120VAC & 12VDC in a short length of conduit?

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120VAC & 12VDC in a short length of conduit?

Postby Bob Hammond » Thu Jun 01, 2017 1:33 pm

Hi, after reading threads here and elsewhere about running AC & DC in the conduit, I decided that the principal concerns were 1) adequate insulation, and 2) parasitic induction of AC interference in the DC wires. I decided to do it since it's a single short run of ~14 ft. All wires are 14ga THNN weith 600V insulation, using Blk/Wh/Gr for the 120VAC, and R/Blu for the 12VDC, with all running through a 1/2" PVC conduit.

I expect that I will be using either DC or AC but not both simultaneously, and my theory is that if no AC current is flowing then there should be no induced interference in the DC wiring. Is that logical? Also, suppose I were to place ferrite chokes on either end of the DC wires; would that suppress any induced AC interference? I was thinking about getting the ferrites from defunct computer monitors.
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Re: 120VAC & 12VDC in a short length of conduit?

Postby Philip » Thu Jun 01, 2017 1:39 pm

I would choose a better wire than THNN. That covering on THNN likes to crack and break if it sees much vibration. Seen it many times in different types industry's I have worked in.

Switch to a automotive type wire. The insulation is designed for vibration.
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Re: 120VAC & 12VDC in a short length of conduit?

Postby Bob Hammond » Thu Jun 01, 2017 2:13 pm

Philip wrote:I would choose a better wire than THNN. That covering on THNN likes to crack and break if it sees much vibration. Seen it many times in different types industry's I have worked in.

Switch to a automotive type wire. The insulation is designed for vibration.


So noted. I did look at automotive type wire, but I did not see insulation specifications. Perhaps I could use an outdoor extension for the AC, and automotive wire for the DC.
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Re: 120VAC & 12VDC in a short length of conduit?

Postby H.A. » Fri Jun 02, 2017 2:46 am

Fi0v.
Last edited by H.A. on Thu Jul 06, 2017 4:09 am, edited 1 time in total.
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Re: 120VAC & 12VDC in a short length of conduit?

Postby Padilen » Fri Jun 02, 2017 5:50 am

I used an extension cord AC and ran it with my DC wiring in PVC. What is induced interference and what is an indicator that you have a problem?


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Re: 120VAC & 12VDC in a short length of conduit?

Postby Bob Hammond » Fri Jun 02, 2017 6:53 am

Padilen wrote:I used an extension cord AC and ran it with my DC wiring in PVC. What is induced interference and what is an indicator that you have a problem?


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I don't have a problem yet; I'm trying to avoid a problem so as to not do the wiring job twice.

Re interference - When wires run parallel to each other in close proximity, AC current can induce voltages in the nearby conductors. This is not always noticeable, e.g. in lights, but in audio/visual appliances (radio, TV, computers) it may become much more apparent. Certain other consumer electronics may also be adversely affected, e.g. a CPAP machine. And in scientific instruments it can be very annoying indeed because it injects artifactual noise into a signal of interest.
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Re: 120VAC & 12VDC in a short length of conduit?

Postby Padilen » Fri Jun 02, 2017 6:59 am

Thank you.
My TV is not effected by my DC fans. I know in some of the TT I've torn down the wires ran together so old stuff must have been less sensitive.


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Re: 120VAC & 12VDC in a short length of conduit?

Postby Bob Hammond » Fri Jun 02, 2017 8:36 am

You've got it the wrong way around. AC affects DC. DC does not affect AC. And your TV might have filters to screen out line noise.
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Re: 120VAC & 12VDC in a short length of conduit?

Postby Bob Hammond » Fri Jun 02, 2017 9:02 am

As pointed out by H.A. above, I mistyped THHN as THNN. I had specified THHN at the store.

Upon close examination of the printing on the wire that I bought, I see that it is rated as MTW, THHN, and THWN, which covers installation in wet or dry locations, is oil and gasoline resistant, and safe at temperatures less than 90C. I think that covers everything but the vibration issue, but I'm not convinced that vibration would be a significant issue for the useful life of this trailer.
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Re: 120VAC & 12VDC in a short length of conduit?

Postby Bob Hammond » Fri Jun 02, 2017 3:32 pm

Here's a 'price-is-right' source for 12VDC power & general audio/electronic parts:

https://www.parts-express.com/cat/12v-p ... ssories/56
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Re: 120VAC & 12VDC in a short length of conduit?

Postby daklein » Fri Jun 02, 2017 4:53 pm

Bob Hammond wrote:Hi, after reading threads here and elsewhere about running AC & DC in the conduit, I decided that the principal concerns were 1) adequate insulation, and 2) parasitic induction of AC interference in the DC wires. I decided to do it since it's a single short run of ~14 ft. All wires are 14ga THNN weith 600V insulation, using Blk/Wh/Gr for the 120VAC, and R/Blu for the 12VDC, with all running through a 1/2" PVC conduit.

I expect that I will be using either DC or AC but not both simultaneously, and my theory is that if no AC current is flowing then there should be no induced interference in the DC wiring. Is that logical? Also, suppose I were to place ferrite chokes on either end of the DC wires; would that suppress any induced AC interference? I was thinking about getting the ferrites from defunct computer monitors.

Would it be nice to run two parallel 1/2 inch pvc conduits, pvc is pretty inexpensive. Being in the same conduit means that whatever is in each end, a junction box or breakers or fuses, is also together. Low cost solution; 14 more feet of pvc, and different boxes on each end maybe.

I would say another principle concern is user error: is this here black wire hot 120v ac, or 12v ground? Is this white wire 120v neutral with load current going back, or +12v? This is a potential high severity error.

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Re: 120VAC & 12VDC in a short length of conduit?

Postby Bob Hammond » Fri Jun 02, 2017 5:14 pm

As stated briefly in post #1, and here more explicitly, the color coding scheme is:

120VAC: Black (hot) / White (Neutral) / Green (Ground)

12VDC: Red(Positive) / Blue (Negative).

At this point, routing a second conduit would be possible but not much fun. But I'll consider it.
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Re: 120VAC & 12VDC in a short length of conduit?

Postby friz » Fri Jun 02, 2017 10:56 pm

I would worry about the ac inducing a 60hz hum in any dc audio devices.

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Re: 120VAC & 12VDC in a short length of conduit?

Postby jondbar628 » Fri Jun 02, 2017 11:30 pm

Bob, Since you don't anticipate operating the DC & AC equipment at the same time there doesn't appear to be an issue here. But if you have room in the conduit, you could run your DC in shielded cable in the same conduit with AC, and ground the shielding to AC earth ground. 16 ga. shielded is widely available at Lowes & HD, and fairly cheap. Two 16 ga runs terminated together carry about the same current as one 12 ga......jd
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Re: 120VAC & 12VDC in a short length of conduit?

Postby bdosborn » Sat Jun 03, 2017 3:56 pm

Mixing 120V and 12V in the same raceway is a big no-no in the real world for safety reasons. It's the guy who bought your trailer and is adding a 12V outlet; he pulls the negative lead off the battery, thinks he's safe but forgets the trailer is still plugged into AC and then cuts into the 120V wire that's run in that 12v conduit. It probably wouldn't ever happen but that is the reasoning behind keeping 120V and 12V wiring completely separate.

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