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I like ceiling fans & EMT
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Discussion Starter · #1 ·
I learned a lot from the last one, hoping the same will happen here! I'm interested in learning the theory behind why some things are bad ideas.

Presume this scenario:

Large series of rooms where audio recording devices are used, ergo more sensitive to line noise.

12 60 watt wall sconces on one 1k dimmer. Powers that be want to add two more sconces to the same dimmer in an area no where near any of the existing sconces. There is, however, EMT running from one of the existing sconces to said area, currently serving several outlets on several different circuits.

What is the reasoning against pulling a hot from the existing sconce to the new sconces, but using one of the neutrals already in the new sconce area, serving the outlets?
 

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Really the risk is just overloading a neutral conductor.

Lets just say the normal circuit gets loaded to 75%.. but then you've stolen that neutral to be the return for another circuit.. and you're loading that circuit to 75%.. that neutral could have 150% of it's rated power going through it.

That equates to heat, the insulation could over heat and break down and you could lose the neutral. Losing a neutral can be a really bad thing, destroying equipment.

If someone's going to pull the hot (switch leg) through the EMT, just pull it's neutral along for the ride.
 

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I like ceiling fans & EMT
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Discussion Starter · #3 ·
Presume one of the neutrals is on an opposing phase. Overloading is no longer an issue, yes?

I was also wondering if there would be issues with weird line noise since there is a dimmer involved.
 

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You can state at Article 300. Only a few types of wire types allow this and small wiring method don't allow it!

Article 300.3 (B) All conductor of the same circuit, and where used the ground... shall be contained in the same raceway.

If you pulled a hot out your creating electrical hystira. The hot and the neutral
equal each other out a solo wire can develop temperature and higher undesired magnetic properties.

It called a boot-leg (grabing a) neutral, if you did what you asking.
 

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You can state at Article 300. Only a few types of wire types allow this and small wiring method don't allow it!

Article 300.3 (B) All conductor of the same circuit, and where used the ground... shall be contained in the same raceway.

If you pulled a hot out your creating electrical hystira. The hot and the neutral
equal each other out a solo wire can develop temperature and higher undesired magnetic properties.

It called a boot-leg (grabing a) neutral, if you did what you asking.
I think the OP is placing the code aside and throwing out a hypothetical situation only for learning purposes. Dangerous conduit heating doesn’t take place until you get into much higher currents. I really don’t see any safety issues unless one of the neutrals opens up and allows both circuits (on the same phase) to return on one neutral. There is also a shock hazard to someone working in the panel who will most likely not know about this parallel neutral.

As far as the dimmer noise question, that would be a trial and error sort of thing to see if it affects this installation.
 

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Recording studio, then let's assume there are two transformers one for lighting another for critical receptacles utilized for sound equipment.

In this case you would have the current from the "bootlegged" lights having to return to the source transformer over the ground.

This ground current is the number one cause of 60 hz hum in sound systems
 

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I like ceiling fans & EMT
Former commercial, occasional (small) residential
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Discussion Starter · #7 ·
No transformers, and all the circuits originate in the same panel.

I've never actually seen built in AC isolation transformers in a recording studio, is this a common thing? Or are you referring to something else and I'm having a slow moment?
 

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While not what we would call common, many of the better studios do have isolation transformers. See page 21 of this Middle Atlantic paper.

"Integrating Electronic Equipment and Power into Rack Enclosures"
'Optimized Power Distribution and Grounding for Audio, Video and Electronic Systems'

http://www.middleatlantic.com/pdf/PowerPaper.pdf
 

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In general, studio wiring systems try to hold separate the lighting circuits (even more-so the ones with dimmers or LED's/CFL's lights) from the audio AC power circuits all the way back to the main panel-board if possible.
 

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I like ceiling fans & EMT
Former commercial, occasional (small) residential
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Discussion Starter · #10 ·
So in the scenario pictured, the circuits serving audio gear connect to the panel pictured, and the lighting circuits connect to a different panel not served by the transformer? Or vice versa?

I've heard of this, but I've never seen it, and presumed it wasnt common. Especially as studios are getting smaller, more numerous, and smaller in budget.

I'm still not following how, in Brian's post, the current from the "bootleg" light ends up traveling back via the ground rather than the neutral?
 

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NJ-IEC
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Recording studio, then let's assume there are two transformers one for lighting another for critical receptacles utilized for sound equipment.

In this case you would have the current from the "bootlegged" lights having to return to the source transformer over the ground.

This ground current is the number one cause of 60 hz hum in sound systems
Is this what they would call "electrical noise?"
 

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So in the scenario pictured, the circuits serving audio gear connect to the panel pictured, and the lighting circuits connect to a different panel not served by the transformer? Or vice versa?

I've heard of this, but I've never seen it, and presumed it wasnt common. Especially as studios are getting smaller, more numerous, and smaller in budget.

I'm still not following how, in Brian's post, the current from the "bootleg" light ends up traveling back via the ground rather than the neutral?
It would return to the panel/transformer neutral then at the neutral ground bond the current would travel on the ground (copper, water pipe, metal duct, grounding electrode conductor all metallic components in the path) to the neutral ground bond of the separate panel/transformer.
 

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I think the OP is placing the code aside and throwing out a hypothetical situation only for learning purposes. Dangerous conduit heating doesn’t take place until you get into much higher currents. I really don’t see any safety issues unless one of the neutrals opens up and allows both circuits (on the same phase) to return on one neutral. There is also a shock hazard to someone working in the panel who will most likely not know about this parallel neutral.

As far as the dimmer noise question, that would be a trial and error sort of thing to see if it affects this installation.
Hypothetical, what's that? The OP is pondering the cause and effect of a
Code breaking application! There's nothing else to it, it's real!

This is an assembly, where people pay attention like inspectors, when they hear of these things. I've walked into site surveys for EE's where I had to walk right out because various construction was already underway.

Guess what they where both assembly's. Yes the EE's had to report my finding to the proper authorities.
 

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While not what we would call common, many of the better studios do have isolation transformers. See page 21 of this Middle Atlantic paper.

"Integrating Electronic Equipment and Power into Rack Enclosures"
'Optimized Power Distribution and Grounding for Audio, Video and Electronic Systems'

http://www.middleatlantic.com/pdf/PowerPaper.pdf

Couple issues with your drawing

As short as possible-LOGIC BEHIND THIS
Is the EGC Termination Bar in the panel isolated?
I contend double neutral is not necessary (NOW I SEE WHY 120/120)
I assume the they assume metallic conduit and do not require a copper EGC in the conduit?
 

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Hypothetical, what's that? The OP is pondering the cause and effect of a
Code breaking application! There's nothing else to it, it's real!
Are you saying we should not explain why something is wrong and just tell him accept the NEC at face value with no explanation?
 

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fertilizer distrubuter
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While not what we would call common, many of the better studios do have isolation transformers. See page 21 of this Middle Atlantic paper.

"Integrating Electronic Equipment and Power into Rack Enclosures"
'Optimized Power Distribution and Grounding for Audio, Video and Electronic Systems'

http://www.middleatlantic.com/pdf/PowerPaper.pdf
Do you have a part number on that isolation tranny. I want to see its specs. I havent seen a 120/120/120 transformer.
 

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Not only do some use isolation transformers, some studios use a balanced power system that has no neutral.
Just like in an operating room they have a 2 pole breaker for each 120 circuit and you measure about 60 volts to ground on each leg.
 

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fertilizer distrubuter
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Not only do some use isolation transformers, some studios use a balanced power system that has no neutral.
Just like in an operating room they have a 2 pole breaker for each 120 circuit and you measure about 60 volts to ground on each leg.
I dont know where to begin so I am going to quietly have a beer and forget about whatever my issue with this statement is.
 
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