risking a fire and creating a bird's nest to save pulling maybe $50 of wire. I'd replace the current wiring with individual #8's simply to bring it up to current code, be rid of the romex, and make room for future cable/telephone/other lines in the raceway. I'm not a code Nazi, but I've repaired far too much bad/lazy wiring.

11-15-2014, 06:08 PM
I don't know the cable size but one thing I have learned, whatever you think is enough, won't be!, I've run a 10mm2 3 core SWA to my garage, think that will take about 53 amps, but it is 300' long, I'd run another cable myself and reuse the old ones for lights or something you may want to switch from the house, I'm forever leaving the lights on,

11-15-2014, 06:12 PM
justanengineer- I edited the original post for some clarification.
Currently pulling (digging) new wiring is not an option.

Option 2 is to keep one of the 14-2's as a 120 circuit and use the other as a 240, both at 15A.

11-15-2014, 06:23 PM
Ok, if its only about a 25ft run, why not run it overhead? A piece of #6 triplex (aluminum) is pretty cheap. At each end, a piece of #6 ser range cable for the vertical run into the buildings. Then, when you do your digging next year, put in some #2 urd cable underground to the garage, its direct burial, no conduit necessary and its cheap.

J Tiers
11-15-2014, 06:26 PM
15A at 240 is more power than 15A at 120, for sure.

problem with overloading 14 ga is NOT fire, that won't happen soon (might eventually). The problems are that the insulation will get old and brittle more quickly with the overheating if running 20A at 14 ga. Also that the voltage drop will be more than it should be, especially on motor starts, so you will be un-doing some of the good of using higher voltage.

11-15-2014, 06:35 PM
those who follow codes...may wish to exit here

I know from many years of observation (including looking in the mirror) that in any given situation a person will either do what's right, or what he thinks he can get away with. Based on the results I've seen, these days I generally try to do what's right. "YMMV".

11-15-2014, 06:58 PM
you said "buried conduit"? Pulling wire is what conduit is all about. Just use the existing wires as the fish, and pull in as large a set of wires as you think you need. :rolleyes:

11-15-2014, 07:00 PM
Not installing it to code is not smart... the electrical code is there help you preserve life and property, not to be an impediment.

Code does not allow you (for very good reasons) to parallel small conductors, so that's out. And.. with 14 guage romex you are limited to 15 amps. Ampacity (the 20 amps you are referring to) is used for derating calculations (temperature, bundling etc), not to extend load. heck, you can get 30, 50 or whatever amps out of a 14awg conductor, but... lol..

If you want to derive 120v from the 240v legs, you need to extend the neutral as well as ground. That neutral must be in the same wiring "conduit", and if romex, you can't use an additional wire from another romex to supplement the 14-2. You most certainly cannot use the ground as neutral.

Are you sure it's conduit all the way, or just at the end transition points to code burial depth? It's not legal to run romex in a conduit system. It is legal to use it for "protection" at transitions or where exposed to mechanical damage if the conduits ends are open (closing them makes "system"). If it's real conduit, rip out the romex and pull in new THNN wires. Even a 1 inch conduit can easily handle 8 awg wiring. Look up the code for subpanels in detached buildings. You need to extend ground and have a ground rod, and unless the panel is close to a door, a disconnect switch at the power entry. Check your local codes.

As suggested by Sparky, run overhead wiring until you can dig.

11-15-2014, 07:13 PM
Option 2 is to keep one of the 14-2's as a 120 circuit and use the other as a 240, both at 15A.

Infinity smarter. Skip the subpanel - no point. You can put multiple outlets on 240 like 120. Keep your lights separate; tripping a breaker that takes out the lights can suck.

11-15-2014, 08:04 PM
Ok I have seen the light (code).

Was tinking that any non standard wiring could cause insurance problems if there were a fire.

For now I will keep one 120 and make the other a 240, both breakered at 15A.

Thank you for all the feedback


11-15-2014, 09:44 PM
... and make room for future cable/telephone/other lines in the raceway.
Phone, cable, networking, etc. wiring cannot share a conduit with power wiring.

Also, if I remember correctly, there is wording in the NEC that states you should not load circuits beyond 80% of their rated capacity. I.E., you should not
pull more than 12 Amps from a 15 Amp circuit and 16 Amps from a 20 Amp circuit, though I'm sure that is mostly ignored.

J Tiers
11-15-2014, 09:56 PM
Phone, cable, networking, etc. wiring cannot share a conduit with power wiring.

Also, if I remember correctly, there is wording in the NEC that states you should not load circuits beyond 80% of their rated capacity. I.E., you should not
pull more than 12 Amps from a 15 Amp circuit and 16 Amps from a 20 Amp circuit, though I'm sure that is mostly ignored.

That's for "long term loading". Used to be poorly defined, but I think the writers have gotten closer with a definition.

11-15-2014, 10:14 PM
Yes, it is now better defined.

On the other hand, many householder breakers won't hold a circuit longer term (hours) it is over 80% of breaker rating. I don't think the 80% issue is deliberately ignored, but there is little consumer awareness of what plugs into an outlet. What is more often "ignored" by homeowner wiring (mostly though lack of understanding of how to apply the relevant NEC) is the derating of wires run though a hot attic, or bundles of wires though conduit, common holes in wood or just proximity.

J Tiers
11-15-2014, 11:46 PM
You do not need to derate for passing through short sections of close proximity. I forget the min length, but it was longer than I thought.

11-16-2014, 12:10 AM
This isn't "the code" and is from 2002, but make for interesting reading.


The code doesn't clearly define exactly what bundling is so the local AHJ has final say. In general it's 24 inches, so consider hole in line through 24 inch spaced rafters or floor joists, and multiple NMB cables pulled though - they all end up in "a bundle" and are often surrounded by insulation. I don't remember the exact details but here it was a max (or derating) of 3 x 12/2 with 20 amp breakers in any one bored hole when the cables aligned and were in insulation. There is another rule for conductors that pass though the hole, but then disperse in different directions or are separated and not in contact with insulation.

J Tiers
11-16-2014, 11:20 AM
The 24" length is based on heat conduction along the wire from the bundle area to other areas outside the bundled area.

This is a valid consideration, that conduction can be verified by test. I would argue that the 24 inch is an average, and that 12 and especially 14 ga should be shorter, 10 ga is perhaps OK, 8 and larger can be longer, but the code writers didn't opt to do that. My argument is that the length is best expressed as length vs cross-section, length of "x" number of equivalent diameters, to properly account for heat flow.

Now, if you have TWO "points of failure" here, first that the wires are bundled for a distance, AND that the wire nearby but outside the bundle is not allowed to dissipate heat, then the assumed conditions are violated. The single derating is not valid because there is no place for the heat to go.

There is probably a valid argument for additional derating *when surrounded by insulation*. perhaps the derate should be applied twice. Once for the heating condition and once for the insulated condition. Then once only for an insulated condition without added heating.

When bundled in raceway, there is an assumption that the surface of the raceway is allowed to dissipate heat. When buried, there is an assumption that the earth will carry away heat. If the conditions are such that this does not happen, then added derate is obviously required.

precedent for the derating is perhaps available in cable tray requirements, where each cable must have free airflow. The tray is open, but "bundling" can occur if the cables are too close, cross over, or have blocked airflow for other reasons (tray not open enough, etc).

I suspect that future code editions will include a double derate or similar requirement, since houses are being built with more insulation these days.

Paul Alciatore
11-16-2014, 11:28 AM
I assume your primary motivation is to have 240 V available in the garage; NOW, not next spring. If you are going to re-purpose the existing cable, re-purpose one of the existing 120 V cables to 240 V. And next spring, get your new box and lay a new cable.

11-16-2014, 12:54 PM
Hate to bring it up but if you should have a fire that is traced back to this electrical wire you will have a major problem with your insurance company paying.