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Re: Contactor question <Control Panels '05 NEC>

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 10:39 PM 8/14/2005, you wrote: 
Original poster: Sparktron01@xxxxxxxxxxx
David

Theoretically yes, however, NO ONE will try to get UL or IEC ratings
on such a contactor design.  Invariably, one contact will lead or lag
(and get over loaded).  Use a contactor rated for the worse case current.

A good rule of thumb is size the contactor as no less then 125% of
main fuse (or branch circuit) over current protection.   Also be aware
that with new 2005 NEC, control panels have to have a short circuit
current rating placarded on the panel, and input (branch circuit) OCP
must have a let through worse case SC current LESS THAN control
panel rating or installation will fail electrical inspection.


Interesting that this is now part of the NEC. That's historically been done as "good design practice" by your friendly local electrical engineer during the short circuit analysis. Kind of like "insulation and BIL coordination" which is the same concept except for overvoltage (i.e. no point in having a 100kV circuit breaker downstream from a 20kV gap).

Most of the big equipment manufacturers actually have design tools to help do the analysis (factors in the impedance of the distribution transformer, etc. to determine SC currents).

Probably part of the overall trend to codify (in the regulatory sense) what used to be "good engineering practice". Good in some ways: it keeps clients from asking, "why are you doing that when it's not required by code and it's costing me more?". Bad, because it makes what is a fairly complex analysis problem appear to be something that can be done as "cookbook". When the cookbook approach fails, it engenders more conservative regulatory requirements, which tends to be antithetical to new and novel stuff.

Such is life.


There have been explosions, fires, injuries and deaths by not following
this design parameter (new article 408 IIRC).

Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA


> Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>
>
> Hi all,
>
> Just a quick question about relay contactors. Let's say that I have
> a 4-pole, 30 FLA inductive per pole rated contactor. Does this mean
> that I will have up 120 amp inductive rating of the contact points if I
> use all 4 of the poles in a parallel or series parallel fashion in a single
> circuit to switch my pole pig transformer under a load?
>
> Thanks,
> David Rieben
>
>