Re: [Amps] 220V wiring: Was Question about safety ground connection

[R.Measures <r@somis.org>]

On Nov 15, 2005, at 3:02 PM, Mike McCarthy, W1NR wrote:

> Everyone.  When talking about a 220V outlet FORGET about what it may 
> or may
> not do at the breaker box.  A 3 prong 220V outlet is 2 hot and GROUND.

My 240V, 40A circuit is wired 2-hot and 1-neutral.  I assumed that no 
"safety" ground wire was needed because neutral is connected to safety 
ground in the circuit breaker box.  Under normal operation, virtually 
zero current flows in the neutral wire.  However, some will argue that 
if a >40A load accidentally appeared on one side of the 240v outlet, a 
dangerous potential would appear between neutral and ground until the 
40A breaker trips.    Determining the level of this potential involves 
the application of Ohm's Law.  Assuming that the asymmetric accidental 
load is 80A @ 120V, the 40A breaker would probably trip in c. 1-second. 
 Under this condition 80A would flow in the neutral wire and one hot 
wire.  Since the neutral wire is #6-ga, which has c. 0.004Ω per ft, and 
it is 99-ft in length, it has c. 0.0396Ωs of resistance.  Thus, at 80A, 
the voltage-drop = 0.0396Ω x 80A = 3.12V ref. to ground - which would 
appear on the neutral pin of the 240v outlet.  Even though a potential 
of several volts is dangerous to some microbial life forms,  could 
several volts be dangerous to primates?

Editorial -- In my opinion, the folks who got paid for updating the NEC 
probably realized that if they didn't make at least some changes, 
eyebrows would be raised by bureaucratic bean counters.   Another 
factor may have been that they knew little about circuit analysis and 
Ohm;s Law.
end

> A 4
> prong 220V outlet (also called 110V/220V) has a neutral added.
>
> You cannot safely connect a 110V device to 1/2 of a 3 conductor 
> outlet.  In
> this case, you are using the ground as neutral and are in violation of 
> the
> National Electric Code.
>
> Breaker panels may or MAY NOT have neutral and ground bonded.  The 
> breaker
> box in my radio room, by the code, has separate neutral and ground 
> busses
> connected to the main entrance with a 4 conductor feeder.
>
> This is what is mandated by the NEC.  It is not dictated by the 
> breaker box.
>
> Your safest bet is to run 4 conductor wire and outlets.  All new 
> ranges and
> dryer circuits must be 4 wire.  As far as I am concerned, all new 
> amplifier
> circuits should be too.
>
> Mike, W1NR
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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