Control/Dimming Dds 9800 rack Power Question

[Apmccandless]

I work for a small private school. Recently we purchased 3 Dds 9800 dimmers from a local theatre. My question is about power supply conductors. The school electrician is going to be wiring them into a portable rack. After consulting with him we have different understandings of what gauge power conductors are required for this purpose. The building is wired with a 3 phase 120/207 5 wire company switch (400) amps. The plan we decided on is to wire each dimmer to 2 phases of power. So dimmer one would be phase a and phase b, dimmer two would be phase a and phase c, dimmer 3 would be phase b and phase c. Which brings me to my question, my understanding of the NEC is that we are required in this situation to supply the 2 phases of power in each dimmer with wire to accommodate 80 amps per phase (per the manual) and neutral to accommodate the sum of the power of the two phases (160 amps) because it will be run on 3 phase power. The school electrician says that the NEC only requires 130% of the amperage of a single phase because that is the most that can be drawn by the neutral in 3 phase power with SCR dimmers. My question is only in regard to the wiring in the rack (from dimmer to camlok) not to the feeder cable supplying the rack. Normally I trust the school electrician he has been a valuable resource but I know he is not as knowledgeable about the theatre sections of the NEC as some of the posters on this forum. Thank you in advance for your help.

 

[JD]

It is not single phase power, even if only two legs are in use on each pack. It is known as bi-phase. The two legs are operating at a phase angle to each other as compared to 180 degrees apart as in a true single phase. This is why your hot-to-hot voltage is not twice your Hot-to-neutral voltage. The amount of potential over-current is less, and many do ignore it. BUT, oversize the neutral for it's full path. Obviously, once the dimmers get wired together at the a-b, b-c, and c-a point, it is straight three phase and should be treated as such.
EDIT: With regards to the single phase X2 statement- this would only be true if you were wiring both hots to the same phase leg. (80 per leg, 160 on the neutral) If it is TRUE single phase power (120-0-120 with 240 between hots) then no neutral up-gauge would be needed as the most found on the neutral would be 80 amps. All channels on full with equal loads would see near 0 current on the neutral. The 130% mentioned is to cover neutral over-current caused by triplen harmonics in dimmer systems running off 3 phase power.
Bottom line: Follow the 130% guidelines for the full path.

 

[Apmccandless]

So if I understand your post correctly the 200% of a phase rule would only apply in this situation if we were planning to connect the rack to a single phase of power using a cam tee? Essentially connecting the black and blue cams to the same phase (without an over current protection device), because of the potential of having 160 amps drawn on the neutral line.

 

[JD]

If you dimmers are operating off a single phase leg (Hot to Neutral) then the neutral must have the capacity to handle the full pack.

Neutral doubling is often used on live systems running off three phase as it is easier to have all of your feeder cables the same gauge as compared to having one that is larger for just the neutral. Also, the most common cam is rated at 400 amps, so it would be undersized if the hots were truly running near 400 amps as well. 200% in this case is overkill, but far easier to deal with on a logistical basis.

 

[garyvp]

It is not single phase power, even if only two legs are in use on each pack. It is known as bi-phase. The two legs are operating at a phase angle to each other as compared to 180 degrees apart as in a true single phase. This is why your hot-to-hot voltage is not twice your Hot-to-neutral voltage. The amount of potential over-current is less, and many do ignore it. BUT, oversize the neutral for it's full path. Obviously, once the dimmers get wired together at the a-b, b-c, and c-a point, it is straight three phase and should be treated as such.
EDIT: With regards to the single phase X2 statement- this would only be true if you were wiring both hots to the same phase leg. (80 per leg, 160 on the neutral) If it is TRUE single phase power (120-0-120 with 240 between hots) then no neutral up-gauge would be needed as the most found on the neutral would be 80 amps. All channels on full with equal loads would see near 0 current on the neutral. The 130% mentioned is to cover neutral over-current caused by triplen harmonics in dimmer systems running off 3 phase power.
Bottom line: Follow the 130% guidelines for the full path.

Agree with this 100%. We have the same dimmers in a hardwired three phase environment. The six phase feeders are metered as is the oversized neutral. When balanced across all phases, at full power the neutral is near zero, but with any dimming the neutral jumps and can exceed pause currents.