New electrical question

[Junkman]

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If the neutral and ground are connected at both places, you can get circulating currents, also known as ground loops, which can cause current in the ground line, which is not desirable from a safety perspective.

paul
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I understand the safety situation, but don't understand how you can have circulating currents or ground loops. Could you please explain this to me. I would like to better understand the reasoning behind the separation. Also, no one mentioned it, but when I was reading the code book, it said that there needed to be two ground rods separated by at least 8' of distance. Mine is hooked to the ground rod at one end and the steel well casing at the other end. Thanks for the help and information... Junk....

 

[techman]

I'll try to explain (don't be insulted if I try to be too simple, I don't know the electrical ability of all people out there).

First there is a voltage drop in any wire carrying current. The voltage drop is the current times the resistance of the wire (V=I*R). When you connect the ground and neutral together in both boxes, the ground and neutral wires are electrically connected in parallel. Given that, when there is current flowing in the neutral leg (which normally occurs), there will be a voltage drop over that wire. In the example of the ground-neutral connected in parallel, the current will flow through both of them, and a voltage drop will occur in both of them, so the ground (safety ground) will rise above "true" ground, which is established by the ground rod and bonding to the house piping, by that voltage drop due to the current.

So in this example, say there is 20 amps in the neutral leg, which causes a 2 volt drop in the neutral-ground wires between the main and sub. Now the ground pin on all of the sockets will be 2 volts above the "true" ground. If you are working in a bathroom with a grounded tool, the metal case of the tool will now be 2 volts above the potential of the metal faucet. If you touched the tool to the faucet, current would flow back through the pipes, because there is a 2 volt difference. This would be considered a ground loop, or circulating current - it is "circulating" through the ground circuit.

This may seem trivial with 2 volts, but consider now that your welder, which is connected to the sub, shorts internally. In the time before the breaker trips (they takes some time to trip the breaker), there could be hundreds of amps of fault current (fault current can exceed the service rating for very short times, limited by the wiring and transformer impedances only). This momentary high current will cause a corresponding high voltage drop as described earlier. If someone happened to be between neutral and ground (like the bathroom example I gave) when the fault occured (the fault can be far from them), they could get a jolt.

I hope this explains it and the reason why the code does not want you to bond a sub.

paul

 

[Junkman]

OK.... I believe that I understand the concept and reasoning, but don't ask me any questions about this in the morning. It is like when I went to the library to get a book on frogs.... the book was 6" thick. I told the librarian that it was more about frogs than I could comprehend in a short time. /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif Once again... thanks for the answer.
Next question..... what size breaker should I use for a 5 HP 220 volt air compressor? It is presently wired with 12/3 and a 30 amp breaker. I believe that it should have a 10/3 wire for 30 amps, so this is the next item to be corrected, but I have no one to blame for this except for myself. I will say that I have monitored the wire temperature by touching it in my hand after the compressor has been working for a while and it has never even gotten warm. Is the 12/3 acceptable and should I change the breaker to a 20 amp???? thanks again.

 

[techman]

In general motor HP is an inaccurate way to size circuit protection, due to the various motor designs/start configurations. The best thing is to look at the motor nameplate for the FLA (full load amperes). This is the rating for the particular motor that should be used to size the breaker. As a rough estimate, a 20 amp CB should be correct. If that matches the motor FLA, 12 or 10 size wire will work.

paul

 

[fractal]

</font><font color="blue" class="small">( Next question..... what size breaker should I use for a 5 HP 220 volt air compressor? It is presently wired with 12/3 and a 30 amp breaker. I believe that it should have a 10/3 wire for 30 amps, so this is the next item to be corrected, but I have no one to blame for this except for myself. I will say that I have monitored the wire temperature by touching it in my hand after the compressor has been working for a while and it has never even gotten warm. Is the 12/3 acceptable and should I change the breaker to a 20 amp???? thanks again. )</font>
Please remember that the purpose of the breaker is to protect the wire. Not to protect your equipment ... to protect the wire.

A 20 amp breaker is the proper size breaker for a 12 AWG wiring.

A 30 amp breaker should have 10 AWG wire.

My calculations suggest that a 5hp motor operating at 80% efficiency off 220v will draw just over 20 amps.

If it were me, I would replace the breaker with a 20 amp breaker. The correct size for the 12awg wire.

btw, is this compressor hard wired or is there a plug involved? If so, what type of plug?

 

[notamustang]

Did you ever find the FLA (full load Amps) on the compressor name plate? The ideal situation is running power to a fused disconect or using a motor rated breaker HACR type. Well thats if the breaker is withen 25' code stuff or use a plug. I personally like have a big handle mounted disco at waist height near the equipment. Its easy to shut off if something goes astray while you are standing there.