Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Cable Specification
References: <[email protected]>
"Joe"  writes:

> OK Troops, I need to learn cable specing for cabling up dimmer packs, so...
> 
> 1.) In 3 Phase cable does the neutral leg have to be heavier than the lives?
> ( I.e on a static load - lights - does the neutral run continuously with a
> full load of 60Amps max - 3 x 20Amps from the 6 circuits of the dimmer
> pack?)

In ideal 3 phase system the load would be completely balanced between
three phases, so in ideal case there woudl be no load on neutral
wire. When the 3 phase system is loaded with linear loads,
no mattr how the load is unbalanced or balanced, the load of
the neutral wire is always between 0A and maximum one phase current.
So the neutral wire does not need to be in any way thicker than
the phase wires. This is the magic of the three phase system.

Three phase system neutral specifications has one special case:
highly non-linear loads. If the load has lots of third harmonic,
then it is possible that in some cases the neutral wire load
gets higher than any phase wire. In worst case this neutral 
wire load could get to something like 120-150% of phase current.
Light dimmer is a traditional form of very non-linear loads
which cause this kind of harmonics. In some cases the neutral
wire needs to be specified to be thick enough to handle the possible
extra current in theutral. Some methods to solve this is to
use thicker than otherwise needed (by current rating only) cable
or doulbe neutral wirings can be used to solve this problem.

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/