Balanced Phases?

[FMEng]

There are electrical systems where phase imbalance becomes an important issue because the neutral conductor is sized to handle a relatively small imbalance current. If changes are made to the system without re-examining the whole system, problems can occur. This has been known to happen in older, large buildings when many lighting fixtures are upgraded without proper engineering being done.

But, in a properly engineered and installed stage lighting system, there should be no worries about phase imbalances, because the electrical engineer should have sized all of the system components for the worst case scenario.

 

[STEVETERRY]

There are electrical systems where phase imbalance becomes an important issue because the neutral conductor is sized to handle a relatively small imbalance current. If changes are made to the system without re-examining the whole system, problems can occur. This has been known to happen in older, large buildings when many lighting fixtures are upgraded without proper engineering being done.

But, in a properly engineered and installed stage lighting system, there should be no worries about phase imbalances, because the electrical engineer should have sized all of the system components for the worst case scenario.

Yes, especially because the Code requires the neutral to be considered a current-carrying conductor for conduit fill and ampacity adjustments when feeding a non-linear load such as a phase-control dimming system. This has the effect of building in "phase imbalance protection".


ST

 

[journeymanjohn]

Ok, to the original question...75 amps on a 2/O neutral?? No problem, as a 2/O (depending on the type of wire/insulation,ambient temp., conduit fill, and application) is rated for about 200amps. Not gonna overload that with 75 amp inbalance.

New National Code (US) no longer allows single phase loads on a 3phase breaker. Of course it also no longer allows three phase loads to be tied to three individual individual breakers. Same applies for single phase loads with two "hot" conductors. The breaker must be designed for the circuit to be fed. No more putting individual branch circuits on a 2 pole or three pole breaker.


We do design and install branch circuit wiring so that the phases are balanced. Typically, we size the neutral as the same size of the feeders.

There are applications where we will oversize the neutral, typically in installations that are known to produce high harmonics...such as office buildings with many electronic loads like computers and electronic ballast for lighting. Of course, we often oversize the ungrounded "hot" wires in these situations as well.

In the situation where your two breakers, wired in series, with the largest rated breaker tripping first...... Get a new breaker, it is worn out most likely. Has nothing to do with the phase inbalance.

 

[STEVETERRY]

New National Code (US) no longer allows single phase loads on a 3phase breaker. Of course it also no longer allows three phase loads to be tied to three individual individual breakers. Same applies for single phase loads with two "hot" conductors. The breaker must be designed for the circuit to be fed. No more putting individual branch circuits on a 2 pole or three pole breaker.

I assume you are referring to the new material for 2008 in 210.4:

210.4 Multiwire Branch Circuits.

(A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire circuit shall be permitted to be considered as multiple circuits. All conductors of a multiwire branch circuit shall originate from the same panelboard or similar distribution equipment.

FPN: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic currents on the neutral conductor.

(B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.

(C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only line-to-neutral loads.
Exception No. 1: A multiwire branch circuit that supplies only one utilization equipment.
Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.
FPN: See 300.13(B) for continuity of grounded conductor on multiwire circuits.

(D) Grouping. The ungrounded and grounded conductors of each multiwire branch circuit shall be grouped by wire ties or similar means in at least one location within the panelboard or other point of origination.
Exception: The requirement for grouping shall not apply if the circuit enters from a cable or raceway unique to the circuit that makes the grouping obvious.

Actually, 210.4(C) allows single phase loads to be fed from a three pole breaker. What is new for 2008 is that all 3 ungrounded conductors of a multiwire branch circuit must be disconnected by a single device per 210.4(B). This specifically disallows common-neutral wiring in a multiwire configuration on three circuits leaving a dimmer rack that have untied single pole breakers.

This may have an impact on retrofits, but multiwire circuits on phase control dimmers were always a bad idea, and only designed by engineers and specifiers who did not know the multiple reasons not to do it.

ST

 

[journeymanjohn]

Honestly, I'm unsure where the code is, but I'll check my codebook tomorrow. I remember the discussion during a code update class. Thought that part was added 2005, bout not using a three pole for a 4 wire (2 H, 1 N, 1G) single phase circuit. You may be correct in this though.