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If i put a 30 amp generater 125' from my main box will it work ?

  • May 02 2012 - Town of Salem
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Don't use #10 wire; it will be too small!  Is it a 120 volt generator, or 240 volt generator?

30 amps at 120 volts, 125 feet is 5.5% drop assuming 80% power factor and PVC conduit and #10 copper wire.  At 240 volts, that is 2.8% drop.

With #8 AWG copper wire (the next standard size up), the voltage drop is reduced to 1.76% at 240 volts, or 3.5% drop at 120 volts.

Building Codes recommend no more than 2% drop to a panel, and no more than 5% drop to the furthest outlet.  Of course, it will depend on what you are running.  Under voltage lights are just a bit dimmer.  Under voltage on a motor may not allow the motor to start, putting it in a "stall" condition, and possibly overheating the motor windings and burning it up (if the fuse doesn't go first).  Most 120 volt equipment will work just fine at 110 volts; maybe even at 105 volts.  Most 240 volt equipment works just fine at 220 volts, and maybe even as low as 200 volts.

But going up a wire size or two really doesn't add that much cost to your project.
  • May 02 2012
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I just wanted to apologize for the incorrect voltage drop numbers provided earlier on 5/2/12...  they are actually a bit higher, as I ran them at 3 phase instead of 1 phase. (2/sqrt(3) difference, due to return current path, and balancing of loads on 3 phase...)

Since I was temporarily banned from the site, I could not post the correction.

#10 awg, 30 amps, 125 ft, 120 volts... 6.35% drop
#10awg, 30 amps, 125 ft, 240 volts 1phase ... 3.18% drop

#8 awg, 30 amps, 125 ft, 120 volts... 4.06% drop
#8 awg, 30 amps, 125 ft, 240 volts 1 phase ... 2.03% drop

#6 awg, 30 amps, 125 ft, 120 volts... 2.6% drop
#6 awg, 30 amps, 125 ft, 240 volts, 1 phase... 1.3% drop

These are calculated using the vector dot-product method listed in the National Electric Code, using the copper wire impedances listed in the National Electric Code, published by the National Fire Protection Association.  80% power factor, copper wire, and PVC conduit were assumed.

You will get slightly different results using ohms law and calculating using either "rated current" or "measured current".  For really long runs and small wire sizes, these other methods of calculation would give you more useful results.  Obviously, one can't have more than 100% voltage drop, so if one is using rated current, the actual current with high impedance path is much lower.  And if one is using measured current, this change in current is already factored into the numbers.

And by the way, I agree with the prior poster that there are a lot more issues to consider than just voltage drop.

Why so far away anyway?  Just the noise and fumes?  Or is one intending to share with a neighbor or another building on site?

And why the selection of a 30 amp unit?  Just what one has?  Or has someone calculated the total of the critical loads one wants to run?
  • May 16 2012
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Something else to consider, if you use a 120/240 volt generator with load imbalance...

Say we consider A-phase as 120 volts, neutral as 0 volts, and B-phase as -120 volts (180 degrees out of phase with phase A) at the generator.

Then say we load A phase to 28 amps, and B phase to 0.5 amps....

Now calculate the voltage at the Panel assuming 80% Power Factor, #6 CU wire, PVC conduit, 125 feet away:

Neutral becomes 1.5 volts, A phase is 118.5 volts; phase A to neutral is 117 volts.

B phase is -119.97 volts.  Add in the shift in neutral voltage; phase B to neutral is -121.47 volts.

So, we have an "over-voltage" condition on phase B.  For some things, the amount of over-voltage is small enough that it shouldn't make any difference.... but consider, motor starting on Phase A for the refrigerator compressor.  This pulls the neutral voltage even further toward phase A.  Thus causing a "spike" in voltage on phase B.  For many light bulbs rated at nominally 120 volts, this could cause the light bulb filament to break, substantially shortening the life of the light bulb.

This may be able to be addressed by using "heavy duty", "service duty" or "130 volt" light bulbs.  But the better solution is to balance the loads.

But with so few items on the generator, and with the loads not constantly running at full load (for example, cycling on and off of the refrigerator compressor motor, and defroster heater), it is very difficult to maintain a balanced load between the phases.

Of course a 120 volt single phase generator addresses the "over voltage" condition... but increases total voltage drop, and cuts in half the power output of the generator.  And introduces the "shared neutral" problem on the branch circuits.
  • May 03 2012
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Just wondering how you are doing your power transfer, and your "load shedding".   And if using a 120/240 volt single phase generator, how you are balancing your loads....  And if only a 120 volt generator, how you are making sure you are not sharing a neutral with two circuits that will be on the same phase leg?

I'm assuming this is primarily for the refrigerator, a computer, and a few lights during a power outage, since 30 amps will not power your whole house most of the time?  Perhaps some minimal electric cooking, such as a microwave?

(I hope you totaled the loads that you plan to be running on the generator.  It is not fun to stall a generator, and even less fun to burn it up).

Also, how are you going to start the generator?  Manually start 125 feet away from the house?  Or will you have a battery in the generator for electric start, and a charger, and some remote start wires?

Also consider the temperature.  If left in the cold without a crankcase heater, it could crack the engine block.  Or is this something that is stored protected from the environment, and then just moved into place when needed?

Remember that you cannot parallel the Generator with the utility service as there is no way that you will get the generator synchronized with the commercial power.  It is not like electronic inverters for a photovoltaic solar power system.
  • May 03 2012
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Profile picture for PeterMckernon
Thanks for the info I think I'll go with #6 copper
  • May 02 2012
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Profile picture for the_country_hick
It should as long as it is wired correctly AND you use the correct gauge wire to control voltage drop (to much is a bad thing) from that distance and amperage.
  • May 02 2012
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