i don't know what kind of extreme situation you are usually working in, but having 380amps on one
leg and 20 on the two other... with a 400a 3ph supply.. this is not only extreme, but crazy...
For transformers, the efficiency at which they step-down the
voltage and provide
current to devices drops significantly when phases are out of balance. A single-phase example would be in a residential application. There are typically three wires coming in the main panel from the service;
Phase A,
Phase B, and a
Neutral.
Phase A and B, relative to
Neutral, are both 120v. Relative to each other, they are 240v. (That is strictly in this example and is not necessarily the case in other countries or other
wire arrangements out of transformers)
If
Phase A and
Phase B are loaded to 60A, the
Neutral wire out of that panel sees zero
current. In this sense, they are balanced. However, if
Phase A is loaded at 20A and
Phase B is loaded at 50A, the
Neutral wire will
pick up the imbalance between the two. Now, the
Neutral is a current-carrying
conductor with 30A of
current running through it.
(FYI, I am not speaking in terms of generators because my expertise is not with them.)
In entertainment industry applications, the
neutral wire can often pass a lot more
current through it than it should. Let's
face it, when people are patching their
light plot, they may be somewhat concerned about how many they have on each
phase, but when it comes to show time, they simply aren't concerned about how many lights are on in a given
cue on
Phase A, or
Phase B, or
Phase C. I've worked
in one space where they were limited in their
power needs, and I did have to worry about imbalances, but typically it is not the factor that decides which lights are turned on in a given
cue, or not turned on.
For this matter, it is widely-accepted that in
theatre,
Neutral wires see a lot more
current than they do in motor applications, as motor loads tend to be quite balanced, whereas lighting applications minute-to-minute can create large imbalances in a
system. To solve this,
Neutral wires on phase-controlled dimming systems are considered current-carrying conductors. That means they are appropriately up-sized to accommodate such large imbalances. Also, it is common for transformers to be installed that are able to handle such imbalances better. Often this is a K-rated
transformer. They are designed such that non-linear loads like that in
theatre that create excess
current running through the
Neutral do not bring a
transformer to an early demise.
(see
http://www.centralyacht.com/library/electrics/kratedtransformer.pdf)
Also, the up-sized conductors for the
Neutral allow more
current to go through them without heating up to a destructive temperature. This is also why on 400A/3p company switches, there are 2 Neutrals. They are connected together on the same paths of conductivity on either end of the
conductor run such that they act as one, really large
conductor. That means a large imbalance can go through them without melting the
insulation down and starting on fire, because now the heat and
current are spread across the copper for both
Neutral runs.
I strongly advise you check out that PDF on K-rated transformers; it should help shine some light on the problems with non-linear loads that leave phases out of balance with each other.
Here is another PDF that describes out-of-phase loads as they relate to the Entertainment Industry. Also check this out (
http://www.saunderselectric.com/PDFs/a-primer-on-power-harmonics.pdf)
But I guarantee the 
