Split circuits in a 3 phase service?

[Ellement]

Just wanted to know if a split circuit(14/3 suppling 2 branch circuits with a common neutral)would respond the same way in a 208v 3 phase situation as it would for a 240v single phase service. Would there be any considerations?
I'm seeing this wiring method in a commercial setting to feed 2 runs of fluorescent light fixtures.

[neil seidner]

In most cases there would be no difference. The only difference is the phase angle between the two hot conductors. Since you are not connecting a load between two hot conductors, but rather between a hot conductor and a neutral, the phase angle is irrelevant.

Some loads such as a range or a clothes dryer, have some of the load connected between a hot conductor and a neutral, and the heating element connected between two hot conductors. These make a difference. If you install a 120/240V range on a 120/208V circuit, it will only produce 75% of its rated heat, and only draw 75% of the rated energy. Installing a 120/208V range on a 120/240V circuit will cause excessive heat related problems.

Another consideration would be harmonics. Harmonics are usually not a problem on branch circuits, but are a consideration on feeders and services. 120/240V circiuts are not prone to problems with harmonics, 120/208V circiuts can have problems with harmonics.

[twh]

4-004(4) treats the 120/208 three-wire circuit differently, too.

ccrash: I'm a little surprised that harmonics are not a problem on a single phase system. I never thought about it, but when you mention it, I think my assumption would have been that the equipment that causes the problem is usually not used on single phase systems. Can you tell me more?

[neil seidner]

TWH: I'll try.

In a single phase system, if there are harmonics present, the harmonics in line 1 will be 180 degrees out of phase with the same harmonic in line 2. When these harmonic currents are added vectorally, they cancel each other out. Thus the harmonic current on the neutral is naturally kept to a minimum.

In a 120/208 3 wire or 4 wire system the harmonic currents on line 1 are not 180 degrees out of phase with the harmonics on line 2. The third harmonic from line 1 is actually in phase with the third harmonic in line 2, and line 3. Thus currents from the third harmonic actually add on the neutral, instead on subtract. The same thing happens with the 6th, 9th, 12th etc, harmonics. These harmonic frequencies are called triplens.

I know Ed has some good diagrams that show how the sine waves from these harmonics line up to be in phase or out of phase, maybe he will post some of them.

[electric-ed]

As ccrash has explained, the triplen harmonics in the three phases, A, B, and C, are all in phase with each other.
They will add rather than cancel on the neutral conductor of a 3-phase 4-wire system. This can overload the neutral if it is not sized to handle this type of load.

Ed

[neil seidner]

Thanks Ed.

[twh]

Okay, I never thought about harmonics on a neutral, either. Drives or inverters, that I have seen, don't use a neutral and ... duh.

Nice explanation and diagram. I hope the trade schools are teaching about harmonics, now. Mid winter, with cabinet doors slamming against you in the wind, and trying to understand the readings on a frozen digital meter is a poor learning environment.