Metal box mounted to wood or drywall

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Danny89

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How does it make since that...
A metal box can be mounted to a wood stud/or floating in drywall(old work box) with a ground pigtail and meet code requirements.
In my mind , path of least resistance would mean if someone touches the metal box it will go through them to ground.

In my mind all boxes should be mounted to metal so path of least resistance isn't a person. Any code notes or article would be helpful. Thanks in advance

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The box and pig tail would be attached to a type of EGC listed in 250.118. Electricity does not seek "ground" so even without an EGC you could touch the box and still might not be a ciruit path.

Roger
 
The pigtail to the box is connected to the EGC in the cable serving it. If there were a fault to the box, like an ungrounded conductor popping out of a backstab, or melting its insulation, and it contacted the box, the low impedance path provided by the ground would trip the breaker.

although you are technically incorrect In stating that electricity takes the path of least resistance, that path is where most of the electricity will go, which is why you do not get electrocuted if you touch the service neutral in a panel. The path back to the source is probably a tenth of an ohm resistance tops the path through you may be on the order of several million ohms.

Back to your box example, if it were not bonded back to the service, and a fault occurred, it could be sitting energized at 120 volts. How badly you would get shocked would depend on your resistance, and the resistance of any path back to the source.

if you are not sweating and are standing on a rubber mat, you would not even know the box was energized. If you are just getting out of the shower, standing on a concrete floor, or turning a faucet on an all metal water pipe system, and flick a faulty light switch, touching the faceplate screws, you are probably going to have a very bad day.

This is more than a theoretical problem with older 2 wire cloth non-metallic cabled houses.

it only takes something like 10 ma across your heart to stop it.

I do not have the code section, however all metallic boxes are required to be bonded. And with the 2017 NEC, virtually every encounterable receptacle outside of a non-industrial installation has to be ground-fault protected. See 210.8.
 
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electrofelon said:
There's no such thing as "path of least resistance" ...
Click to expand...
At the risk of being labeled a nit-picker ...

Whenever there are two or more parallel circuits -- which is to say, in any electrical installation more complex than a one-bulb flashlight -- one of the paths will be the path of least resistance, one of the paths will be the path of most resistance, and all the others will be somewhere between the two.

But here's the usual point of confusion: When current flows through the path of least resistance, it doesn't cease to flow through the other paths. Current flows through ALL parallel paths simultaneously, and the amount of current flowing in each path is inversely proportional to the resistance of each path. (Ohm's and Kirchhoff's laws)

- - -

But back to the original question:

The center tap of the transformer feeding the house is grounded at the service entrance. The metal box is bonded to this same point by the pigtail and the equipment-grounding conductor, assuring that the two will be at the same voltage. (nominally zero) The only way current would flow through a person touching the box is if they were standing on something that was at a different voltage. If there's no voltage differential between your hand and your feet, no current will flow through your body. (Ohm's law again, with voltage=zero)
 
electrofelon said:
There's no such thing as "path of least resistance". If there was, parallel circuits wouldn't work.
Click to expand...
Someone already replied with most of what I had to say on this:

drcampbell said:
At the risk of being labeled a nit-picker ...

Whenever there are two or more parallel circuits -- which is to say, in any electrical installation more complex than a one-bulb flashlight -- one of the paths will be the path of least resistance, one of the paths will be the path of most resistance, and all the others will be somewhere between the two.

But here's the usual point of confusion: When current flows through the path of least resistance, it doesn't cease to flow through the other paths. Current flows through ALL parallel paths simultaneously, and the amount of current flowing in each path is inversely proportional to the resistance of each path. (Ohm's and Kirchhoff's laws)

- - -

But back to the original question:

The center tap of the transformer feeding the house is grounded at the service entrance. The metal box is bonded to this same point by the pigtail and the equipment-grounding conductor, assuring that the two will be at the same voltage. (nominally zero) The only way current would flow through a person touching the box is if they were standing on something that was at a different voltage. If there's no voltage differential between your hand and your feet, no current will flow through your body. (Ohm's law again, with voltage=zero)
Click to expand...

Current flows through all available paths, but in proportion to resistance of each path. If the box in question becomes energized by an ungrounded conductor, the bonding jumper/EGC may have a resistance of less than an ohm, if you touch it but have resistance to other grounded object of 20K ohms, you will see very little current, the EGC will carry nearly all the current, will be high enough to trip the OCPD and will limit duration of whatever current is flowing through you. If you have 20k ohms across you and apply 120 volts you will get 6 mA current through you. That is high enough you will definitely feel it, is also high enough it can trip GFCI's. The EGC is likely carrying hundreds or even thousands of amps though during this event, but is over in milliseconds because that high current tripped the OCPD.
 
Even in a place such as Jamaica, where we run pvc conduit in concrete... they require the metal boxes to have grounds attached to them... they might be the only metal inside that section of concrete othe than the copper wires, but still... they need grounded to the system, via a pigtail or a bonding screw...
not sure that this helps you any... but it is code even if you use an all plastic cover such as an Internet or telephone plate... I have to run a ground wire with the telephone and cable and internet cables in the low voltage conduits, for bonding the outlet boxes... simply as a just in case...
 
Adamjamma said:
Even in a place such as Jamaica, where we run pvc conduit in concrete... they require the metal boxes to have grounds attached to them... they might be the only metal inside that section of concrete othe than the copper wires, but still... they need grounded to the system, via a pigtail or a bonding screw...
not sure that this helps you any... but it is code even if you use an all plastic cover such as an Internet or telephone plate... I have to run a ground wire with the telephone and cable and internet cables in the low voltage conduits, for bonding the outlet boxes... simply as a just in case...
Click to expand...
NEC does require bonding such boxes containing power conductors, does not require it (as a general rule) for the communications applications though.
 
kwired said:
NEC does require bonding such boxes containing power conductors, does not require it (as a general rule) for the communications applications though.
Click to expand...

that is what I know myself... the amount of power going to be introduced into the metal box in a fault when you have a tv cable, an internet cable, a telephone cable and an hdmi cable inside a plastic conduit should be rather low, and all the cables are themselves shielded...
Yet, the local code I must go by says I must run a 10 guage green coated or green and yellow coated wire through the conduit and connect it to any metal boxes and to any metal lids on such boxes. Because the conduit runs through concrete, if it was metal I would still be required to run the ground wire and bond the boxes, in case the metal conduit rusted...

Of course, no one can explain why, just tht it must be in case of lightning on the telephone or cable tv lines...
 
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