220v wiring for a future saw

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Forum topic by HokieMojo posted 03-04-2010 08:21 PM 2561 views 1 time favorited 11 replies Add to Favorites Watch
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2104 posts in 3968 days

03-04-2010 08:21 PM

so I’m looking to add 220v (or 230/240v depending on what my service allows) in my garage. I’ve got a non-working 8 inch jointer that I can either wire for 120 or 220, so I’m thinking I’ll do 220. This will allow me to run my DC on a seperate 120V circuit.

In addition, I’ve currently got a ridgid benchtop table saw. I looked at the specs and it has a 15 amp 120 volt motor. If I were to plan for the future (10-20 yrs from now) and want to wire up this circuit so that it could run a full 5 hp cabinet saw, would that cost substantially more than if I were to wire for a 3 hp? Unfortunately, the cable would need to be about 80 ft long from the breaker to the socket. As a benchmark, I looked at the powermatic 5 hp saws and it looks like they are 21 amp 230 volt motors. I’m trying to plan ahead so that I can sheetrock and insulate over the wiring once I’m done and not need to cut into the wall down the road.


11 replies so far

View Alex Lane's profile

Alex Lane

542 posts in 4130 days

#1 posted 03-04-2010 08:52 PM

I’m not a fully trained electrician, so if someone with more experience sees an error in my logic or my suggestions, please let me and HokieMojo know. But I’m quite sure this is correct.

Typical electric motor ratings are always given AT FULL LOAD. That includes the motor RPM and Amp draw. Sometimes a motor might draw 75% to 85% of its full load current, but as soon as it achieves its full RPM after a second or two, it will not use as much amperage. If you are cutting a monstrous 2” deep x 1” wide dado in solid Bubinga (please don’t do that!!), then maybe your motor will pull its full 21A and drop down to the RPM (typically 3450) on its nameplate.

As far as a long 80-foot cable for the Voltage supply, get the heaviest gauge wire you can afford (4 to 6 gauge would probably be ideal, but as long as the specifications for the wire give it an Amp rating HIGHER than your most powerful tool, it will be fine). When it comes to electricity, Amps are Amps are Amps, regardless of the Voltage. What I mean is: if one tool draws 21A at 120V, and a different tool draws 21A at 240V, the wires carrying the current to each tool are under the same ‘load’. The 240V tool, however is twice as powerful as the 120V tool, because it would draw 42A at 120V. Clear as mud, right?? ;-D

I believe motors can actually draw more Amps than they are rated for on the nameplate. As the RPM drops while under a heavy load, the motor will draw more and more amps to compensate and keep its speed up. If it cannot keep up the RPM, it can potentially reach what’s called a “locked rotor” condition (totally stalled), where the Amp draw is VERY FAR above what the motor is rated for, thus possibly melting wires in the motor’s coils and heating up the supply wire to the motor, causing a serious fire hazard. That should all be prevented by a breaker panel, but if there is no breaker, serious damage and fire can occur.

A longer cable also tends to diminish the voltage that reaches the motor, thus causing the motor to draw more Amps than usual in order to maintain its load (i.e., the heavy dado cut). It’s always better to be safe and get a wire that will handle the load your machine draws, PLUS SOME EXTRA, in case of excessive amp draw. The Breaker panel will prevent extreme amps to be drawn, but it will only trip the circuit if your motor stalls or the wire leading to the motor is too small and causes excessive amp draw.

I hope this helps!!

-- Lane Custom Guitars and Basses

View WayneC's profile


13800 posts in 4337 days

#2 posted 03-04-2010 08:58 PM

Also, think about your shop layout and how you will place the shop. I would like to turn my saw around from where it is now. The only thing stopping me is the need to run a new plug. You may want to add lighting and 110v drops as well. Are you installing a sub-panel in your shop?

-- We must guard our enthusiasm as we would our life - James Krenov

View HokieMojo's profile


2104 posts in 3968 days

#3 posted 03-04-2010 09:14 PM

thanks for the replies so far. I think this is going in the direction I need it to in order to decide what I want to do.

I posted a separate wiring question over here with some more back ground, in an attempt to not confuse two separate issues, but I was not planning on adding a subpanel. It would b nice in that I could turn off the “shop stuff” from the panel, but I think it would basically equate to a very expensive “master switch”. Maybe I’ll end up going that route depending on what a panel would cost when I price things at HD this weekend, but I do have 6 open slots on my current panel.

As for drops, I assume that is an outlet that hangs from the ceiling. I’m not planning on those because the ceiling has already been sheetrocked (although I swear it looks like they “forgot to put in insulation”) and the garage door would probably interfere with them hanging down.

View LeeG's profile


40 posts in 3261 days

#4 posted 03-04-2010 10:11 PM

Here is a link to a calculator that shows what gauge wire you need for various volt-amp combos with various distances

Here is a link to some NEC docs about it or HERE the whole NEC

-- Lee in Phoenix

View TopamaxSurvivor's profile


18425 posts in 3916 days

#5 posted 03-05-2010 03:17 AM

Here’s the deal from the code allowances for motor wiring. 120 volt 20 amp circuit is good up to 3/4 hp.

20 amp 240 will hold 2 hp, 30 amp 240 is 3 hp, 60 amp 240 is good for 5 hp.

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

View HokieMojo's profile


2104 posts in 3968 days

#6 posted 03-05-2010 04:31 PM

This really is some great info everyone. I really apprecaite it.

You didn’t quite come out and say it, but what I’m hearing is that a 5hp saw would probably require a significantly bigger investment. I guess if I’ve been successful with my benchtop saw so far, a 3 hp cabinet should more than do the job for a hobbyist, no matter how much time I plan to invest in woodworking down the road.

View TopamaxSurvivor's profile


18425 posts in 3916 days

#7 posted 03-05-2010 11:11 PM

5 hp is definitely more expensive to wire. It requires #8 cu circuit conductors.

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

View williams's profile


53 posts in 3257 days

#8 posted 03-06-2010 01:55 AM

By my read, #8 is rated for 24amps max. For 60amps you need min #4. For longer distance you may need bigger. Starts to get real expensive.

-- William, Brighton, MI

View TopamaxSurvivor's profile


18425 posts in 3916 days

#9 posted 03-06-2010 02:46 AM

#8 Cu THHN is good for 50 amps, but there are a gilion factors that can affect what your actual allowable amperage will be. MOtor circuits are allowed to be fused over the wire size to compensate for the starting surge. The bottom line is what the National Electrical Code requires and /or allows.

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

View panther's profile


59 posts in 3483 days

#10 posted 03-06-2010 03:03 AM

i would call a fully trained electrician and tell him to make that install, some things you can do, some things you hire people for , just remember, no one wants to see a fire truck at their shop

-- you must live for something or die for nothing (rambo)

View HokieMojo's profile


2104 posts in 3968 days

#11 posted 03-06-2010 06:35 AM

i’m thinking the easiest thing is to just give up on the dream of a 5 hp saw someday. I’m reading more and more info as I prep for this project, and I just don’t think I need the additional 2 hp, or the extra expenses associated with it (saw cost and infrastructure costs). I’ll prepare for a 3 hp saw someday in the distant future. Thanks all!

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