Electrical Question: Extension Cords

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Forum topic by bringitonhome posted 05-04-2010 10:24 PM 2689 views 0 times favorited 10 replies Add to Favorites Watch
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28 posts in 3069 days

05-04-2010 10:24 PM

Ok I need some help from you electrical-type folks.

I see in a lot of tool manuals some caveats about using extension cords – specifically in reference to length and gauge. Usually they warn that too long a cord will result in a drop in voltage and burn out the motor. Most things i have read don’t recommend using anything over 50 feet – at 12 or 10 gauge max. I have 20 amp service on the exterior of my house, and my shop is in a detached garage just over 50 feet away. I just started acquiring big tools. (got a jointer, contractor saw on the way, etc). I had planned to use a temporary extension cord from the house when I’m working out there. So now I’m confused.
What I don’t get – If I had the 20 amp service run to the shop, there would still be 50 or so feet of 12 gauge wire running under the ground to a receptacle there. What’s the difference if it’s inside an extension cord or inside NM cable? Same gauge, same distance.

*scratches head

10 replies so far

View TopamaxSurvivor's profile


18266 posts in 3670 days

#1 posted 05-04-2010 10:36 PM

You need to run bigger wire for your circuit to compensate for the voltage drop. Run 10 or use a #10 cord.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View Michael Murphy's profile

Michael Murphy

453 posts in 2999 days

#2 posted 05-04-2010 10:38 PM

Here are some answers from WIKI
Sounds like Topamax is right. Never doubted it.

-- Michael Murphy, Woodland, CA.

View CharlieM1958's profile


16274 posts in 4212 days

#3 posted 05-04-2010 10:40 PM

I love electrical questions. You guys wait while I get the popcorn started. :-)

-- Charlie M. "Woodworking - patience = firewood"

View bayspt's profile


292 posts in 3698 days

#4 posted 05-04-2010 10:41 PM

The only differance would be that most extension cords are going to be braided wire and NM is going to be solid wire. There is a bigger voltage drop on the braided, but they use it for extension cords because of the flex. If it where me and I was only going to use it temporary, I would get some 10-2 or even 8-2 NM and make my own extension cord to cover the distance. This is going to give you the least drop possible over the run to your shop. Just don’t forget to put it inside when not in use.

-- Jimmy, Oklahoma "It's a dog-eat-dog world, and I'm wearing milkbone underwear!"

View TopamaxSurvivor's profile


18266 posts in 3670 days

#5 posted 05-04-2010 10:43 PM

I’m with you Charlie. I’m done and watching now :-)) I can hardly wait!!

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View Knothead62's profile


2584 posts in 2955 days

#6 posted 05-05-2010 02:53 AM

You are supposed to run a heavier gauge wire for long distances. I was told by an electrician that you don’t worry about voltage drop until you get to about 100 feet. If you are going underground, use the appropriate rated wire or run the wire through conduit. Schedule 40 under ground level, Schedule 80 above ground. I would run the wire for the convenience instead of running an extension every time I went to the shop. I ran an extension cord to a mini-shop in Louisiana. A real pain, especially if it was raining.
A good electrical supply house should have all the supplies and information you need.

View Mark Whitsitt's profile

Mark Whitsitt

86 posts in 2973 days

#7 posted 05-05-2010 04:04 AM

good comments from the others… my only thought is that if you’re getting into multiple tools, 20 amps may not be enough… my dust collector sucks up 20 amps all by itself when it starts up… same thing for my compressor (both not running simultaneously). If you’re going to use a table saw and a dust collector bigger than a shop vac, 20 amps is not going to be enough, imho

-- -- "there are many good reasons to use old hand tools, but moral superiority is NOT one of them..."

View richgreer's profile


4541 posts in 3069 days

#8 posted 05-05-2010 02:37 PM

I’ll pick up on Jawhorse’s point and suggest that you really need 2 (20 amp) circuits to your shop if you are going to be running 2 machines at a time (e.g. TS and DC).

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View bringitonhome's profile


28 posts in 3069 days

#9 posted 05-05-2010 06:08 PM

Hm – well, I’m not about to tell the wife we have to rip up the 2-year old patio and walkway so I can get more power in the garage :)

I have 15 amps out there already for the lights and (hopefully) DC. This 20-amp extension would be dedicated to whatever tool I’m using. I guess I’m going to try a 10-gauge ext cord.

View Yekrub's profile


47 posts in 2995 days

#10 posted 05-05-2010 06:42 PM

One thing to remember is what ever breaker (plug-ing) your running your extension cord from, that the wife and kids are not also running something from the same breaker at the same time as your equipment. For instance, you run an extension cord from a wall socket in your basement thru a window and out to the shop. Your midway thru cutting a large sheet of plywood when all of a sudden the dehumidifier decides that there is too much moisture in the basement and turns on and throws the breaker. It sucks and is unsafe to have to restart this cut after a lot of cusing and the travel thru the house to the breaker box.

I would suggest installing a small breaker box inside your shop that the electrical service runs to. This would only need to be a 2 breaker switch for most application purposes. You can pick one of these up at the local hardware or from Craigslist fairly cheap. This way, if you should happen to overload the breaker you only need to reset it in the shop instead of going across the lawn (possibly in the rain), down to the basement (possibly passing the wife who will ask you to do something else then what your currently doing), and reseting it at the main power box. Installing a permanent line to the shop is much safer and less time consuming in the long run. I would suggest using a 10ga wire just for the extra benefit and the cost difference is only a few bucks. I recently bough 50’ of 10/2 romex and I almost positive it was under $50. There are different types of wire for underground/outdoor applications and interior applications. The use of conduit is highly suggested in case you decided later to install a purgola around that 2 year old patio and dig down to set a post and hit the wire with the shovel.

**edit: I just remember the worse part of what I wrote above, If installing this tye of service you must have a licensed electrician do the work per building code and safety. Luckily my grandfather and father were so I learned a lot from them when I was growing up. Also my father-in-law is one so he oversees and checks my work quite thoroughly.

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