-----Original Message-----
From: Michel Jullian
Thanks Terry. The above is quite right but in the case of typical
rectifier+capacitor loads the low power factor is due to shape rather
than phase
of the current (brief capacitor refill current pulses at voltage
peaks). But
this increases RMS current all the same, and RMS current is what heats
the wire:
Pw=Rw*Irms^2. This is a fact. If you care to reread my previous post in
this
light you will realize you have unjustly criticized your electrical
dudes :)
<><><><><><>
Well, sure, if it deviates from a sinusoid. I was trying to use this
as an example. My bad.
I'm sorry for this rant which is really off topic; but, we're facing a
real problem in many of our contracts. In our industry, we often must
budget five years in advance. In the past five years we have seen
tremendous rise in commodity prices. Today Cu is trading at the $3.60
per pound area. This is up from about $1 per pound two years ago.
The NFPA requires branch circuits to be rated for the KVA load which
includes real and reactive power. However, reactive power does not
contribute to ohmic heating. The NFPA was changed because PF were not
always known so they took the safe approach.
Here in the transit industry, we know our PFs for all our loads;
however, I can't convince them that they do not have to increase their
conductor sized to meet this NFPA requirement. Besides, we're not even
subject to the NFPA. Because we're talking hundreds of miles of 750
kcm Cu conductors here, we're talking about millions of wasted $$.
<rant over>
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