Multiple conduits

tsoura

Hi Guys,

Running some new wiring for a small home audio recording studio. When I was an audio engineer, we would never run an audio cable parallel to a electrical cable for fear of interference.

I’m running a couple of electrical lines (12/2) flexible conduit and I need to run them parallel as well on the same beam in the basement. It’s 2 different circuits one going to the basement shop and 1 going to the audio studio on the floor above.

They will run next to each other for about 14 ft.

Does anyone think this might cause any interference?

Is it ok to run to electrical lines parallel on the same beam?

One more thing. Is it ok to have 2 different circuits going into the same junctions box?


Thanks for your help,


T

InPhase277

Is the conduit steel flex? If so you are fine. Besides, interference on parallel electrical cables is really more like a myth. Everyone talks about it, but no-one has ever really witnessed it. The reason being, 15 and 20 amp branch circuits can't pass enough current to generate a field powerful enough field to induce a significant voltage in nearby lines.

Yes.

You can have as many circuits going into the same box as the volume of the box allows. One thing to remember, however, is all the grounds should be tied together and then bonded to the box (if it is metal). But keep all your neutrals separate.

tsoura

thanks again In Phase......


The conduit is MC Lite Metal Clad Cable with green insulated.......but it says Aluminum armor.

Does that make a difference? Should I take it back?


thanks again...

InPhase277

Nah. Steel conduit keeps any fields inside the conduit and contains leakage. However, it is a basic electromagnetic principle that the net magnetic field of an electric circuit is zero if the two circuit conductors stay together for the length of the circuit. Since the black and white conductors are all in one cable, there will be no net effect. And your electronic equipment most likely has some form of power conditioning built in, as well as any you may choose to add externally.

You are OK.

tsoura

thanks again for your help....

Billy_Bob

As to wires run inside grounded metal conduit or "shielded" wires, this keeps the "noise" inside the conduit or protects the wiring inside the conduit from outside "noise". This principal is called a "Faraday Cage".

More on that here...
http://en.wikipedia.org/wiki/Faraday_cage

The above is the same thing with "coax" cable. It has a "shield" around the inside wire to keep the "noise" out.

So far as running two different circuits into the same box, this can be a safety hazard for someone working on the wiring. They may turn off one circuit and think that power is off, yet power is still on for the second circuit.

In the past I have used tie bar breakers for this and placed a note in the breaker panel that these breakers were for two 120V circuits which both go to xx box. So if you turn off one circuit, you have to turn off the other at the same time. Safe.

Now I believe I read in the 2008 NEC that this is now required? That if you have 2 circuits to same box, then both breakers tie-bared. And if 3 circuits to same box, then 3 breakers tie-bared?

Billy_Bob

Well I have nothing better to do tonight, so thought I would go "code hunting" in my book...

Well I opened the 2008 NEC and I SWEAR it opened to the exact page I was looking for! What are the chances of that happening?

Anyway look at 210.4 (B) [New entry].

nap

You're going to have to provide a section for that.

The only new thing for 08 I am aware of is in a MWBC all the hots must use a common trip breaker or tied to cause the same action.

In the past, anytime 2 circuits were on a common yoke of a device (split duplex recep) those circuits had to be tied together.

It is common to have multiple circuits in any given box. in residential it is not as common but in commercial, it is done all the time.

Billy_Bob

We both posted at the same time!

nap

That is the section I was referring to.

it isn't a new section though. They altered the verbiage from the 2005 code.


it does not require all circuits that pass through a j-box to be on common trip breakers. It requires all hots of a MWBC to be on common trip breakers regardless where they run.

Billy_Bob

It seems the intention of that rule was as the following tutorial says...

"This rule prevents people from working on energized circuits they thought were disconnected."

(From: http://ecmweb.com/nec/electric_branch_circuits_part/
about 1/4th way down page.)

Where I have used tie-bar breakers especially is where you have say 4 switches in the same box (4 gang switch), and one or more switches are on separate circuits. I can just imagine someone needing to replace a switch, turning off the breaker for one of the switches, and assuming that power was off for all of the switches.

InPhase277

It still only applies to circuits that share a neutral, multiwire branch circuits. What it is trying to prevent is someone from opening a neutral on a circuit that is fed from different breakers. When the neutral is connected, the circuit you are working on appears dead. But once you open the neutral, the voltage that was on the other shared circuits will now appear on harmless-looking white wires. And if it is a 3 phase circuit, then you might end up damaging equipment that's connected to the other two hots, as well as shock yourself.

Billy Bob, your idea is noble, but is really impractical for anything other than a MWBC. What if you had a 6 gang switch box with five circuits in it? I don't know where to find a 5 breaker handle tie.

Billy_Bob

If it is a residential 6 gang box, then I think it would be a good idea to place a warning note in the panel.

nap

as inphase 277 states, your intent is noble but 277 is correct about the shared neutral. All folks performing electical work need to understand electricity to some degree. A non-contact voltage detector should be a minimum requirement for all. It would alert a person to a hot circuit BUT it does not indicate on a loaded neutral. That is where you are going to get hurt.

btw; the code does not seperate resi v. commerical/industrial for this section. it applies to all installations.

do you realize there is also a relatively new code section that requires a disconnect within or immediately outside a flourescent fixture (long tube type basically) and one of the reasons is to specifically not require all the lighting in the are to be turned off when servicing a single fixture? They want you to have light to work. Your install would turn off all lights in the area. Contrary to what another rule attempts to do.

Marvin Gardens

There are line filters to keep noise out of the line. Some say these don't do a thing and other swear by it.

There cheap Chinese wall warts on the line they can throw a lot of interference into the system. I have had the oscilloscope on there and watched the interference change as I plugged in and out. The non UL rated ones seem to be real bad. UL ones rate much better.

If you want pure sine wave then a line filter would work.

My guess is that if you treat it like low voltage wiring and keep it way from other lines and only plug in the higher quality electronics then you would be fine.

By the way, some of the higher end surge suppressors also condition the line.