In my opinion a "major portion of the load" is when the nonlinear load exceeds 50% of the branch circuit or feeder load. This would be based on the load on the phase conductors. I have to ask the question that I always ask when this subject comes up. Why do we continue to install 3 phase wye systems for these types of loads? There is a problem with harmonics and excessive neutral currents when these types of loads are installed in wye systems. This problem is addressed by the use of oversized neutrals, K rated transformers, harmonic filters, panels with double neutral bus and other expensive "fixes". Why don't we just use single phase transformers with a 120/240 volt secondary to feed these types of loads. The neutral problem does not exist on 120/240 volt single phase systems. Don
#83162 - 01/17/0309:49 AMRe: Super Neutral Type MC Cable
I'm sure others can provide better insight, but heres my take: In a balanced 3-phase, 4-wire, wye system, the neutral currents cancel each other out and the result is zero current flow on the neutral conductor. Increased neutral currents can result from equipment that draws an asymmetrical current wave, such as computer equipment or SCR driven equipment. These currents are additive as opposed to canceling out and can exceed the ampacity of standard sized neutrals. The same problem may arise with certain ballast loads due to 3rd-order harmonics, which also are additive. The solutions can include individual neutrals for each circuit, or a single oversized neutral with adequate ampacity.
#83163 - 01/17/0310:07 AMRe: Super Neutral Type MC Cable
I'm not up on this subject at all, but I see that Redsy mentioned separate neutrals for each circuit.
In the photo, the cable on the right appears to have a striped wire (red and white), while the center cable has a larger neutral. Does derating become an issue when more conductors are added, as opposed to one larger conductor?
#83166 - 01/17/0302:55 PMRe: Super Neutral Type MC Cable
Where a common neutral would only carry the unbalanced load current, and therefore not be considered a current carrying conductor(per 310.15 (B)(4)), individual neutrals would each carry their respective load current. The total number of current carrying conductors will increase and derating factors will apply.
[This message has been edited by Redsy (edited 01-17-2003).]
#83167 - 01/17/0303:30 PMRe: Super Neutral Type MC Cable
I can forgive and overlook the thin pinstripe of red that the double neutral NM Cable has, but the MC cable's red striped neutral (as far as I can see) has a much larger stripe and would cross the line for me on being compliant with 200.6(A) unless there are three red stripes and subsequently three white stripes present.
-Virgil Residential/Commercial Inspector 5 Star Inspections Member IAEI
#83168 - 01/17/0304:32 PMRe: Super Neutral Type MC Cable
This is an interesting one. I have found the key to understanding nonlinear loads centers on how one describes the nonsine wave current. It helps to start by looking at each half-cycle of voltage that the power company provides, that is put across the load that is connected to one hot conductor of a branch circuit. The voltage can be thought of, essentially, as a clean sine wave with a frequency of 60 Hz. 60 Hz is the base frequency. . .the fundamental, or the "first harmonic".
Now, if the load on my branch circuit conductor is an incandescent light bulb, or a resistance heat element, as the voltage rises and falls through the sine wave, the current in the bulb or the element will rise and fall exactly the same way, in the shape of a clean sine wave. The current, at any instant in time, will be exactly that calculated by Ohms Law. The resistance of the load stays constant, and, as the voltage rises, the current rises in direct proportion. I=V÷R.
Set inductive and capacitive load to the side for the moment.
Let's jump straight to loads that receive their voltage, not directly from the branch circuit voltage, but rather from the output side of a silicon controlled power supply (SCPS). The SCPS, during any one half cycle, turns on and off (in its simplest form), and generally turns on and off a bunch of times in a half cycle. When the SCPS is off, no current flows, when it is on, the current that flows is a function of the logic chip(s) of the SCPS. The half cycle wave form of the current going through the branch circuit to the SCPS is a bunch of pulses more like square waves than any thing else.
This half cycle of branch circuit load current is now very complex, compared to the simple sine wave of the resistive load current. With the help of good math, electrical engineers learned to think of this half cycle of pulses of current as the sum of a lot of currents in the shape of pure sine waves that are multiples (harmonics) of the fundamental frequency (the Fourier Series, so named for Joseph Fourier near the turn of the 18th century, a French mathematician and friend of Napoleon).
In fact, the pulses of current that a silicon controlled power supply draws during a half cycle of voltage are made up of an infinite number of current sine waves of every multiple of the fundamental frequencies of the voltage.
When the neutral current of the branch circuit is then combined with the current of other branch circuits in a multiwire branch circuit neutral, some of the currents add to each other.