[BC] Electrical Code

[Phil Alexander]

On 21 Oct 2005 at 11:35, Davis, Jack L. (KTXL) wrote:


> The three phase Wye connected power is actually 208 volts leg to leg and 120
> volts to neutral.  So it is perfectly legitimate to use any leg to neutral
> for a single phase load.  The code problem gets to be one of fusing, the
> three phase is most likely fused (or circuit breaker) at a high value and
> the wiring on the single phase load is not capeable of that kind of current.
> If you fuse the single phase load at a proper value there should not be a
> problem.  If the three phase is Delta connected (Not common anymore) then
> the voltage is 240 volts leg to leg and one of the leg to leg connections is
> normally center tapped to form the neutral and 120-0-120 for single phase
> load.  Either end of the center tapped transformer winding to neutral is 120
> volts and is usable. The third leg (often reffered as the "wild leg") can not
> be used to neutral as it varys wildly depending on the load and current
> unbalance.

While all of the above is quite true, there is a simple solution. Install a
single three phase disconnect and wire a smaller single phase disconnect as
a branch of its load. Then, when the three phase switch is pulled, all is
dead. I would go so far as to add a shrieking "howler" on the three phase
switch so that if the low power single phase is pulled, a relay falls and
the big box starts screaming like a banshee until it is also pulled. Thus
the small box is there for meeting the intent of the code that each wire
be fused with a fuse correct for its ampacity, and the large three phase
box provides single switch safety for personnel.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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