[Amps] 220VOLT LINE...HAMS CAN BE DANGEROUS

[Jim Smith]

----- Original Message ----- 
From: tongaloa 
To: Ian White, G3SEK 
Cc: amps at contesting.com 
Sent: Thursday, December 11, 2003 7:56 AM
Subject: Re: [Amps] 220VOLT LINE...HAMS CAN BE DANGEROUS


All I know is my wiring passed inspection, with compliments.
240V 3 phase to 440V 3 phase transformer, tie in to distribution, tie in to
the big Blanchard grinder. Did not consult the NEC, just followed same
scheme that the licensed guy did when he wired up one of the other machines.
Except for the transformer because the other machine was 240VAC.
My lights passed too. 64 banks of 4x4' flourescents. For that, I looked 
at the book :-)

OK you electricians, here are some questions. What is VA rating of a 240V
100A 3 phase circuit? 

  Are you sure it's actually 240V and not 208V? Either way, multiply the amperage times the voltage times the square root of three (1.732). This comes to 36,025.6 VA at 208V, 3 phase or 41,568.0 VA at 240V, 3 phase.

By what % should it be derated for an inductive load
with a power factor of 0.8? 

  The circuit does not need to be derated. If you mean the motor draws 100A, it does not draw more than this under normal load, but only 80% of the power supplied is doing work.

What voltage rating wire should be used with
a 440VAC three phase motor. 

  The common dual rated THHN/THWN wire is rated for 600V. This will work fine.

Does the power factor affect the choice?

73,
-bob

  No, it doesn't. However, you should note that conductors feeding motors, and the overcurrent  devices protecting them are sized differently than other conductors. See article 430 of the N.E.C. for details. In short, the conductors must be rated for at least 125% of the full load to allow for the higher starting load. This higher starting load is usually about 6 times the full load (about equal to the locked rotor amps) for a brief moment in an "across the line" start. The overcurrent protection device can be rated for up to 200% of the full load, increasing up to 225% if it still trips.

  Don't forget the overload protection for the motor as well. This is different from the overcurrent device, and is usually provided in a motor starter, or fusible disconnect switch.

  Jim Smith, KQ6UV