Re: OT - Electrical question

Posted by Gerald J. on September 24, 2007 at 09:06:50 from (67.0.103.120):

In Reply to: OT - Electrical question posted by CB in central NY on September 24, 2007 at 08:21:19:

First realize the electric loads tend to grow, you add more than you started with.

Second remember that unless you have special motors (repulsion start, induction run) the starting current is 6 times the running current. If you have 9 volts drop at running current, you will have 54 volts drop at starting. That motor will take so long starting that it will likely burn up and will do a poor job handling tasks that require short bursts of peak power. And with the terminal voltage 9 volts low, the motor current will be 8% high for normal rated load. An induction motor hasn't the privilige of taking less power when the voltage is low, the AC motor maintains a near synchronous shaft speed and so the shaft load is the same no matter what the voltage (within reason) and the motor with a low supply voltage has to draw more current to run the load. Low voltage tends to be the death of motors.

Third, its a lot harder to put the wire in place than to buy it larger so its generally easier in the long run to put the wire in once and to make it large enough.

In my machine shed I set up a woodworking shop 60' from the 200 amp service entrance. The outlets are wired with #12. Running a shop vac and radial arm saw on the same circuit, it took the radial arm saw 5 or 6 seconds to get up to speed. I ran a #10 extension cord from an outlet mounted at the service panel. That improved things but then with both of them starting on the same circuit, I tended to trip the 20 amp breaker either while starting or while sawing. I have a 50 amp 4 wire circuit for welder or whatever, running the length of the building so on the shop end, I hung a temporary 100 amp panel and wired outlets to it so I and put saw and vac on separate phases and I run a SHORT #10 over to the saw. Now it gets up to speed in a hurry, without showing 25 volts drop while starting. I wired the saw table with a relay to run the vac so the vac always starts when I start the saw. I can get a lot more work done, but the run to the 50 amp 4 wire outlet is #6.

Being cheap on wire size is bad economics, worse after the first motor burns out from the low voltage.

Gerald J.

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