On Thu, 10 Nov 2005 22:40:32 -0800, Esther & Fester Bestertester
wrote:

Thus spake :

You have 2 issues. Foiust, is the motor supposed to be running on 230
or 208? That couild be the extra current.
If the motor is really exceeding the nameplate amps I would
investigate futher.

Nameplate: 230V, 16.5A.

I presume that if an autotransformer boosts the voltage
10%, it must draw 10%(?) more current in order to do so. Is
this correct? (Nothing is free.)
FBt

Correct, except that there will also be an additional bit of current
draw due to losses in the autotransformer, which will probably be over
95% efficient:

For 208V in, 230V out at 16.5A out to motor and 95% autotransformer
efficiency:

Iin = (16.5*(230/208))/.95 = 19.2A

Which is slightly under your measured 20A, which indicates that you
are running slightly over 100% load on your motor if my voltage
guesses are right. Hopefully the nameplate service factor is at least
1.15 continuous (if the motor runs continuously).

BTW congratulations on responding to one of only 2 posters who
understand your situation (the other being Tom Horne of the Tacoma
Park VFD). But consider that anyone responding to your question here
cannot be aware of all pertinent circumstances such as possible
derating requirements for high ambient temperature or more than 3
current carrying conductors in a conduit, which a competent local
electrical designer would take into consideration, as well as
considering the length of the wiring run and resulting voltage drop.

Since you did not seem to understand the very basic fact that the 90 C
wire ampacity rating is provided only to allow for various derating
requirements, and the usable ampacity cannot exceed the rating for the
conductor at the max temp ratings of the connections at either end
(probably 75 C max for your circuit breaker, which will be labeled
with temp rating), I suggest you find a competent local electrical
designer or inspector to examine your installation. The $100 or so
could save a lot more in the long run.