D20/Bench Focusing question.


Active Member
We have D20 dimmers on our Sensor SR48 rack. This 20 is supposed to mean 20 amps, but I am not sure if this is for the whole module, or the 2 channels on the module.
Which is it?
And is it just given that it is at 120V, or is voltage specified somewhere? (the rack gives a range of voltages that it will take...I think...from last time I looked.)

Also, how are all the modules and the rack hookep up? I'm pretty sure the modules just slide in.
Are there eleventy billion wires in the back to connect to the electrics?
(do they have connectors on them?)
Also, since our circuits repeat on the batten, are there just 2 plugs going to the same dimmer module, or are the wires parallel circuited in the drop box...?

Also, there has been a lot of advice given on bench focusing, but (you can call me stupid) I am not exactly sure what that is, and have not been able to find the answer.
Is this just focusing the bulb to match the focal point of the reflector? Should this be done with every new bulb?
What is the difference between the 2 knobs on the back. (On a S4. The inside knob, and the outside knob.)
I have read a lot about how to set up the fixture on a boom, and point it at the cyc, but I don't know what to do to the fixture. :mrgreen:

You guys rock. :grin:


CB Mods
In response to your first question about dimmers, the ETC D20 Dimmer module is a dual dimmer module with two 20 amp dimmers on board. In the back of your rack the dimmer connects to a power in from the feeders to the rack, power out to the raceways/drop boxes/etc. in your theatre, and a data connection to the CEM (control module).

The input voltage of your rack was set when it was installed and is usually controlled by a transformer that feeds the dimmers. Generally when the racks are installed they set the input voltage at a point where the output at the circuits is 120v or 115v. This usually means that the input voltage is slightly higher to account for voltage drop over long cable runs. Most dimmer racks are fed with three phase power so when you look at the voltage readout on your CEM you should see something like: "A: 115 B: 117 C: 115" This is just telling you the current voltage the rack is receiving on each phase.

As for how the rack is wired to your system, yes, there are lots of wires that come out of the rack and get fed to each raceway, drop box, and wall pocket. Out of the rack should only be the hot leads. All the neutrals and grounds generally come from another location as they don't go through the dimmers. For your repeating circuits, they have to be wired in parallel because if they were in series you would need to have something plugged into every outlet for a circuit to work.

Now on to your bench focus question. The idea behind bench focus is exactly what you said, to center the lamp in the reflector to produce and even field (unless your designer wants hot or dark spots). In theory you should bench focus for every lamp because the action of changing the lamp can throw off the bench. On the S4 you have two knobs, the smaller inside knob controls the vertical position of the lamp (how deep into the reflector the lamp is). The bigger knob is the locking knob for the horizontal alignment of the lamp (left/right top/bottom), so you loosen that, then move the lamp and then tighten it down.

If you read the S4 users manual they tell you how to bench focus the light, but the general idea, as I mentioned before, is to create a bright, even field with no hot or dark spots. This is easiest to do if you can set up your lights at a 90˚ angle to a white or neutral grey surface. Focus the beam on the surface, then adjust the lamp.


Well-Known Member
As for how the rack is wired to your system, yes, there are lots of wires that come out of the rack and get fed to each raceway, drop box, and wall pocket. Out of the rack should only be the hot leads. All the neutrals and grounds generally come from another location as they don't go through the dimmers.

I think you are mistaken, ETC has a neutral buss in the rack for a reason. If you have a GFCI rack the neutrals must match up to the same circuit as the hots. You might be able to get away with grounding else where but I cant think of a reason why one would want to.


CB Mods
As I really haven't taken apart my new Sensor+ racks, it is quite possible that I am mistaken. I try not to pick through the wires coming out of my racks unless I need to since with 380 dimmers there are a lot of wires. I would imagine though with one rack, like the original question asks about, if he has 4 wire 3 phase feeding the rack, all of the neutrals come from that connection, thus neutral for all three phases is routed to one neutral connection. So in my case with 4 racks each with it's own 3 phase feed the neutral for each rack goes to the corresponding power tap. So it wouldn't surprise me if the rack has a neutral bus in the back to divide the one neutral feed to each of the 96 circuits in the rack. I suppose what I should have made more clear is that the neutral does not actually feed through every dimmer.


Well-Known Member
As I really haven't taken apart my new Sensor+ racks, it is quite possible that I am mistaken. QUOTE]

Every Sensor rack gets main neutral feeders - it's in the back of the rack, behind the main phase buss bars and load lugs. Thus all circuit neutrals come into the rack, as per code. As per ETC Spec.. (and usually local code) ea. and every branch circuit requires a neutral.

As to the original question, you may note, if you see how the dimmers are labeled in the rack, that it reads - top to bottom (this is on a typical permanent install 48 or 24 module rack - which is 96 or 48 dimmers), as:

Dimmers 1-2
Dimmers 7-8
Dimmers 13-14


Further down, in the middle is:


Then the lower third:


And so on.

The reason is the rack has 3 main phases feeding power, w/ ea. phase providing 1/3 the power to the rack. To save horizontal room, the main buss bars have A phase on the top third, B phase in the middle and C phase in the lower third.



Active Member
So, on the parallel circuited repeated drop boxes question... Are the repeated dimmers circuited together at the drop box, or at the back of the rack? (So, would it be possible to get another rack, and re-wire it back there without messing with the electrics?)

Also, in balanced racks like SteveB said, is this just so that the rack remains balanced when certain users do not use all 96 channels?

And, about how much voltage is lost over the lines to the drop boxes with about 40 feet of cable? (Our circuits trip with 2250 loads, when the rack supplies about 2300w. Does the voltage loss drop the wattage more than 50w? And how much?)


You can not think of the dimmer connections as plugs. In a 120V situation, there are three wires: hot neutral, and ground (220V is two hot, 1 ground). AC power is alternating current. In the US, AC power alternates at 60 hertz(cycles) per second, or faster than the human eye can detect a pulsing in the light. Sensor racks use triacs which control the sine wave to dim the voltage of the light. Only the hot leg in connected to the dimmer. The neutral goes straight to the neutral bus bar.

As for your question regrading double numbered circuits, that is difficult to answer. Electricity follows the path of least resistance, so the hot legs are either joined at the dimmer or in the connectors strips attached to battens. I guaranteed that they are not wired in series. Wiring in series is like old Christmas lights, when one lamp goes out, the entire string dies.

If you want add additional racks, that would depend on available power and the wiring of the current circuits. Contact the ETC dealer that set up the system and they will be able to tell you.

The reason dimmer racks are not sequential is pretty much what you answered. Sensor racks have 2 modes: standard and 3 phased balanced. Install racks are set to balanced because more offend than not, you will not be using every circuit available.

As far as voltage drop is concerned, google is your friend: http://www.google.com/search?q=calculate+voltage+drop

Matt McCormick
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