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Electrical Wiring FAQ (Part 1 of 2)
Section - Testing grounding conductors and grounding electrodes.

( Part1 - Part2 - Single Page )
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	Testing grounds is a tricky and somewhat dangerous process.
	Testing for continuity is not enough.  Nor is simple resistance
	testing.  We will outline some possible approaches, but if
	you're the slightest bit uncomfortable, don't even think of
	trying these procedures.

	For a ground conductor to be good, the resistance must be
	"low".  It must also be robust enough to withstand an overload
	long enough to allow the fuse or breaker to trip.  The
	electrical code states, as a general principle, that the
	resistance of the grounding conductor be such that 4-5 times
	the current of the breaker rating will flow.  For example, if
	your breaker is 15A, the grounding conductor's resistance
	should be low enough to permit 60-75A to flow - around 2 ohms
	maximum at 120V.  For comparative purposes, 1000' of 14ga wire
	is 2.5 ohms.

	The difficulty in older homes is that the grounding conductor's
	condition may be that even though the resistance is < 2 ohms, a
	ground connection may blow out before the fuse/breaker goes,
	leaving the case of the appliance that just shorted out live.

	Therefore, you have to measure both the resistance and it's
	ability to stand up to load.

	One simple way to perform a "real" test is dead short the hot
	to ground and see if the fuse or breaker trips.  This is,
	unfortunately, _extremely_ dangerous.  The fuse might explode.
	The breaker may malfunction.  You may get sprayed with molten
	copper.  You may start a fire.  You may get electrocuted or
	blinded.  So don't even think of trying this.

	One moderately safe approach is to connect a 100W lightbulb
	between hot and the ground you wish to test.  The lamp should
	light fully.  If you have a voltmeter, test the voltage between
	the ground and the neutral.  You should see less than 2 volts.
	If the voltage is much higher, or the lamp dims, disconnect it
	quickly - the ground may be overheating somewhere.  The ground
	should be checked for poor connections.

	Testing a grounding electrode is a somewhat different matter.
	The codes aim for a dirt-to-electrode resistance of 25 ohms or
	better.  One moderately safe way is:

		- turn off the main panel
		- turn off all of the breakers
		- disconnect the grounding electrode from the rest of
		  the system.  (often just a bolt in the panel)
		- connect a 5A fuse between the output of one 15A breaker
		  and the grounding electrode.  (use a 5A automotive fuse
		  in a pigtail holder)
		- turn on the main breaker and the single breaker connected
		  to the 5A fuse.
		- if the 5A fuse blows, your ground is good.

User Contributions:

Dev
Report this comment as inappropriate
Dec 21, 2011 @ 12:00 am
In a fire protection circuit, circuts are shown witha no example 6,8,4etc. what it mean?these circuits are connected between smode detector,junction box etc
kevin
Report this comment as inappropriate
Dec 24, 2011 @ 12:12 pm
My daughter dropped a small necklace behind her dresser. The necklace crossed a plug terminal and shorted the receptacle.
I bought a new receptacle and installed the same. I still have no power I suspect there could be a bigger problem,this is aluminum wiring.
I've killed the breaker and call an electrician but am curious as to what happened.P.s. there is a dimmer switch on the same circuit.
dennis
Report this comment as inappropriate
Feb 24, 2012 @ 11:11 am
Regarding new construction wiring and running 12/2 and 14/3 wire in the same box.

I have multiple switches to lights. Ran 12/2 and 14/3 into switch box and inspector wrote correction needed.

What should I have done instead?

thank you
dennis
Robert
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Nov 26, 2012 @ 9:21 pm
Does a grounding electrode facilitate the operation of a OCPD, to clear a ground fault ?
@dennis
Report this comment as inappropriate
Mar 18, 2013 @ 10:10 am
Assuming you are installing two switches in a two switch box, you probably should have used 14/2 and 14/3 instead of replacing 14/2 with 12/2. If you are only installing one switch in a one switch box, you should only have one cable in the box.
P k
Report this comment as inappropriate
Jan 26, 2014 @ 10:10 am
I prefer to use nothing smaller than12 awg /the smallest sized wire on a circuit determines the allowable ampacity
Ex: 15 amp-14awg. 12awg-20amp only rule for thumb other factors such as continuous load,heating and others if you do not know the safe NEC rules then please call a qualified journeyman Electrician better be safe

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Top Document: Electrical Wiring FAQ (Part 1 of 2)
Previous Document: Bonding requirements
Next Document: Why is one prong wider than the other? Polarization

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Last Update March 27 2014 @ 02:11 PM