ECN Forum Forums Code Related Discussion NEC & other Code issues lighting load on nuetral

There's been many times in the past where I have seen 4 wire homeruns carrying 3 277 volt lighting circuits,in some situations all three circuits contain a large amount of lights in each. Is this a code violation, just bad practice or acceptable?

I don't think its a violation. It can be a bad way to wire fluorescent lighting. The neutral load can become unbalanced due to harmonics.

I have heard that many people consider the nuetral of the circuit you describe as current carrying, due to the non-linear loads of flourescant lights. You might ask your electrical engineer if 310.15(4) applies. The thing to remember, however, is that at 90 degrees a #12 conductor is good for 30 amps, but only for the purposes of derating. Since you are describing a situation that may be 4 current carrying conductors in a raceway, you are permitted to use 30amp rating to derate...80% of which is 24 amps. Since 240.4(D) requires that a #12 conductor be supplied from a breaker not larger than 20 amps, it really is a moot point until you have 10 current carrying conductors in a raceway.

Hope this helps

The modern electronic ballast has solved the problem of harmonics. Buy the expensive kind, or run #8 white wire for the neutral home runs (super-neutral) like they do for cubicles.
But, we have run the 4-wire home run, or even the 8-wire home run for years with no ill effects. (until harmonics)

Earl,
I thought that the electronic ballasts cause even more harmonics than the magnetic ballasts. That being said, I've never seen a problem with a neutral caused by harmonics on a multiwire lighting circuit.
Don

As mentioned, the Common Grounded Conductor on a 3Ø 4W Wye system will carry at least the same level of Current of that found on the Ungrounded Conductor with the highest Load Current - for L-C connected Loads.

Under some circumstances, the Common Noodle will carry little to no Load Current (for L-C loads). These are very specific in nature, and mostly apply to Pure Resistance Loads.

For most applications, figure the Common Noodle to carry at least what the largest Load Current of any ØA, ØB or ØC line is.
Example:

Multiwire (4 Wire) circuit, 20 amp rating, reactive loads (no load is pure resistance only), THD is <1% overall:

ØA Load = 8 Amps,
ØB Load = 10 Amps,
ØC Load = 12 Amps.
Common Grounded Conductor will carry ± 12.3 Amps.

Another example:
If Loads with THD of 33% overall are connected to the same systems + Multiwire Circuitry, and:
ØA Load = 10 Amps,
ØB Load = 10 Amps,
ØC Load = 10 Amps.
Common Grounded Conductor will carry ± 20 Amps.

Harmonics are produced by many types of loads besides Fluorescent Lamp Ballasts. SMPS for Computers, AC Induction Motors, Linear Power Supplies, UPS Equipment, AC Coils for Relays and Contactors, Dimmers, and even TVSS devices are generators of Harmonics.
Magnetic (Reactor-Core type) Ballasts generate Harmonics, as well as Hybrids and straight Electronic Ballasts - for both Fluorescent and HID Lighting.

Multiwire Circuitry is Code Compliant, and if done properly, can be beneficial.
Installation requires knowledge of load types, and for the best installations / designs, those factors should be applied to determine maximum loads + conductor sizing.

Scott35