Re: too small a wire

Posted by JMOR on January 03, 2010 at 09:38:52 from (72.181.166.239):

In Reply to: too small a wire posted by Tx Jim on January 02, 2010 at 07:47:29:

George Marsh said: (quoted from post at 10:24:17 01/03/10) JMOR said: (quoted from post at 17:35:39 01/02/10) Dan-IA said: (quoted from post at 11:47:29 01/02/10) I hear this trotted out at me regularly, an outright accusation that my outdoor extension cords and trouble lights are too light a cord to be running electric motors or battery chargers. In this particular case I was running my electric log splitter off a trouble light. Between it and the extension cord I think I might be running 40 feet or so of 14ga wire and the electric log splitter runs the motor intermittently (only when the ram is pushing) and it is a 1500W motor on the splitter. So is there an obvious and simple way to know if I'm overextending my wire? I havent noticed the wire getting warm and don't check voltage drop or anything, if it runs I use it. The obvious & simple way is to measure the voltage at the motor to be sure it is within the motor's specified voltage operating range. Done! Now, your 1500watt motor on 110volts would result in 13.6amps, and 13.6 amps thru 80 feet of 14GA wore (2-way path) will result in 2.7 volts loss or drop in your wire. Typically 5% is maximum allowed for the total circuit, which included both your extension cord & the wiring feeding the receptacle that it is plugged into (some design for 2 or 3 % on motor loads). 2.7/110 X100= 2.4%. You look OK to me. But remember that I don't know what you have ahead of that receptacle. Motors do have a starting current surge much higher than the run current & it is expected & allowed that the 5% will be exceeded during start up, however, if the start up drop is too great, the motor may not start at all. This is particularly a problem on motors starting while under a load such as some air compressors. In such cases, larger than 'standard practices' wiring may be required. And, finally, a motor which is supplied a lower voltage than designed for, will run hotter than if correctly supplied. If you are fortunate enough to have power delivered to you place at 125volts, then you have more room for loss (voltage drop) than if delivery is 110volts, too. It is a p!zzer there there are no simple answers isn't it? I guess there sort of is a simple answer: get a big old honkin 10GA wire and over kill the sucker, then your done. The problem with your math is that you don't account for lock rotor amps, the starting amps of a motor, which is 5 to 10 times the running amps. Lock rotor amps depends on what the type of motor you are using and what is connected to. If you have a pump or air conditioner and your lights when it starts up, it is because of lock rotor amps. If you get a good amprobe, you can measure LRA and FLA.

Yes, a good thought, George, but not a problem. Wire size feeding a motor is never based on LRA or starting surge. Just as an example, we will take a look at a 3 ton condensing unit, with a 50 foot wire run: LRA=76amps, RLA=14amps, specified breaker size=25amps, wire gauge=10.

NOW, if one were to select a breaker and wire size based on LRA/start surge amps of 76, one would install a 90AMP breaker and #2 to #4 gauge wire (depending on specific temperature rating of wire insulation).

Just a little common sense or my favorite "test of reasonableness" would tell a person that virtually the entire Service Entrance capacity (100A) of this little 1100 square foot home would not be consumed by a 36,000BTU air conditioner with wiring equal in size to the Service Entrance wiring.

If still not convinced, go out and look at the name plate data on your condensing unit. No, George, no problem with my math.

Let us take this a step further, with this fellows 1500Watt motor. That 1500w is 13.6 amps at 110 volts. Using your numbers of 5 to 10 times run current for the LRA/start surge, we arrive at 67.5A or 136Amps. IF this fellow were to size his wiring based on your estimated 136 amps, he would need ought or double ought (0 or 00) wiring. IF based on your smaller-end estimate of 67.5A, then he needs the smaller of the two sizes estimated for the air conditioner exercise (#2).

None meets the test of reasonableness. Further more, such an "over-kill" would offer ZERO protection for the wiring in the motor or within the air conditioner condensing unit which would instantly go up in flames when a motor failed and the 90amp breaker allows 90 amps into the units wiring which was designed to be protected by a 25amp breaker.

-----------No, George, no problem with my math.

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