[BC] Electrical Code

[Phil Alexander]

On 25 Oct 2005 at 1:05, Dave Karr wrote:


> While logical, assuming the above would be located external to the served 
> equipment...that wouldn't pass an inspection here...

Perhaps I'm out of date, but I recall it being legal at one time if the
small box's leads were short.

> The feeder branch created by the first three-phase fusible disconnect and 
> the subsequent 'feeder tap' by the single phase fusible disconnect cannot 
> simultaneously supply the three phase load directly without the addition of 
> a second fusible disconnect for the three-phase load.
> 
> In other words, a "branch feeder" (conductors that supply power to 
> equipment) cannot simultaneously be used as a "feeder tap" (conductors that 
> supply power to other fusible disconnects).

Sounds to me like an unsafe scheme to sell another disconnect switch, however,
that being the case, why not include a contactor following the single phase
disconnect and power it from a transformer across two of the three phases?
> 
> Tap conductors are allowed to be sized smaller according to the downstream 
> fusible disconnect's rating as long as the distance between the tap and 
> disconnect fall inside guidelines.
> 
> 
> This code issue could be circumvented if the fusible disconnect for the low 
> current single phase load were made part of the equipment itself.  Perhaps 
> a small Eaton-Heinemann/Airpax/ABB/etc. panel or DIN rail mount breaker 
> could do the trick.

IIRC, that is essentially what Harris did in the MX-5

The sensible thing to do is put a breaker in the Tx and feed a CV xfmr with
one phase of the 3 phase to drop to 120 the same as might be done with a
large motor starter.

Then there is only a single disconnect for the "appliance" a/k/a Tx and
anything done inside it is, in most cases, exempt, isn't it?


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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