Replies

1. 
   DanH | Jul 22, 2005 05:38pm | #1

   3 volts drop running a shop vac on #10 seems a tad much, but it's hard to say for sure without running the numbers.

   Also, if it's an electronic voltmeter it can be confused by noise on the line (particularly on an "unloaded" line, or the noise caused by certain types of motors). To get a better measure, measure the difference between a lightly loaded circuit and a heavily loaded circuit, using pure resistive loads if at all possible (see below).

   More of a worry, really, is that you have only 110 unloaded. That is about ten volts lower than you should be getting. What do you read in the barn or wherever the service entrance is (separately measuring both sides of the 240V line)?

   What I would suggest is that you get some nice heavy loads -- electric space heater, toaster, the wife's iron, etc -- and plug them in. You should not be able to produce more than about 5 volts drop without exceeding the circuit rating.

2. 
   bladeburner | Jul 22, 2005 05:54pm | #2

   Without going into a lot of technical stuff:

   1. you should be reading 120V (no load) at the panel in North America.

   2. 110V has a min loss transmission lenght of about 50 ft over 12 ga wire. 240V can travel 4 times longer. That's why some people think a 1hp 240v motor is stronger than a 1hp 110v motor.

1. 
   Walker1 | Jul 22, 2005 06:30pm | #4

       Joe.  Question;  Would a tablesaw motor converted from 110volts to 220 have more power under load?  Or is using half the amps. the only benefit?

               Walker1

   Edited 7/22/2005 3:41 pm ET by Walker1

1. 
   bladeburner | Jul 22, 2005 10:59pm | #5

   All convertible 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 is wired. It is amperage that creates heat, and because the amperage in each coil is the same for both wiring configurations, there is no difference in the heat produced by either wiring configuration. 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 circuit, 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.

3. 
   junkhound | Jul 22, 2005 06:12pm | #3

   a. define 'good' meter -  calibration certification?  As others said, 110 V may not be accurate or indicates other problems.

   b. shop vac nameplate amps?  For sake of argument, say max 15 amps (probably not).  150 ft round trip 10 Awg is 0.15 ohms, so at most you should have a 2.25 V drop plus another 1/10 volt from the plug-in conection.

4. 
   CPopejoy | Jul 23, 2005 12:44am | #6

   Bud,

   Well, I'm gonna take your statements at face value.  It's quite possible that the voltage at your service is 110, if you're out in the sticks, or you measured the value when the load on the distribution grid was heavy.

   Anyway, a few strokes on the 'ol calculator tells me that the voltage drop in the situation you described is 2.7 percent.

   That's not bad, not bad at all, given the load from the shop vac (reasonably heavy), the distance from the service, and the intervening subpanel in the barn.  The NEC recommends that voltage drop from service to point of utilization be not more than 5 percent.  There is no requirement or limit in the NEC concerning voltage drop; it's an efficiency thing, not a safety issue.  And the NEC's focus is safety--not design, efficiency, or convenience.

   I wouldn't worry about the voltage drop at the garage.  I'd be more concerned about damage to the wiring from the lightning strike.  You could fix the stuff that's obviously damaged, and wait to see if something else fails (sooner or later) as a consequence of the overvoltage.

   On the other hand, it wouldn't hurt to have a qualified electrician take a look at the system.  For instance, a visual inspection of the panel (for signs of arcing/flashover, burnt conductors, etc.), a check for frozen breakers, a check of the utility service drop neutral (or service lateral neutral, if underground distribution) under load, and maybe even some insulation resistence testing (with a megohmmeter, or megger) could detect damage to the wiring before it fails (and possibly causes collateral damage).

   Also, if your power company offers it, look into getting the whole-house surge protector installed at the electric meter.  Some utility companies offer  installation and lease of a good whole-house surge protector (which they install between the meter and the meter socket); one or two good strikes will destroy it, but it'll save your house wiring.  Then the PoCo replaces the surge protector as a part of the deal (you pay a couple of bucks a month on your electric bill). 

   An alternative is to have a surge protector installed in your breaker panel, ($200 for the unit and maybe $250 to install).  Then when it gets fried, it's your responsiblity to replace.

   Of course, a whole house protector might not cover all the bases.  With 75 feet separating the main panel and the barn and then some distance to the garage, if there's a lightning strike very near your house, a pretty good slug of energy could be picked up by your underground feeder.  The solution there is a breaker panel mounted surge protector at each outbuilding. 

   That's expensive, so it depends on how much lightining you get.  Of course, the cheap way to protect stuff is just to unplug it when Zeus starts to pitch lightining bolts.  Not always practical, and definitely not convenient.

   Regards,

   Cliff