Armored Cable and paper fillers

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AudioGuy

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Is there a specific code requirement that Armored Cable use paper fillers around the conductors?

I'm doing research into voltage induction onto the grounding wire from the current carrying conductors. Basically when the ground wire is equally spaced from the hot and neutral, there is very little voltage induced onto the ground. Normal Poly-wrapped MC is very good in this regard because the conductor spacing is consistent. However I'm looking for a configuration that allows Isolated Ground (IG). HCF-AC seems promising since the safety ground can be carried through the sheath and the internal green wire can be the IG. However the HFC-AC I've seen has paper fillers and these perform poorly due to the inconsistent conductor spacing. Does anyone make an HCF-AC that uses poly wrap?
 

infinity

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Location
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Journeyman Electrician
I don't think that you'll find type AC cable without the paper conductor wrapping. If I remember correctly it's part of the listing requirement for the manufacturing of the cable.
 

tom baker

First Chief Moderator
Staff member
Its not a code requirement, its a listing requirement.
AFC, who makes AC cable, has a new MC cable to compete with the Southwire MC AP cable. So until the MC AP was introduced, only AC in health care had the two ground paths, but you could get MC with two EGCs, but not allowed for health care applications. MC AP in health care has the two ground paths.
Why are you concerned with the induction onto the ground?
 

hurk27

Senior Member
If an IG is bonded at one end via a NEC compliant method, 60hz can not be capacitively coupled into the IG conductor, if your are installing an IG conductor that is isolated at the supply end from the service grounded conductor, then not only are you violating the NEC but also setting yourself up for other problems.

An IG has to be bonded somewhere to the service grounded conductor, or it can not be used. just running it to a ground rod is not compliant and dangerous.

Read through the links I posted above for some good info.
 

AudioGuy

Member
Its not a code requirement, its a listing requirement.

Ah, that's it

you could get MC with two EGCs, but not allowed for health care applications.

Yes, I've looked at the 4 conductor MC, but the conductor arrangement would have to be changed. Normally it's (as you go around) black, white, green, green/yellow. This puts one ground closer to the hot than the neutral, and the other ground closer to the neutral than the hot. Both grounds will have voltage induced on them. If the conductors were arranged Black, green, white, green/yellow. then both grounds would be the same distance from the hot and neutral and not have voltage induced on them. A friend of mine talked to AFC and they would make a custom run for a large enough order, but that's not practical for my specs. AFC also pointed out that the greater separation between hot and neutral would emit more magnetic leakage. However that can be offset by using steel armor instead of AL


Why are you concerned with the induction onto the ground?

Reduce ground voltage differences between two separated locations, where low voltage signal cables also connect equipment together. Analog video is sensitive to noise voltages in the millivolt region, analog audio in the microvolt region. We do have means of dealing with this, such as balanced interfaces, (signal) transformer isolation.

In active balanced interfaces every volt of noise that must be canceled takes away a volt of signal headroom. Since these voltages are often very narrow spikes, they can have high peak values yet read very low on RMS or average reading meters.
 

AudioGuy

Member
If an IG is bonded at one end via a NEC compliant method, 60hz can not be capacitively coupled into the IG conductor,

Quite correct. However the coupling mechanism isn't capacitance, it's mutual inductance. Please pardon the length, but this excerpt from an article I wrote for an audio industry newsletter should explain the issue well.

When current flows through a conductor a magnetic field is built up around it. For alternating current this field collapses and reverses each half cycle of the power frequency, thus it is a moving magnetic field. A conductor placed within a moving magnetic field will have a voltage induced upon it. This is the principle of the transformer.

When two conductors carrying current of the same magnitude and phase but flowing in opposite directions are in close proximity, their fields repel each other, which distorts both fields. While this interaction is complex, there are two important points we need to know. First at a distance from the two conductors, the field is considerably reduced through cancellation. Second, a third conductor located equal distance from both conductors, will not have a net voltage induced onto it, since the two fields are trying to induce equal and opposite voltages onto it.

This is the case for a typical power circuit where the hot and neutral form a series current loop. Since the current is the same everywhere in a series circuit, the current in the hot and neutral wires should always be the same magnitude but flowing in the opposite direction.

In a typical power circuit a third wire is added to serve as a safety ground. This wire may also serve as the signal ground reference for equipment it is powering. This wire normally should not be carrying any current and should not be generating a magnetic field of its own. However as a conductor it can have a voltage induced onto it by magnetic fields it may encounter along the way.

The NEC and other electrical codes require the ground wire to be run the same route as the power wires for the circuit it is serving. This means it will be in close proximity to the current-carrying, magnetic-field-producing power conductors. If, by a matter of its physical location, it is closer to one current carrying conductor than the other, than it will have a voltage induced upon it. The voltage induced onto the ground wire is added to the ground reference at the source panel to become the ground voltage at the load end of the circuit. It creates the ground voltage difference we are trying to avoid.

There are two ways to keep the voltage induced onto the ground wire to a minimum.
1. Maintain exactly equal spacing between the ground wire and each power conductor. This can be done with varying effectiveness by twisting the ground together with the hot and neutral or by cable construction with the ground wire held precisely equal distance from the hot and neutral.
2. Alternate the exposure to each side of the current loop and thus each polarity of the magnetic field, so there is no net addition. This can be done by twisting the hot and neutral together into a pair and running the ground wire separate from this twisted pair.

Read through the links I posted above for some good info.

While I haven't read those threads today, I'm quite familiar with the stand on IG taken by Mike Holt and others on this forum. At the considerable risk of launching a firebomb which may be better addressed elsewhere, I'll just say this. They do make some good points about IG and how, particularly when poorly implemented, it can result in poorer performance in the very area it is meant to fix. However, I see some basic misunderstandings of the real purpose of IG, and total lack of insight into the means and methods of keeping ground voltage differences low.

It was not my intent to start a thread on these issues, but anyway....

if your are installing an IG conductor that is isolated at the supply end from the service grounded conductor, then not only are you violating the NEC but also setting yourself up for other problems.

An IG has to be bonded somewhere to the service grounded conductor, or it can not be used. just running it to a ground rod is not compliant and dangerous.

I thank you for your diligence in correcting the many ignorant thoughts that come from so many in my industry. I'm also working to educate people toward avoiding the foolish and dangerous practices you mention above. I trust by now you've seen your assumptions are ungrounded and we need not discuss them further ;)
 

nakulak

Senior Member
I have always been fascinated by the great lengths taken to mitigate hum and such.

It would be nice if you pointed to some links (your newsletter ?).
 
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