Just in from Square D [RE-wrenches]

Drake Chamberlin - Electrical Energy solar at eagle-access.net
Sat Mar 10 18:10:53 PST 2001


<x-flowed>At 11:53 AM 03/10/2001 -0800, you wrote:
>There are all sorts of faults which could develop.
>
>At 10:12 am -0800 10/3/01, Drake Chamberlin - Electrical Energy wrote:
>>
>>The code requires overcurrent protection in ungrounded conductors.
>
>As I understand it, this is because there could be a fault earth circuit 
>back to the other pole behind the breaker (and this would not already have 
>blown a fuse/tripped a breaker, because the system is floating).

Both wires carry electrical potential relative to each other.  The two 
circuit wires are just two energized, ungrounded conductors with an 
unspecified relationship to ground.  If the system is ungrounded, neither 
wire would be grounded (or earth).

If the positive wire were to accidentally become grounded, then there would 
be a negative potential relative to ground on the other wire.  It would 
become like the positive ground on an old auto or tractor.

With ungrounded systems, one conductor accidentally coming in contact with 
ground wouldn't blow the breaker.  It would take both wires touching either 
ground or each other.  A 2 pole breaker wouldn't solve the problem, either.

>>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.
>
>Hmm but who is to say where the fault is going to happen - which end of 
>the wire?  Or right at the breaker?

That is a good point.  If the fault happened next to the breaker, it might 
not trip  This could be a problem, especially if a short occurred directly 
in the breaker box.  If the short happened a couple of feet away from the 
box in a run of 2 wire, #12 cable, the 5000 amp limit should not be exceeded.


>>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.
>
>that's a bit fascist.  What about circuits with 1 amp fuses?

This can be a bit of a grey area, but wires that are strictly control wires 
can be smaller and have a small fuse to protect them.  The number 12 rule 
is for wires that supply a load.

Where I've seen this get into grey areas are with El Sid pumps for DC 
pumped, hot water heating.  The pumps pull an amount of current that is 
consistent with the amount of power pulled by control circuits.  Yet they 
are actual loads.

Many installers use thermostat wire fused at very low amperage for this 
purpose.  It is not code, but it works so long as the run is not too long 
or not too many pumps are on one wire.

Drake


>Hugh
>
>Scoraig, Scotland
>http://www.scoraigwind.co.uk
>
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