One Wire

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domnic

Senior Member
Occupation
Electrical Contractor
I see one hot wire going to a transformer (7200 volts) this is a camp site . above the hot wire is a grounded wire. if the load on the hot wire is 20 amps how much current is on the grounded wire. and how current is going to earth.on average . the soil is clay reading on 8" ground rod is 15 oms in this camp site.
 

wbalsam1

Senior Member
Location
Upper Jay, NY
Will you please reshape the question? I'm assuming you are discussing the secondary voltage side of the xfmr, but need to know more. Thanks. :smile:
 

barbeer

Senior Member
You have never seen on of those 15/20 amp 7200 volt circuits?:smile: Off the cuff I would say this circuit is lending 20 amps to the grounded conductor, but that conductor has far more amps than 20 as there are many taps on this same line?!
 

tryinghard

Senior Member
Location
California
domnic said:
I see one hot wire going to a transformer (7200 volts) this is a camp site . above the hot wire is a grounded wire. if the load on the hot wire is 20 amps how much current is on the grounded wire. and how current is going to earth.on average . the soil is clay reading on 8" ground rod is 15 oms in this camp site.

Current through earth will only exist according to resistance and if there is a parallel path with the grounded conductor, I=E/R.

The grounded conductor could be connected to earth in multiple locations and the resistance between these connections at any given moment will determine the current. The earth will not keep the same resistance at any moment it is likely changing, moist, dry, hot, cold, rain...This is why we cannot use earth for an effective ground fault current path, 250.4(A)(5) & 250.4(B)(4).
 

mivey

Senior Member
I'm guessing 10-100 milliamps through the earth. So I'm guessing 19.9 to 19.99 amps on the grounded conductor and 0.01 to 0.1 amps through the earth.
 

brian john

Senior Member
Location
Leesburg, VA
1. There are two conductors going to this single phase transformer.
2. How did you measure the ground resistance?
3. More importantly how did you measure the HV current?
 

domnic

Senior Member
Occupation
Electrical Contractor
wbalsam1 said:
Will you please reshape the question? I'm assuming you are discussing the secondary voltage side of the xfmr, but need to know more. Thanks. :smile:
I'M TALKING ABOUT THE PRIMARY CURRENT.
 

zog

Senior Member
Location
Charlotte, NC
domnic said:
I see one hot wire going to a transformer (7200 volts) this is a camp site . above the hot wire is a grounded wire. if the load on the hot wire is 20 amps how much current is on the grounded wire. and how current is going to earth.on average . the soil is clay reading on 8" ground rod is 15 oms in this camp site.

Single or 3 phase?
Delta or Wye connection on the primary of the transformer?
What type of cable? Are you sure it isnt a concentric grounded nuetral cable?
How did you measure the ground rod resistance?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
domnic said:
I'M TALKING ABOUT THE PRIMARY CURRENT.
HOW SHOULD WE KNOW?

Without measurements, it's all a wild guess. We need one more parameter to add to your numbers of 20a line current and 15 ohms ground resistance.

You could tell us either the electrode's current or the system neutral resistance, and then we can answer the original question. But then, so could you.
 

mivey

Senior Member
tryinghard said:
Most of it maybe returning on the grounded conductor and some current might be in earth :)
Most? Some? Pull some assumptions out of the air and take a stab at some numbers. That will give us something to do until we get more information. :smile:
 

brian john

Senior Member
Location
Leesburg, VA
Without testing any answer would be a SWAG.

As for the primary current response, I was assuming from the question the poster was measuring primary current. As the knowledge to calculate primary current from secondary current measurements did not appear to be with in the grasp. WAS THIS PC enough?
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
080620-2016 EST

Between poles are there one or two primary wires. If there is only one primary wire between the poles, then the earth or a wire in the earth has to be the other side of the primary circuit. Note: also the primary neutral wire my be the same wire as your low voltage secondary neutral wire. This will be bundled with the hot wires to all the houses supplied by your transformer. Then it will continue on and connect with the next transformer's secondary and primary neutral.

Put a probe in the ground, use a screwdriver, at your transformer pole. If the earth is being used as one conductor for the primary, then you will see a substantial voltage gradient as you move away from your transformer pole. Be careful. Use insulated probes and maybe other protective means. Put another probe 10 ft away from the first and below your primary power line. Measure the voltage difference. If this is millivolts, then there is not much ground current. If volts over this short range, then be very careful.

The extension wire from probe 1 to your voltmeter at probe 2 needs to be insulated.

Move the second probe further away in the same direction along and under the primary wire. Your results will determine how far you separate the probes, and your safety concerns.

Report back.

.
 
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