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Electrical Advice Needed

| Posted in General Discussion on June 28, 2002 08:51am

My home is wired like most homes in the U.S. I have a 100amp split phase 240v service. I have a contractors saw that I can run on either 120 or 240. If I choose to run it on 240 I would use two hots and a ground but the question is should these be in phase or out of phase. If they are out of phase which phase goes on which side.

If I decide to do it I will hire an Electrican so its not a matter of me trying this on my own. I have a definite fear of working inside a power panel. I’m just one of those people who for some stupid reason would like to know the answer.

By the way, I also know that the only advantage in converting is the lowered line loss which should result in less heat build during motor startups. All of my 120 runs in the shop are less than 50ft from tool to panel and they are all home runs.

Thanks in advance to all that respond

Bernie

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Replies

  1. Piccioni | Jun 28, 2002 10:33pm | #1

    Ok, as a guy who does all of his own electrical work, I hate to say this, but, based on how you phrased the question, you might want to ask an electrician to do it. Nonetheless, here is the story.

    240 volts is two out of phase 120. So, on the left side of the panel is one phase, on the right side is the other. If you connect both hots to the same power bus, you basically get a fat 120 volt line, not 240.

    If you have a breaker panel (I have no clue what to do with a fuse panel) they cleverly sort of zigzag the phases so if you plug a 'tied' double breaker (one where the two breaker switchy things are connected together) what you get off of one pole of the breaker is one hot, and the other pole is the other hot.

    So, what you do is you run a cable with one black wire, one red wire, and one ground wire. The black wire goes into one part of the double breaker, the red wire goes into the other. It doesn't matter which which is which, because its AC, but I like to be consistant.  Ground goes to ground.

    I use a 12 guage line for my saw. There are several advantages to 240, but I figure you get less power loss because the current used is halved. This means the line doesn't 'sag', and I think it seems to bothers the lights less when I switch on the saw.

    Besides, real men use 240.

    ps: somebody is gonna say 240 doesn't have phases. It do.

    pps: shut the power off before going near it. 240 bites.

  2. 4Lorn1 | Jun 28, 2002 10:46pm | #2

    Unless there is something profoundly strange you only have two hot legs. If you have three disregard this advice and please post back. Use both for 240v. It doesn't matter which leg goes to which terminal.

    These legs are 180 degrees out of phase with each other with each being + or - 120v to ground maximum. When one is at the top of the sine wave, +120v, the other is at the bottom of the wave 180 degrees out of phase and at -120v.  

    +120v - (-120v) = 240v

    Be safe. Remember that the cost of an electrician is a fraction of the cost of an ambulance ride and emergency room visit. There are several good electrical manuals on the market that will repay their cost many times over in peace of mind alone.

    1. Bernie3265 | Jun 28, 2002 11:10pm | #3

      Not to worry, as I stated in my previous post, I have no intention of doing this myslef. I just wanted to know. Thanks for the responses.

      Yes, I do have three wires but like all normal households this is a split phase 240v feed. Two are hot 120's that are 180 out of phase and the other is the neutral from the center tap at the transformer. It is tied to the ground buss bar at the primary panel and not tied together at the subpanel inside the shop, (single ground circuit). This is a home shop so theres not much justification for 3 phase.

      The reason I am interested is because I'm confused about dual voltage motors. If both field coils are designed to run on 120v single phase then splitting the coils from a parallel circuit into two series circuits with a common ground in my minds eye is not running it on 240 but instead two in phase individual 120v circuits. Resistance in the fields remains the same so the current requirment doesn't change in the individual coils. To me this doesnt buy you much except maybe a faster rise time to required current which would mean a faster motor start and a little less heat. No one seems to be able to fully validate that fact though.

      Thanks again.

      Edited 6/28/2002 4:11:20 PM ET by Bernie

      1. junkhound | Jun 29, 2002 08:00am | #4

        HOLY SMOKES!  so much misinfo in this "short" post here its time to call in Mr. sparky.

        Hey AJ (aka Mr. sparky), you still around anywhere, can you add any ascerbic comments???

        example.  "If you connect both hots to the same power bus, you basically get a fat 120 volt line, not 240"  UGH!!!!BullSBSBS etc - you get a big flash, and tripped breaker.

        Serious practical advise: run all the motors you can off 240. Most, but not all, present day motors have only one start winding, that is designed to run off 240, so a 120 connection results in really pathetic startup. Motors where you switch 6 or more leads to change over voltages are OK either way, depending on line voltage drop etc, etc.

        Phase me up, Scotty!!!  apparently D- or flunk on the freshman EE couse in power systems to most.

        1. Piccioni | Jul 02, 2002 03:23pm | #8

          Uh, gee, I guess I missed something in the 13 years I spent designing computers. You connect 2 wires to the same power bus, you get on 120 volt line. Why would you get a flash? no short (no neutral even)?I'll assume you misunderstood.

        2. JohnSprung | Jul 02, 2002 11:58pm | #9

          > Serious practical advise: run all the motors you can off 240. Most, but not all, present day motors have only one start winding, that is designed to run off 240, so a 120 connection results in really pathetic startup. Motors where you switch 6 or more leads to change over voltages are OK either way, ....

          Aha --- Thanks, Junkhound.  I didn't know that about the start windings.  That must be why I've been having such grief with the new 2 HP Baldor on my Unisaw.  It blew some 30 amp slo-blo fuses when I tried to start it. 

          Since the house only has a 120 volt / 30 amp service, I've had to find another way around the starting surge problem:  I use a drill motor driving the fan end of the Baldor's shaft to get it up to 1200 RPM before I hit the main power, and it run up the rest of the way to 3450. That turns out to be enough to keep the surge down to where the fuses survive.

          (I made a cam-out linkage that disengages when the big motor starts, sorta like those screws in the men's room stalls that you can put in with a straight blade driver, but can't back out.  The hand crank for starting an old time car worked the same way - I put one on a VW a long time ago.)

          -- J.S.

          1. User avater
            BillHartmann | Jul 03, 2002 01:04am | #10

            John

            You have poor wiring in addition to the lack of service.

            I start my 1 1/2 hp on 120 20 amp circuit all the time without a problem, while running a 3/4 hp dust collector at the same time.

            It would be interesting to put a volt meter on the line while you start the saw.

          2. JohnSprung | Jul 03, 2002 01:29am | #11

            > You have poor wiring in addition to the lack of service.

            Yes, alas most of it's ahead of the meter where I can't legally deal with it.  I've installed a 30 amp fuse and re-arranged some circuits so it feeds only two receptacles.  I have a total of 50 feet of AWG #10 between the fuse box and the saw, all new.

            > I start my 1 1/2 hp on 120 20 amp circuit all the time without a problem, while running a 3/4 hp dust collector at the same time.

            Once the 3/4 hp is up and running, it draws very little current.  How much the 1 1/2 draws on starting will depend on how much inertia and friction it's up against.  That can vary a lot.  If you're on breakers instead of fuses, they may also deal a lot better with starting surges.

            > It would be interesting to put a volt meter on the line while you start the saw.

            Unfortunately all my meters are digital now.  This really calls for an analog meter with good ballistics to see what's going on.  Or better yet, a storage scope.  Make that a battery powered storage scope, so you can see what happened after the fuse blows.  ;-) 

            I can start and run the motor if I take the belts off (the saw doesn't work too well that way). The light dims significantly when I do that.

            -- J.S.

          3. junkhound | Jul 03, 2002 08:25am | #12

            John: re- BattPower storage scope:  I sprang for a Fluke 123, great to have around - it cost more than some of my cars though! Have seen some used ones on the net for under $400.

  3. CPopejoy | Jun 30, 2002 03:17am | #5

    Bernie,

    Here's what's considered standard terminology on this subject.

    You have a conventional (North American) residential 240/120 three-wire service.  As someone said, two of the service conductors are from the secondary winding of a stepdown transformer, and the third comes off the center tap of that secondary winding and is grounded at the pole.  These are not two "phases", they are two poles.  The power being provided is single phase.  A lot of people mis-use the term "phase" when talking about electrical power.  Call the two ungrounded conductors  "poles" or "legs".  And as said before, it doesn't matter which of the two poles of the 240 is connected to a given  motor terminal.  It does matter in three-phase power, by the way.

    Also, I'll be willing to bet that you have a 240/120V, single phase service.  This is either a meter socket or meter/main with a circuit breaker panel, or a service entrance panel ("all-in-one"--meter base, main breaker, and branch circuit breakers).  It is not a "split phase" panel.  It has two buses.   These are the things shaped like giant interleaved combs with alternating teeth bent up (these are called the tabs or stabs, and the breakers clip onto them).  Your panel is not a "split bus" panel--those haven't been made for a long time.  A split bus panel has two pull out fuseholders on one part (for range and AC) and below it a standard two-pole bus as described above.  The buses are split between types of overcurrent protection device (fuse and molded case circuit breaker).

    As far as a motor being able to run on 120V or 240V, to switch from one to the other, you've got to change the internal motor wiring (basically running some of the windings in series on 240 that were in parallel on 120).  It's no big deal, because the insulation in the motor (windings) is designed to handle 250V (or sometimes 600V).

    There are a couple of benefits to running 240, and both relate to the fact that at 240V, you get the same energy at the load at half the current.  One, as you mentioned, is that you get less heat produced in the motor.  Another is that you lose a lot less power in getting the energy to the load.  The heating of the motor feeder is proportional to the square of the current.  So you spend a lot less energy heating the wires between your panel and the motor when you use 240V, than at 120V.  The other benefit is that you can use smaller conductors  as a feeder, since the current is half of what is would be at 120.  Now, neither of these savings (less I2R loss [feeder heating] and reduced cost for the wires) is huge, but then again, it costs almost nothing to re-wire the motor to run on 240. 

    You're smart to keep your hands out of the panel.  With many panels, even with the main breaker off, there are still energized parts in the panel.  If a slip of a tool shorts the poles, you get 100A at 240V, which will vaporize part of the tool and the panel, spraying molten metal in your face.

    When your electrician is in the panel, ask him to check the grounding and bonding connections.  This shouldn't take long if things were done right when the panel was installed.

    Good luck,

    Cliff 

           

    1. Bernie3265 | Jul 02, 2002 12:11am | #6

      Thank You Cliff.. I called an electrician and had him come out and tell me exactly what I had. It was well worth the time and the money to get the show me kind of advice that this person was willing to give. It took a while to find someone willing and I finally had to go through my brother who is a local GC. He gave me the name of a friend. Unlike the folks here on this forum, most of the electricians I called simply said "we don't train people unless they work for us". With that kind of attitude I never bothered to tell them the I was only trying to make an informed decision and they would likely do the work.

      Anyway, your info combined with others has allowed me to now make that informed decision and move forward with changing everything that I can over to 240. That will be the DC, the Drill Press, the Radial Arm Saw, the Table Saw and the Band Saw. Had it just been one piece I wouldn't bother but with all of these it really starts to make sense to me know

      Thanks Everyone

      Bernie

      1. junkhound | Jul 02, 2002 07:41am | #7

        - predict you will be much happier with how your tools start, no more waiting ten sec for the RAS to come up to speed.

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