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Author Topic: What does it take to run AC and signal in the same box?  (Read 4013 times)

Tim Weaver

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What does it take to run AC and signal in the same box?
« on: March 28, 2018, 04:52:11 pm »

From an inspector's standpoint what would make it Kosher to have a box-end snake that had power and xlr in the same chassis?

Specifically I'm thinking of a powered speaker "drop box" that would house 2 edison, 2 powercon, and 4 XLR, being 2 channels of in/out.
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John Ferreira

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What does it take to run AC and signal in the same box?
« Reply #1 on: March 28, 2018, 05:39:19 pm »

From an inspector's standpoint what would make it Kosher to have a box-end snake that had power and xlr in the same chassis?

Specifically I'm thinking of a powered speaker "drop box" that would house 2 edison, 2 powercon, and 4 XLR, being 2 channels of in/out.

If you are running mic levels in the XLR, parallel to AC you may get the 60Hz blues.
« Last Edit: March 29, 2018, 04:02:41 pm by John Ferreira »
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Tim McCulloch

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Re: What does it take to run AC and signal in the same box?
« Reply #2 on: March 28, 2018, 05:44:53 pm »

From an inspector's standpoint what would make it Kosher to have a box-end snake that had power and xlr in the same chassis?

Specifically I'm thinking of a powered speaker "drop box" that would house 2 edison, 2 powercon, and 4 XLR, being 2 channels of in/out.

Code doesn't allow mixing of "services" in the same box, but IIRC the box can have a divider so long as each service remains on its respective side.  The idea is the divider makes each side its own "box" for the purpose of splicing or terminating.  The sides cannot share a common cable entry or exit, I think.

Not sure of the current Code as my book is from a previous decade... but Chapter 3 is Wiring Methods & Materials, Articles 312 and 314 cover meter bases to junction boxes.  I think what you're looking for is in there.
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Tim McCulloch

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Re: What does it take to run AC and signal in the same box?
« Reply #3 on: March 28, 2018, 05:50:32 pm »

If you are running mike levels in the XLR, parallel to AC you may get the 60Hz blues.

I don't think I'd want to run the snake on top of 1200 Amps of lighting dimmer feeder for 300 feet but we routinely run our 250 ft snakes with attached 10/4 AC service for FOH and in at least one venue the mic and drive snakes are on top of both audio and lighting feeders for about 40 feet of the run.  No inductive hum.

This is another of Olde Things that has gotten better over time as circuit designs have improved (like Pin 1).  I remember when it Was Not So.
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Jonathan Johnson

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Re: What does it take to run AC and signal in the same box?
« Reply #4 on: March 28, 2018, 06:41:00 pm »

Code doesn't allow mixing of "services" in the same box, but IIRC the box can have a divider so long as each service remains on its respective side.  The idea is the divider makes each side its own "box" for the purpose of splicing or terminating.  The sides cannot share a common cable entry or exit, I think

The same goes for conduit (what the code calls "raceways") and outer jackets (as discussed in another thread recently). "Low voltage" and "high voltage" cables can't be in the same cable, unless the low voltage conductors are insulated to the same voltage rating as the high voltage conductors. When in a common jacket, the entire cordset is derated to the lowest voltage rating of the insulation on any conductor.
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Frank DeWitt

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Re: What does it take to run AC and signal in the same box?
« Reply #5 on: March 29, 2018, 02:03:13 pm »

A sothers have said, there are boxes made for this with partitions in them.  My favorite is Ace Backstage.  There was a great white paper on what happens when you run AC next to audio lines.  The bottom line is that the ideal is Hot and Neutral twisted using a drill motor, then puled through conduit with a ground (The ground not twisted.  Outlets are isolated ground.  (The safety ground connection on the outlet is not connected to the box or the conduit.  (Both are grounded but not at each box.)  The worst thing to run is Romex.  Hot and Neutral are carefully kept parallel.  Not good.
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Tim Weaver

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Re: What does it take to run AC and signal in the same box?
« Reply #6 on: March 29, 2018, 03:36:03 pm »

Iím really not worried about 60 cycle hum. In my long experience it hasnít been an issue no matter how careless I am running snakes. Iíve always taped power to my main snake for over 20 years and have never had a problem with it.

All you hens go back into the kitchen and cast those bones around another myth.
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Jonathan Johnson

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Re: What does it take to run AC and signal in the same box?
« Reply #7 on: March 29, 2018, 03:46:24 pm »

Iím really not worried about 60 cycle hum. In my long experience it hasnít been an issue no matter how careless I am running snakes. Iíve always taped power to my main snake for over 20 years and have never had a problem with it.

All you hens go back into the kitchen and cast those bones around another myth.

There's a saying that kind of applies: the dose makes the poison. In other words, running power next to signal lines can be a problem when very low-level signal lines (i.e., microphone) are run adjacent to high-current power lines (hundreds of amps) over very long distances (hundreds of feet) AND the signal is then amplified. Running a few amps of power next to shielded signal line for a few tens of feet will result in unnoticeable effects.

It's just like you can be burned by sun exposure if your skin is exposed for several hours. But incidental exposure doesn't do any harm at all (and may actually provide a benefit.)

Electrical code provisions for isolating low and high voltage lines has little to do with signal contamination and everything with ensuring that insulation failure does not result in the low voltage line being energized to a high voltage.
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John Ferreira

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Re: What does it take to run AC and signal in the same box?
« Reply #8 on: March 29, 2018, 04:06:32 pm »

No myth here.
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John Ferreira

Dan Mortensen

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Re: What does it take to run AC and signal in the same box?
« Reply #9 on: March 29, 2018, 04:10:21 pm »

When in a common jacket, the entire cordset is derated to the lowest voltage rating of the insulation on any conductor.

Which means there can be no voltage inside the common jacket higher than the derated voltage?

Dang, that's what I've been trying to get someone to say in that other thread.
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