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From: smb@ulysses.homer.nj.att.com (Steven M. Bellovin)
Newsgroups: misc.consumers.house,sci.electronics
Subject: Re: Grounding old electrics
Keywords: Electrical, Ground, House, Hints
Message-ID: <12073@ulysses.homer.nj.att.com>
Date: 24 Aug 89 02:11:18 GMT
References: <2171@netcom.UUCP> <5057@teklds.CAE.TEK.COM> <26924@amdcad.AMD.COM>
Distribution: misc
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 55

In article <26924@amdcad.AMD.COM>, phil@diablo.amd.com (Phil Ngai) writes:
} In article <5057@teklds.CAE.TEK.COM> brucem@teklds.CAE.TEK.COM (Bruce McAlary) writes:
} |I have done some re-work of older 2 wire systems, code allows you to wire the
} |neutral to the ground screw to on 3 wire plugs to satisify the ground. If you 
} |examine a 3 wire system, the ground and white neutral are attached to a common
} |point in the breaker box, thus you are accomplishing the same thing by wiring
} |a jumper from the white neutral to the ground. Make sure you have the
} 
} Didn't we just go through this? As I understand it, this is misinformation. 
} 
} As Henry Spencer likes to say "NEUTRAL IS NOT GROUND!"
} 
} I believe that to do it right, you need a separate ground wire, of
} ampacity at least equal to the hot and neutral wires. Otherwise, a
} short from hot to ground could end up burning out the ground wire
} before the breaker blows. Tying the socket's ground and neutral
} together and only using a neutral wire exposes you to the voltage drop
} across the neutral wire which can be substantial. 

As I recall the NEC, the ground wire does not have to be the same size;
it won't be carrying the current for that long...  However, you're
correct for 12 and 14 gauge wires; it's only for the larger wires
that the smaller size will do.

On your main point, you're quite correct, as far as I know.  However,
many older houses, though equipped with 2-prong receptacles, actually
have a ground conductor.  If the house was wired with Romex -- and I've
seen it in a house built ~1950 (it may have been as late as 1955, but
I doubt it) -- there is quite likely a ground conductor inside the box.
If the wiring is BX -- more likely in a house that old -- the metal
sheathing of the BX is considered an adequate ground in the U.S.
I believe that Canadian electrical code require a separate ground wire
even with BX, but I've never done any wiring there...  Either way,
check it!  Using a voltmeter, verify that there is ~120 volts between
the hot (black) wire and the outlet box.  If there isn't, that outlet
is not groundable without running a new line to the breaker box.

I've said a lot about GFCIs in the past; let me add one more comment
here.  The instructions I've seen with GFCIs have stated that they
can be wired up to replace ungroundable outlets.  That seems right
in principle -- they're measuring current flow in the hot and neutral
wires, not the potential to ground.  But check -- your mileage may
vary.  The biggest problem in installing a GFCI is likely to be room --
GFCI outlets are quite bulky, and you may have trouble fitting them
into a box, especially if there are other devices downstream.  In that
case, you may want to contemplate using GFCI circuit breakers instead.
If you update the panel box, as someone else suggested might be needed,
that can easily be done at the same time.  Or it's easy enough to do
yourself, though I confess that working inside the main breaker panel
makes me a bit nervous.  Other than the usual cautions -- and turning
off the main breaker! -- figure out what brand to get; breakers must
be compatible with the box (called a "load center"), and there are at
least two or three different types.

		--Steve Bellovin