Veteran Contributor
Posts: 118
Registered: ‎11-12-2009

120/240 volt

I have a question about 120 vs 240 volt. I'm shopping for a table saw and I know most contractor/hybrids are usually wired for 120. I think I understand the concept that a 120 volt draws 15 amps when using, and a 240 volt(or maybe its 220v) draw half of those amps(7.5). My question is it more beneficial to have the 220, and if it is, is there a specific cord, and wiring required(I know there is but just don't know where to start). Please tell me too if I'm totally off about my understanding voltage and amps.

Senior Advisor
Posts: 1,082
Registered: ‎10-23-2009

Re: 120/240 volt

     Basic answer is "yes - just as there are specific wiring requirements for 110, there are requirements for 220." and "no, you are not (totally) off on your understanding". And if you have 220 readily available, it MAY be more beneficial to go to 220. But it's a bit more complex than you state in your first item.

     Acceptable amperage is linked directly to breaker AND wire size. But regardles of that, all electric stuff follows Ohms Law which says current (amps) equals the potential difference (volts) divided by the resistance (ohms).

     I'm a natural gas pipeline guy and (to me) it means if you increase the pressure you get more gas out the other end, which stands to reason.

     Be careful, though, when comparing volts to amperes. Just because it's 110 (or 120) doesn't means it'll draw 15 amps. Smaller motors will draw less. It's just that there are practical limitations for electrical motors and 15 amps is about the upper end for stuff we'd use (at 110 volts).  

Ted in Michigan
Go Buckeyes!
Posts: 524
Registered: ‎10-23-2009

Re: 120/240 volt

[ Edited ]

Hey John

Voltage is a pressure that pushes current through a resistance. For a given voltage, current flow will decrease as the resistance increases. There is no real benefit in terms of machine performance from having a 220 volt motor versus a 120 volt motor. A motor wired to operate on 120 volts will draw approximately twice the current as a motor wired for 220 volts. The amount of current drawn is determined by the motor characteristics and its actual load when in operation.


Some motors are dual voltage and can be wired to operate on either 120 volts or 220 volts. The motor nameplate will state the voltage and current draw the motor will use. Wiring and circuit protection installed to supply the motor must be sized for the current load. Wire capacity is defined in terms of ampacity, the amount of current a conductor can carry without damage to the insulation. Wire size is chosen to safely carry the current load, and the circuit breaker or fuse is sized to protect the wire. All of this is defined in the National Electric Code. If you have questions about an installation, contact an experienced electrician. 

Good luck!

Posts: 1,626
Registered: ‎11-05-2009

Re: 120/240 volt

A couple thoughts on 220 vs 110. IF the 220 is available I would get wired for it....folowing codes of course. The big argument over which is better boils down to what kind of amps is your service? Suppose you have 100 amp 110 service and you have an AC or Heat running and 3 other 10 to 15 amp devices, you are at say 70 amp nominal load and then you turn on the table saw. The lights dim a bit for a second or two and then all is normal. Starting amps ran up close to the 100 amp load and then settled back to running and now at a nominal 85 amps. At 220v the amp load drops roughly in half so the same service and the same AC or heat and 3 other devices are running at a nominal 35 amps and this leaves the reserve number at 110v, 30 amps, now doubled to 70 amps  at 220v. the kilowatt usage number won't change but the reserve capacity is greater. I'm no expert and this may not be a real good comparison story but the idea should come through. The lower your service is in amps...the more 220v will help. Make sense? I hope I didn't muddy things up.

Never do anything you don't want to explain to the EMT.

Senior Contributor
Posts: 161
Registered: ‎10-23-2009

Re: 120/240 volt

Your understanding of volts to amps is fine. what you need to know is to use 3 wires + ground. Meaning 1-white 1-black 1-red 1-green . your 220 volts will be the black and red. the green is your ground. The white can be used to make 120 volts 1-white and 1 red or black plus the green for ground. the differance is in the plugs, make sure if you are using 220 volts your 120 volt extentions will not fit.Marvin
Senior Contributor
Posts: 237
Registered: ‎10-23-2009

Re: 120/240 volt

No, changing from 125 to 240V will not really give you anything extra as long as the amp requirement is met on the 125v line.  You might notice a little faster start to full speed, but, so what?  So, if you don't have a 240V circuit, don't waste the money.  If someone tries to tell you 240 is cheaper to run ... tune them out.  Cost is identical.

Esteemed Advisor
Posts: 1,648
Registered: ‎10-20-2009

Re: 120/240 volt

3-j wrote:

No, changing from 125 to 240V will not really give you anything extra as long as the amp requirement is met on the 125v line.  You might notice a little faster start to full speed, but, so what?  So, if you don't have a 240V circuit, don't waste the money.  If someone tries to tell you 240 is cheaper to run ... tune them out.  Cost is identical.


I've switched a couple of my 120v saws to 240v.  In addition to faster startups (which, I agree isn't worth the changeover), I also noticed faster recovery from bogging under heavy load and less bogging in general, which is an advantage in thicker materials.  Every circuit and every motor will respond a little differently.  If you've got easy access to 240v, there's no harm done and some potential benefit.  If your 120v tools are performing great on your 120v circuit, and 240v is expensive or difficult for you, there's probably no need.

"I've gotta stop treating this stuff like it grows on trees"
Senior Contributor
Posts: 253
Registered: ‎10-30-2009

Re: 120/240 volt

I've been through similar before, whether to change my Jet contractor saw over or not.  I have an electrician friend that is helping me.   I already have 220 to my shop so that is not a problem.  He tells me that I will see no REAL benifits to changing other than helping out on bogging,


That said...let me give you a senario that I came up with that has not been mentioned.  I sometimes roll  my saw out of the shop onto my trailer ( I have mobile bases on everything in my shop)  Then take the saw up to the house and do work right there on the trailer running it off a receptical I have on the utility pole.  I have done the same thing doing work for other friends or at the church.  Point being----there is not always a 220 receptical available if you ever plan do do something similar.


Also, suppose you plan to upgrade.  Some folks don't have access to 220 so you will face a problem on resale.


Keeping both of those in mind if your saw is convertable between the 110 and 220  you have no resale problem nor if you do not plan to move it elsewhere you will have no problem.  Personally, I would not waste my time changing it over.  Just use the correct blade and rate of feed of your stock.


Alternative if you do plan on moving it for work a used bench top saw for that.  I am going to sell my Jet ( 110/220) and have already bought a Unisaw (220) and picked up a tolerable benchtop for $30.


And yes,  I know 220 is the same as 240, but I'm just an ol' country boy and around here no one refers to it as anything buy 220.  LOL

Veteran Contributor
Posts: 125
Registered: ‎10-23-2009

Re: 120/240 volt

Okay, I understand that 120/240 makes no difference theoretically in operation of a saw.  (E - I x R and all)


Practically though.


A 1.5 HP saw (say a Jet JWTS-10) on 115VAC draws 12 Amps.  On a typical 15A circuit that is 80% of the rated limit.  That is the max you should normally use.  But if you are like me and others , you also have the shop- vac, and maybe a radio nad maybe a light on the same circuit.  Pushing 90% probably. 


If I switch it to 240 V on a 15 A circuit, it now pulls 6 Amps. (40% of the limit) and the other equipment is on the sandard 115 circuit, pulling 10% (hypothetically).  All are happy, wires are cooler, and world hunger is solved. 


Well maybe not that much, but you get the drift, the loads are reduced on the house wiring, extra current is available for startup etc.



Honored Advisor
Posts: 1,309
Registered: ‎10-24-2009

Re: 120/240 volt

Understanding dual voltage 120/240 volt motors is prefaced by an understanding of how electricity is delivered to a house.  Coming in from the pole are three wires consisting of two 120 wires and a common.  Voltage measured across the two 120 volt wires will read 240 volts while the voltage read between either 120 volt wire and common will read 120 volts.

All convertable 120/240 motors run on 120 volts internally. There are two coils each running 120 volts and using 1/2 the 120 volt amperage (The coils act as a resistance and split the amperage). All you do when you re-wire the motor to run on 240 is change the wiring connecting of the coils from parallel to series. When wired for 240 volt operation, one 120 volt leg and its associated amperage is routed to each individual coil rather than a single 120 volt line providing 120 volts to both coils. The same voltage and amperage runs through the individual coils no matter how it it wired. It is amperage that creates heat, and because the amperage in each coil is the same for both wiring configuations, there is no difference in the heat produced by either wiring configuation. The motor is perfectly happy with either voltage and doesn't even know you made the change.

The only advantage to re-wiring for 240 is that it reduces the amperage in shop wiring running from the breaker to the wall outlet.  This means that the voltage drop in the wiring is lessened.  If your wiring is properly sized for the amperage and run length, voltage drop will be minimal and well within the operation range of any good motor.  Voltage drop will be almost equal if the wire size is the required size for each different motor amperage.  Only if your wiring is inadequate for the higher amperage of 120 volts will the motor run better when you convert it to 240.  In this case, upgrading the 120 volt wiring one size and making it a dedicated curcuit, will accomplish the same as installing a 240 volt circuit and wiring the motor for 240.

If a motor coming up to speed very slowly or is tripping a breaker during start up or when under normal load, you either have other loads on the circuit, or the circuit is undersized for the amperage or the run length. The fixes are: remove the other loads from the circuit or upgrade the circuit. To upgrade the circuit, either rewire with heavier wire and a larger 120 volt breaker, or convert the circuit to 240 volts which has the affect of lowering the wiring amperage draw. Either of these solutions will equally fix the problem. Again, the motor doesn't care and won't perform differently as long as it gets clean power.



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