Re: why two neutral camlok inputs
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The Code calls for the
neutral terminal and
neutral feeder of a
portable 3-phase
phase-control dimmer rack to be sized at 130% or 200% of the
phase conductors See below for more discussion of 130% vs. 200%). This is because of the non-linear nature of a phase-control
dimmer rack that produces
neutral overcurrents.
On a 400A feed, there is no readily available
portable conductor above 400A, since table 400.5(B) gives 4/0 in free air a
rating of 405A. In addition, an
E1016 Cam-Lok is only rated at 400A.
So, a second parallel
neutral conductor is required, hence the second
Cam-Lok. Generally, electricians just add another 4/0
neutral, which is more
conductor than is needed, but helps if the two neutrals are not of exactly the same length.
Why isn't a second
neutral required in the
conduit feeding the 400A
company switch? Because those conductors have a lower
ampacity to begin with (and are therefore larger) and in addition are de-rated to 80%
ampacity because there are four current-carrying conductors in the pipe. The
neutral is always considered a
current carrying
conductor when feeding a
non-linear load. The four wires in the
conduit heat
as a system, and the
neutral experiences overcurrent only when the
phase conductors
are at less that full current (when the
system is dimmed and the
current waveform is most non-sinusoidal).
On the other
hand, the
portable single
conductor feeders are pushed to their free-air
rating, are not in a pipe, and are not in contact with each other, so they
heat as individual conductors. The
neutral conductor therefore has no overhead to deal with
neutral overcurrents. So, we add a
neutral.
Note that a three-phase
dimmer rack without internal conversion to single
phase (240/120V 3W+G) requires a double size
neutral terminal. Why? The assumption is that when the
road electrician is faced with only a single
phase 3
wire +G service, he or she is going to connect two phases of the rack to one
phase of the
company switch. This means that the
neutral could be loaded to 2X the
phase current.
BTW,
sine wave dimmers do not require oversize neutrals because they are not non-linear loads.
Clear as mud?
ST