Hi John:
> >For 3% voltage drop at maximum rated load, the
> >source resistance is about 0.2 ohm. So the
> >maximum rms current would be system nominal
> >voltage divided by 0.2 ohm, or 600 amps for 120
> >V and 1150 amps for 230 V.
>
> For 230V, there are two modifications. One is that roughly the same
> *power* is distributed as at 120 V, so the system impedance would be
> doubled. But, in addition, the supply voltage tolerances are greater
> than 3%: in Europe the tolerance is about double, so, overall, the
> system impedance could be four times that 0.2 ohm. But in fact, 90% of
> supplies are below 0.47 ohms (see IEC 60725, under amendment).
I disagree that the same power is distributed at
230 V and 120 V.
If the breakers are 230 V/16 A and 120 V/15 A, then
the power distributed at 230 V is roughly twice that
at 120 V.
My subject was *not* supply voltage tolerance, but
system *design* goal for percent voltage drop at max
rated load (the circuit-breaker rating).
I really believe that EEs in Europe design supply
systems to 3% voltage drop at rated current, 16 A,
just as they do in North America. The reason I so
believe is that the wire sizes for 15 A (NA) or 16 A
(Euro) circuits are nearly the same. The same size
wire at (nearly) the same current would give (nearly)
the same percent voltage drop.
(If the system source impedance is 0.47 ohms, then, at
230 V, the system voltage drop would be slightly more
than 6% at maximum rated load.)
We really weren't talking about voltage, but about
the maximum current into a fault in cord-connected
equipment. The voltage tolerance can be ignored for
this purpose.
> That still gives 490 A, which would be embarrassingly big, but it apples
> at the 'point of common connection', not at the wall socket and still
> less at the equipment.
My comments were for the current available at the
wall socket. In North America, building wiring is
designed for maximum 3% voltage drop at maximum
rated current.
In NA, at the point of common connection between the
building and the electicity supplier, the electricity
supplier maintains a nearly constant voltage. He
does so not only by his system design, but also by
dynamically adjusting the voltage of the source.
Best regards,
Rich
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