Just in from Square D [RE-wrenches]

[Drake Chamberlin - Electrical Energy]

<x-flowed>At 10:20 PM 03/09/2001 -0800, you wrote:
>wrote:
>
> > An ungrounded system will have a double pole breaker opening both the
> > positive and the negative, which will do a much better job of
> > suppressing arcs.
> >
>Yup, I will admit that I am not well versed in this area of the code,
>but I do remember that at specific voltages, (low Voltage) DC systems
>could go ungrounded, but you had to provide current interrupt/disconnect
>(or was it current limiting?) on both the Pos and Neg.  Wonder if it
>would meet code to use a standard QO box, and bring the Pos in on one
>breaker leg and the Neg. in on the other breaker buss, then just use two
>poles?


The code requires overcurrent protection in ungrounded conductors.




>I once had a Square D guy tell me that a 15 amp QO breaker sould NOT be
>used on a circuit supplied with a 12 gauge wire.  He said it was just as
>bad as using a 20 amp breaker on a circuit supplied by a 14 gauge
>wire.   The large conductor changes a bunch of stuff that makes the
>breaker break when it should?   We're often lowing resistance as far as
>possible.  Cool?

The resistance varies with the length of the run.  It doesn't take a very 
long run of #12 wire to limit the surge of low voltage current to well 
within the bounds of the 5000 DC amp interrupt capability of the QO breaker.

Another issue is that article 720 states that for circuits operating at 
under 50 volts, number 12 wire is as small as we are allowed to use.

ARTICLE 720 -- Circuits and Equipment Operating at Less than 50 Volts

720-4. Conductors
Conductors shall not be smaller than No. 12 copper or equivalent. 
Conductors for appliance branch circuits supplying more than one appliance 
or appliance receptacle shall not be smaller than No. 10 copper or equivalent.

Drake 

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