neutral current on grounded parts

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jes25

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Location
Midwest
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Electrician
In a typical resedential service in my area you have a imc riser pipe out of the top of the meter can. The utility drops power overhead. The neutral is bonded to the meter can. The messanger/neutral is tied DIRECTLY to the imc riser pipe. A 3-wire cable than feeds the main disconnect where the main bonding jumper is instaled.

My question is it seems to me that a small amount of neutral current would flow on the meter can and imc pipe, is this normal or OK? :confused:
 
Re: neutral current on grounded parts

It is very normal and okay in that its not a common source of reported problems. There are possibly several parallel paths back to the source, namely the grounding electrode system. However, a properly sized grounded conductor and properly made terminations should eliminate the potential of objectionable current over the metal parts. Section 250.6(B) provides options if you are seriously concerned.
 
Re: neutral current on grounded parts

Thanks brian, But could you explain how the ground rod/ metal water pipe (my grounding electrode system) is a parallel path. I seriosly doubt 120V is going to push through a 100 plus feet of dirt back to the source

[ July 21, 2004, 10:43 PM: Message edited by: jes25 ]
 
Re: neutral current on grounded parts

That would be true if Earth acted as one path, but it is also a whole bunch of parallel paths as well. If you remeber back in theory class, the total resistance of several resistors in parallel will always be less than the least resistive path.

So once the current gets through the highly resistive electrode to Earth connection, there can be an infinite number of ways the current can get back to the source.
 
Re: neutral current on grounded parts

I have never put my amprobe on a ground electrode but I assume we are talking about 1 milliamp or less.
 
Re: neutral current on grounded parts

Jes25 it is just simple ohm's law and parallel circuits. Lets say your neutral impedance is .1 ohms in parallel with a 25 ohm ground electrode, with a neutal load current of 10 amps. That would give you 9.6 amps on your neutral, and .4 amps on the ground electrode.
 
Re: neutral current on grounded parts

Jes,
And if you are using a metal underground water piping system that is common to other buildings for the grounding electrode, it would not me uncommon to have 20% or more of the neutral current on the water piping system. In you original post, I would expect at least 20% of the grounded conductor current to be flowing on the raceway, assuming that the clamp at the top is a good electrcial connection to the conduit.
Don
 
Re: neutral current on grounded parts

jes25,

Your suggestion of putting the clampon on the grounding electrode conductor is a good one. Try it, and [head scratch] think about the conductive paths that are present.[/head scratch]

The PoCo transformer will have a connection to earth at the base of the transformer.

If more than one electrical service is run off that one PoCo transformer, the neutrals of those other services will be connected to earth and to any piping systems that are common to the buildings (especially metal water lines, as well as coax sheaths and Telco grounds and maybe even metal gas lines).

This was a real eye openner for me a couple years back, as I had never given it much thought.

Bottom line, the unbalance current on the PoCo side of the Service Disconnect usually has multiple paths.
 
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