Cable, Dimmers, and such

[Synchronize]

I need to threefer 6 2.4 kW dimmers (575 watt lamps). Each run from dimmer to lighting position is between 75' - 100'. This cool fancy table says that 12ga can only run 82' under a load of 15A. Unfortunately for me, I only have 12ga to work with. I'd really like to run one line from dimmer to position and threefer at the position. So a 75' cable would be ok, but not a 100', correct?

If I have to run 100' can I threefer at the dimmer and then run three 12ga cables to the position? I'm assuming that the load splits three times at the threefer so the 15A would turn into 5A per cable thus making a 100' 12ga cable ok?

I'd actually really like to run multi and then threefer from the breakout, but I'm not sure how to calculate the load on the cable. Lets say we are looking at 1 line in the multi. The multi runs 75' from dimmer to close to the position. We threefer at the breakout and then run 3 25' 12ga cables from the threefer to the position. Am I correct in assuming that the 75' multi run is ok because there is 15A per line and that's within acceptable boundaries; then we threefer at the end of the multi and the load breaks up into three 12ga lines each carrying 5A so the remaining 25' is within acceptable boundaries?

I hope that makes sense... It's late and I'm not sure I'm even making sense to myself anymore.

 

[DuckJordan]

from what it sounds to me is you have three fixtures you need to power from 100 ft' away that need to be powered from the same dimmer, The problem is the 100' run. even if you only ran 99 feet plus the three fer then the 25 extentions you'd need its still not going to go well, your cable will become hot because the reason why 100 ft runs are the max at 15 amps is because of the load put on the core wires. it doesn't matter if i only have a 2 ft piece of 12 gauge in a line of 10 gauge its still going to get hot just because its not meant for that much Ohms (if i am thinking correctly)

Unless I'm completely wrong, haven't taken my electrical course for a while yet.

 

[derekleffew]

Promoting this, somewhat convoluted, predicament to QotD. Standard rules apply.

1. So a 75' cable would be ok, but not a 100', correct?

2. If I have to run 100' can I threefer at the dimmer and then run three 12ga cables to the position?

3. I'm assuming that the load splits three times at the threefer so the 15A would turn into 5A per cable thus making a 100' 12ga cable ok?

4. Am I correct in assuming that the 75' multi run is ok because there is 15A per line and that's within acceptable boundaries; then we threefer at the end of the multi and the load breaks up into three 12ga lines each carrying 5A so the remaining 25' is within acceptable boundaries?

 

[jeffsw6]

If I have to run 100' can I threefer at the dimmer and then run three 12ga cables to the position? I'm assuming that the load splits three times at the threefer so the 15A would turn into 5A per cable thus making a 100' 12ga cable ok?

Your assumption is correct. The voltage drop will be a bit less with three separate runs of cable supplying your instruments. Note that you will increase the number of current-carrying conductors in your conduit/cable tray/parallel run, so this may have implications for conductor sizing and operating temperature. If you use multi-cable regularly, you are no doubt already familiar with these issues.

Note that your voltage drop calculation with a single 10/3 run and 15A load over a 100 foot span falls to 3%. One run of 10/3 will cost less than other wiring methods. But if you want the instruments to have individual home runs to your dimmer/patch bay for flexibility, the additional cost of copper may be acceptable.

 

[n1ist]

Voltage drop in the cable is proportional to the current flowing in that cable. There are two choices here - you can either split the wiring at the dimmer end (using either a 3fer and three cables, or a 3fer feeding a multi with a breakin) or you can run a single cable and 3fer it at the load end.

3fer at the dimmer, 100' to load: Each cable will have 4.8A load, for 1.8v drop.

75' single circuit cable, 3fer, 25' to load: The 75' cable will have 14.4A for 4.1v drop. The 25' cable will have 4.8A for 0.5v drop. Each load would see 4.6v drop

So splitting at the dimmer end is the way to go. As for whether you can use a multi instead of 3 cables, there is derating to take into effect. I'm not sure on this, but I think it should be fine. You will have 12 current-carrying wires, each with 4.8A flowing and 0.7V drop, so each wire will be dissipating 3.4 watts over the 75' length from the dimmer to breakout. For all 12, that's a half a watt per foot.

/mike

 

[epimetheus]

I'm going to reply here without doing the math, which may not be the wisest thing...

My read of the OP's post is that he's got 6 circuits with (3) 575W lamps each at a lighting position approx. 100' away from his dimmers. This sounds like a prime situation is use a 100' socapex with three-fer's on the breakout. Each circuit has 15A of load, well within the capacity of the dimmers. To be honest, who cares about a little voltage drop in a lighting circuit. I would think that if that little bit of lost intensity is a problem, then the design is lakcing in the first place. Considering load diversity (that is assuming that all lights will not be at full at the same time, for any length of time), 12 AWG conductor is fine, I think. There's a Steve Terry article around here somewhere that discusses the ampacity of multi-circuit cables. I'm sure DL will add the link to my post eventually. [EDIT BY DL: See the wiki entry Diversity, Electrical.]

The end result - voltage drop isn't much of a concern to a resistive load such as a theatrical lamp. The added cable resistance will only reduce the current seen in the cable, not increase it. Now if these circuits were powering movers, this would be a completely different post.

Edit: Seems I was a little eager to reply...oops.

 

[derekleffew]

Well, it seems the rules regarding professionals waiting a week to reply are out the window. So be it.

...This sounds like a prime situation to use a 100' Socapex with three-fer's on the breakout. ...

See NEC Table 400.5(A)(3). How many Current-Carrying Conductors are there in a six-circuit multi-cable? What is the derating factor?
How is the Production Electrician supposed to know if all 18 575W (115V or 120V? Does it matter?) lamps will be on at full at the same time? Does 400.5(C)/310.15(C) apply?

Dear NEC Panel: Table 400.5(A)(3) refers one to Tables 400.5(A) and 400.5(B), which don't seem to exist. Should this read Tables 400.5(A)(1) and 400.5(A)(2)?

...Now if these circuits were powering movers, this would be a completely different post.

Would it really? Why?

 

[n1ist]

There are 12 current-carrying conductors in a 6-circuit soca. Derating is 50%, starting with 20A (from 400.5(A) column A, giving a 10A maximum. Three 575W loads will pull 15A (at 115V) or 14.37A (at 120V), so either is over the rating. I don't see how any of the 310.15 exemptions apply as the different circuits aren't coordinated.
/mike

 

[Chris15]

Now if these circuits were powering movers, this would be a completely different post.

Would it really? Why?

Would there be / what difference would it make if the ballasts in those movers were magnetic or electronic?

 

[STEVETERRY]

Dear NEC Panel: Table 400.5(A)(3) refers one to Tables 400.5(A) and 400.5(B), which don't seem to exist. Should this read Tables 400.5(A)(1) and 400.5(A)(2)?

Congratulations--you just found an error in the 2011 Code. It should indeed read Tables 400.5(A)(1) and 400.5(A)(2).

And on this other subject of 6-circuit multicable ampacity, I think I read something recently that might be applicable. Now where was it? Oh wait, it was here:

Diversity, electrical - ControlBooth

ST