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Author Topic: What does it take to run AC and signal in the same box?  (Read 3286 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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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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Art Welter

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Re: What does it take to run AC and signal in the same box?
« Reply #10 on: March 29, 2018, 05:50:32 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.
I'm no code expert, but I  have rarely seen any audio device with interior dividers, and they are still UL listed, presumably code compliant.

That said, I've also never had an electrical inspector look close enough at audio gear to determine if it has a UL listing...
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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 #11 on: March 29, 2018, 06:15:46 pm »

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.

Also posted this in that other thread:

Here's what seems to be the relevant section of the National Electrical Code, from Chapter 3, Wiring Methods and Materials:

Quote
300.3 (C) (1) 1000 Volts, Nominal, or Less. Conductors of ac and dc circuits, rated 1000 volts, nominal, or less, shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway.

And here's a related article based on a different section of the code.
« Last Edit: March 29, 2018, 06:22:24 pm by Jonathan Johnson »
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brian maddox

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Re: What does it take to run AC and signal in the same box?
« Reply #12 on: March 29, 2018, 06:16:33 pm »

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.
...

This^^

I have this conversation often with less experienced A/V types on show sites whenever there is any kind of buzz or other audio issue.  "Well the Audio is like super close to the Power backstage.  That's what causing it..."

Uh, no....

I've chased several [tens of... ] thousand [s of] buzzes and it's been induced buzz from a power cable maybe twice.  maybe...
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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 #13 on: March 29, 2018, 06:44:18 pm »

No myth here.

Near-mythical.

Statistically very low chance... until it happens to you, then it's 100%.

It takes a fair bit of induction but it *can* happen.  I'm with Brian Maddox, I'm not sure I can count such incidents on more than 2 fingers in over 35 years, though.

More often where there are hum/buzz issues I find ground/neutral swaps or inappropriate ground/neutral bonds.
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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 #14 on: March 29, 2018, 10:40:37 pm »

I just avoid Henrys all together.
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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 #16 on: March 29, 2018, 11:01:22 pm »

I just avoid Henrys all together.

Yeah, but then a whole bunch of milliHenrys gang up on you!  ;)
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Kevin Graf

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Re: What does it take to run AC and signal in the same box?
« Reply #17 on: March 30, 2018, 10:16:35 am »

Interestingly the Bill Whitlock and the Dale Shirk papers have somewhat different view-points on Romex. I suspect that Bill Whitlock tested real SouthWire brand Romexģ and Dale Shirk tested generic Non-Metallic (NM) cables.
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Re: What does it take to run AC and signal in the same box?
« Reply #18 on: March 31, 2018, 12:28:13 pm »

I've chased several [tens of... ] thousand [s of] buzzes and it's been induced buzz from a power cable maybe twice.  maybe...

I've experienced it many times, when people not watching have excess mic cable coiled on or near soca running to self-powered speakers or laying on feeder. It's low-level hum induced into the mic lines, and only in mic-level signals.
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Jean-Pierre Coetzee

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Re: What does it take to run AC and signal in the same box?
« Reply #19 on: April 06, 2018, 05:06:45 am »

I've stuck sensitive clamp on meters around both our temporary and permanent power lines here and have never measured any current from them, doubt that it will cause an issue but then again we are at 240v and thus significantly less current...
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Steve M Smith

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Re: What does it take to run AC and signal in the same box?
« Reply #20 on: April 06, 2018, 07:27:10 am »

No inductive hum.

Or some you are not aware of as common mode rejection hides it. Either way, not a problem!


Steve.
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Bob Leonard

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Re: What does it take to run AC and signal in the same box?
« Reply #21 on: April 06, 2018, 01:45:09 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.

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.


It's not hum that will shut down the show. It's the one in a million chance that a real inspector see's the box and tells you to remove it. Or maybe the slight chance of fire or electrocution. This is the very reason you'll be very hard pressed to find an outlet cover that supports both A/C and low voltage audio or video signals. Not allowed in this state under any circumstance.
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