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240 volt /120 volt which cost less

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Forum topic by hotstick posted 06-13-2009 04:38 PM 7192 views 0 times favorited 19 replies Add to Favorites Watch
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hotstick

7 posts in 2657 days


06-13-2009 04:38 PM

Topic tags/keywords: resource cost cutting tip tablesaw bandsaw lathe planer

I have read and listened to the debate of 240 volt over 120 volt feed to our power tools for some
time. As far as they go, they are correct in the calculation of the power used. However, no one
has mentioned the cost of the power used. It is all in the way we are charged for the power we
use. Our residential services are mostly single phase 120 / 240 volt services. The meters that we
are charged by read the power used on the leg that is carrying the most current. Therefore, if we
use a table saw that requires 14 amps at 120 volts and nothing else on the other leg we are being
charged for 3.360 kilowatt hours of power per hour we use the saw while we actually only used
1.680 KWH. There is a simple and entirely real reason for this seemingly rip off charge. The
other 1.680 KWH we did not actually gain any benefit from went to waste as heat in the
transformer on the pole where our service originates. If we only pay 10 cents per KWH, the
difference in what we actually used on 240 verses 120 is 16.8 cents. Not much, huh? No, it sure
is no big reason to go to the expense of rewiring our shops just to save 17 cents, but how many
hours do we run our tools in a year? What will it cost our environment to pay the cost of all the
excess energy generation we could be saving by converting to 240 volt use where ever possible?
Granted, I am a professional master electrician and have access to better prices and my labor is
free for myself so my conversion was much lower cost. Not only have I converted all my power
tools possible to 240v, I have also built and attached magnetic starters on everything including
my router tables. If I can be of any help is sharing information on mag starters, just say so and I
will be glad to give any instruction I can.


19 replies so far

View Scott Bryan's profile

Scott Bryan

27251 posts in 2566 days


#1 posted 06-13-2009 05:45 PM

Thanks for the info. I did not realize that there was even a cost difference between the two. I just naturally assumed that 120 and 240 used the same total amount of electricity.

-- Challenges are what make life interesting; overcoming them is what makes life meaningful- Joshua Marine

View Bill Davis's profile

Bill Davis

226 posts in 2668 days


#2 posted 06-14-2009 12:48 AM

I believe your statement “The meters that we are charged by read the power used on the leg that is carrying the most current” is in error. The electric meters measure total power consumed. You are charged for what you use, not for double in the example you gave. I think you better go back and check the books on that one. Or talk to your local electric company. And you are not charged for “kilowatt hours of power per hour” but rather there is a rate per KWH. You are charged for the Kilowatts used per hour.

View TopamaxSurvivor's profile

TopamaxSurvivor

15064 posts in 2420 days


#3 posted 06-14-2009 01:15 AM

I have to agree with Bill, a KW hr is a KW hr no mater what voltage you take it at.

Why do you bother to put mag starters on small tools?

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

View Catspaw's profile

Catspaw

236 posts in 2560 days


#4 posted 06-14-2009 02:37 AM

It would seem to me, then, that I could use 15 amps on one leg, and draw 14 amps on the other and only be charged for the 15 amps. I suppose it could be said that the other 14 amps is more or less taken into account in what you are charged. Sounds like some one would have gone to alot of trouble to design a difficult system.

Also, mag switches are simply a way to use a cheap little switch to turn on big current with a cheap relay instead of using a really big expensive switch (relays are much better and cheaper at handling large currents.) They certainly don’t have any components that help a motor out. It is kinda nice if the power goes out so your saw doesn’t keep going on and off…(that just happened to us two days ago….went on and off about 3 or 4 times in about 10 seconds.)

-- arborial reconfiguration specialist

View Bill Davis's profile

Bill Davis

226 posts in 2668 days


#5 posted 06-14-2009 02:57 AM

A Professional Master Electrician ought to have access to some clamp-on amp meters and other measurement equipment to test it out. But in reality I still think your conception on how a KWH meter measures kilowatt hours needs some study. It has to include the total wattage consumed at the load.

View BigJimAK's profile

BigJimAK

30 posts in 2035 days


#6 posted 06-14-2009 10:09 AM

You are correct Bill, it needs more study. Just as scales in the store used to charge you by the pound are carefully regulated, so are power meters.

Jim
EE and wanna-be woodworker :-)

-- Jim in Alaska

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BigJimAK

30 posts in 2035 days


#7 posted 06-14-2009 10:26 AM

That said, there are some advantages to utilizing 240 over 120 for your large tools if the draw is high and/or you have a long run (such as runnig to a disconnected garage).

  • A motor that draws 30A at 120 would draw 15A at 240 so you can use smaller wire to feed it since wire size is based upon current (at these voltages).
  • If you are feeding a distant subpanel, the current will be halved so the voltage drop in the wire will be halved (for the same size of wire).

Most of us have a combination of 120 and 240V tools. If you are setting up a subpanel for your shop (I am in the process of upgrading the power in my shop to a subpanel), you may be able to get away with a smaller main breaker and wire feeding it than if everything was 120V (which would require twice the amperage).

When you choose a 240V breaker, any 120 volt tool utilizes one or the other of the legs (120V to neutral) for its current. Breakers for 240V are designed so that if either leg trips, it trips the other. This means that too much current on one leg can cause you problems. If you are wiring your shop and know where your large-draw 120V devices are going to be (and which ones may be used together), you can optimize your electrical supply by balancing it.

For example, if you have a (assume all 120V) DC that you run with your table saw, jointer and planer but don’t run more than one of the three tools at a time, you would benefit from having the tools on one leg and the DC on the other.

Jim

-- Jim in Alaska

View Bill Davis's profile

Bill Davis

226 posts in 2668 days


#8 posted 06-14-2009 03:59 PM

Jim your above discussion is excellent and accurate except for one error in fact.

“•If you are feeding a distant subpanel, the current will be halved so the voltage drop in the wire will be halved (for the same size of wire).”

The correct statement would be:

•If you are feeding a distant subpanel, the current will be halved so the voltage drop in the wire will be cut by four (for the same size of wire).

The formula is V(drop)=I*2/R which results in a decrease/increase in voltage drop by a factor depending on the SQUARE of the current. In other words if you double the current in the wire, as you would going from 240V to 120V, the voltage drop would be FOUR times greater.

Bill
EE and Woodworker

View EdWood's profile

EdWood

12 posts in 2059 days


#9 posted 06-15-2009 07:37 PM

I have been reading this thread with some curiosity. While I cannot comment on exactly how the electric companies bill power usage I can comment on simple Ohms Law. I know I was taught the formula more than 30 years ago but I do not think Ohm’s basic Law V=IR has changed. Therefore the voltage drop on a line is directly proportional to the Current Resistance. So for example pulling 20 Amps through 50’ of 10 gauge wire will result in 0.0509 ohms® 20A (I) = 1.018V. Halving the current will reduce the voltage drop to 0.509V. Now honestly I do not think switching to 220 just to reduce the IR drop is probably not worth the cost. Especially since typically smaller gauge wire is used for 240V circuits with higher resistance thus canceling voltage drop savings. For reference say you switch to 240V with 12/2 wires and your current drops from 20A to 10A in the above example. The voltage drop at the load would now be 0.811V. Not a huge difference. However running a circuit at 240V does allow smaller gauge wiring to meet safety codes and thus save money and makes since for large load items like motors.

Ed

-- Edwood - http://edwoodcrafting.blogspot.com/

View Bill Davis's profile

Bill Davis

226 posts in 2668 days


#10 posted 06-15-2009 07:47 PM

I appologize for my mis-statement in my previous post. I was thinking of power loss quadrupling when the current is doubled. I stand corrected. I vote for the properly stated Ohms law as clearly stated by EdWood. Thanks EdWood!

View sikrap's profile

sikrap

1062 posts in 2103 days


#11 posted 06-15-2009 08:08 PM

Wow!! Okay, as a non-electrician who is currently in the process of building a new garage in which I will house all of my tools, let me ask a VERY basic question. Is it worthwhile for me to set the new shop up for 240 to run my TS? I will (hopefully) also have a bandsaw and the usual smaller tools (drill press, routers, sander, etc.). I’m not terribly concerned about the cost of the wire since, I’m more concerned about the sub-panel and the efficiency within the shop. Thanks!

-- Dave, Colonie, NY

View Paul's profile

Paul

357 posts in 2334 days


#12 posted 06-15-2009 08:52 PM

Sikrap
I would set up any new shop area for the option of 220v. Given the option I would run any easily capable machine at 220 (some equipmet comes without the option and you must use 220) but I would not personally go to the trouble of a major rework of shop or equipment to convert them to 220. (but that’s just me).

-- If you say 'It's good enough', it probably isn't.

View spaids's profile

spaids

699 posts in 2438 days


#13 posted 06-15-2009 09:13 PM

It seems to me that the nice big TS’s with the 3 and 5hp motors are coming at 220 only. Am I wrong?

-- Wipe the blood stains from your blade before coming in.--

View sIKE's profile

sIKE

1271 posts in 2498 days


#14 posted 06-15-2009 09:26 PM

I would concur with making sure you provide for 220v in a new shop. I put several drops in during the construction of my shop. Plan to run TS/Jointer/Dust Collector off of 220v eventually.

-- //FC - Round Rock, TX - "Experience is what you get just after you need it"

View sikrap's profile

sikrap

1062 posts in 2103 days


#15 posted 06-15-2009 11:13 PM

Thanks all!! I apologize if I hijacked the thread, but it looked like those that were commenting were knowledgeable. It looks like I’ll be running 240 and a sub-panel with its own master breaker.

-- Dave, Colonie, NY

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