Advice on specs for long power cord for saw

[BeachLakeWoodWorker]

I've gained access to an old wood shed at my neighbors' to set-up my Ryobi 3100. Only catch is that I have to run a power cord 75+ feet from an outdoor outlet, out to the shed. Also, I foresee possibly running a shop vac for dust control and some fluorescent lights off the same cord in the future. Any guidance, on what specs I need to look for in a cord that long to run my table saw, is greatly appreciated. Thanks.

John

[LCHIEN]

I've gained access to an old wood shed at my neighbors' to set-up my Ryobi 3100. Only catch is that I have to run a power cord 75+ feet from an outdoor outlet, out to the shed. Also, I foresee possibly running a shop vac for dust control and some fluorescent lights off the same cord in the future. Any guidance, on what specs I need to look for in a cord that long to run my table saw, is greatly appreciated. Thanks.

John

well the issue is that the saw will run on 120V and currents appropriate to the load. So what works fine for you one day will not work when you want to load up the saw and try a big cut, or make a number of cuts and the saw overheat from current starvation.

So the proper answer is that your saw is rated for 15Amps and needs access to all of them with low voltage drop to guarantee to run properly will heavy loads.

So you are about to hear what you don't want to hear and that is:

1) you can't run other items (shop vac) on the same circuit much less the same extension cord. You need another circuit for lights and shopvac.

2) You need an extension cord with less than about 5 volts drop over 75 feet at 15Amps. Which means at least 12 gauge (or bigger, e.g. 10 ga.). That's assuming your outdoor outlet is close to the breaker panel and there''s no appreciable loss in the branch wiring.

[Tom Hintz]

I have a chart that shows the typical cord size/lengths at the link below. I usually try to go a bit heavier just to be safe but safer yet is to get an outlet installed where needed. A nice plan but not always in home workshops.

http://www.newwoodworker.com/ref/xtntincrds.html

[BeachLakeWoodWorker]

Power cord

Tom and Lichen,

Thank you both for the feedback - much appreciated.

[sacherjj]

I noticed a 100' 10 gauge cord at the BORG on Monday. I don't recall the price, but I know it wouldn't be cheap.

[TheRic]

I would question what else is on the outside outlet. Lot of times they are on the same circuit as other stuff. In an older house you might find several inside rooms on the same circuit as that outside outlet. They just run that outlet off the wiring on the otherside of the wall. Heck a corded drill, might be enough to blow a breaker (or fuse) if alot is on the same circuit.

[Pappy]

If your circuit breaker box is outside, add a dedicated circuit and bury wire from the box to the shed. I have a 30 amp giong to the playpen and wil have one tool, DC, and the compressor running off and on at the same time. So far, no problems.

[messmaker]

I would put a plug on the end of some underground wire of sufficient size and bury it to the shed you need. Put a GFI plug on the female side and then you can use whatever plug you want.Solid wire conducts a bit better than twisted so you should not suffer as big a drop plus you won't have to roll the e.cord out every time. I would not live with this forever but would work for the short term.

[just4funsies]

Solid wire conducts a bit better than twisted so you should not suffer as big a drop...

I think I remember reading about something called "skin effect", which essentially says that the majority of electrical flow in a wire occurs on its surface or "skin", and that ampacity, or current-carrying ability in wire is more dependant on this than on cross-sectional area, thus yielding better capacities for stranded wire than for solid, given the considerably greater surface area. It's been a long time since I dealt with this subject directly, so I can't be more definitive than that, but perhaps a more knowledgeable member will weigh in on it.

[jziegler]

If I remember correctly, skin effect is not a factor at lower frequencies. If it were, I suspect that house wiring would be done with stranded wiring. If I had some textbooks at work, I could look this up, but those books are at home.

Basically, as far as I know, the reason that stranded wire is used for cords/extension cords is flexability. Current capacity has nothing to do with it.

Jim

[LCHIEN]

A little bit of knowledge is dangerous

and leads to misleading statements

your local EE checks in:
solid wire is only sometimes a "Bit" better as far as losses,
Solid wire of gauge "X" implies a specific OD of the copper.
Stranded wire is mde of 7, 19, 26, 65, or 105 strands of smaller standard gauge wire to an approximate size near a larger gauge. 16 ga. for example is
solid: .0508" OD, 2583 circular mils area
stranded equivalents to 16 ga. (that a manufacturer would call 16 ga)
7 x 24 ga, 2828 circ mils
65 x 34ga, 2579 circ mils
26 x 30ga, 2600 circ mils
19 x 29ga 2426 circ mils
105 x 36ga 2625 circ mils
the dia. runs from .058 to .060 and is larger because of the gaps between the round strands.

so you see some are better and some are worse (the larger mils being less resistive) but we're talking about less than 2-3 % differences. The circular mils represents the area of the copper only, not the whole cross section area including gaps. I don't think they usually publish the stranding specs on extension cords, but if you buy the bare wire they will.

Incidentally, the choice of 7, 19, 26, 65, 105 is not arbitrary, theat's the number that pack into a round bundle in layers with minimal gaps. For example 7 strands, is a center conductor and 6 outside conductors packed in a hex pattern. 19 would be 12 more conductors wrapped around the 7 in an additional layer completely filled, etc.

Stranded is used for extension cords because of flexibility, the more strands, the more flexible.

Skin effect only applies to radio frequencies. - the whole of the cord will be conducting for all practical purposes in an extension cord. Power is at 60 Hz, you don't have to start worrying skin effect until around 100,000 Hz.

[BeachLakeWoodWorker]

thanks

Thanks everyone for the feedback.

[messmaker]

Thank you for clearing up my misconception. I stand corrected. Lchiens' wealth of knowlege never ceases to amaze me. I am sure glad he is one of us.

[Holbren]

I had a similar situation where I needed a 30' extension cord for all my tools (one at a time) and ended up making one. Went to the Borg and didn't see what I needed for 220V so I bought some 8 gauge wire for I think .69 per foot and the plug/receptacle. The 8 gauge wire was cheaper than the 10 gauge for some reason.

For that long of a run and current load, I would go thicker than 12AWG and that probably means making something.

Skin effect is more for high frequencies than 60Hz AC. A good example would be speaker wire where you get to 20KHz and above. Then there is always theory vs. actually hearing it in the audio example. I'm sure for thinks like RF it can be measured.