different voltages in the same conduit

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  #1  
Old 02-11-07, 02:56 PM
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different voltages in the same conduit

Is it acceptable to run two different voltages in the same conduit...also when running pvc underground does it need to be completely weather tight...Thanks
 
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Old 02-11-07, 03:23 PM
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It depends on the voltages involved. 120 volts and 240 volts are fine. 120 volts and low voltage (such as television, cat-5, telephone, etc.) are not allowed together (nor would you want them together anyway).

Underground conduit is not watertight by nature of the condensation that takes place.
 
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Old 02-11-07, 04:24 PM
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The voltage is irrelevant to a certain extent. The insulation rating of the wire is what is important.
There is nothing illegal with running a 12v control circuit in the same conduit as a 240v motor feed, provided ALL the conductors in the conduit are at least 300v rated.
 
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Old 02-11-07, 05:44 PM
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It is perfectly ok to run different voltages in the same conduit, however you must take into account that once you reach more than 3 current carrying conductors in a conduit a deration factor will come into play (Table 310.15(B)(2)(a) NEC 2005. Also you must watch your conduit fill. This means that if you have 4-6 current carrying conductors then you must derate them by 80%. That would be 80% of their actual current capacity, not the capacities given in 240.4(D) so if you have 12awg thhn and you are going from a 75deg C rated lug to another 75deg C rated lug then and you have 6 current carrying conductors in the pipe then each 12 awg wire is only good for 20amps. Which in this case is OK per the table.
If you decide to run PVC it must be Electrical Grade PVC and have the proper cement used. You would also be required to run a grounding conductor with the wires sized per Table 250.122 of NEC2005 to protect any items from becomming energized.
When you are talking different voltages in the same conduit, what type? 120 vs 240 vs 460 or 120 vs 24v?
I don't recomment the low voltage (24vac or vdc) run with higher voltages as the higher voltage will induce a voltage into the lower one and can cause problems with sensitive electronic equipment.
 
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Old 02-11-07, 06:23 PM
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Just a point of clarification from the last post. Termination temperatures are NOT considered when derating.
So a THHN conductor uses the 90c column to figure the derating, NOT the 75 column.
Derating is a very complicated issue since there are many factors involved. It is rarely a consideration in DIY work.

I also do not recommend very low voltage mixed with higher voltage in typical applications. Basically any residential application.
My point was it is not flat out illegal as many people think.
 
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Old 02-11-07, 06:48 PM
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speedy petey:My point was it is not flat out illegal as many people think.
==============================

Well, sort of.

There are circuits that are not allowed to be comingled with lighting and power circuits even though they are required to have 600 volt rated insulation.

There are also unmarked (regarding insulation value) that are intentionally not marked so as to avoid confusion and misuse as a power circuit cable.

These, as well are not allowed to be comingled.

There are a variety of rules that would need to be considered before determining if a lower voltage cable is allowed to be comingled with a typical power circuit conductor to determine the legality.

and the rule you referred to (I believe) does not mention 300 volt rating but merely that any conductor must have an insulation at least as great as the highest voltage (to ground) present.
 
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Old 02-11-07, 07:56 PM
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The question as asked was about voltages, not different systems. My point was that many folks simply say "No, it can't be done. Period.", and this is not so.

Also, I referred to no "rule". I just used an example of two different voltages. I use 12v and 240v so I used the 300v insulation rating as an example. I never meant to imply this was any specific code.
 
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Old 02-11-07, 08:12 PM
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Not meaning to argue petey. your info is generally above reproach. Got to ask though, how are you going to have different voltages unless you have different systems? and of course you are correct when you state the blanket statement of "illegal" is incorrect.

Maybe the OP needs to clarify what he means by different voltages and what the different voltages are supplying. I would tend to think he did not mean he had motor control circuits in mind.

==================

petey:There is nothing illegal with running a 12v control circuit in the same conduit as a 240v motor feed, provided ALL the conductors in the conduit are at least 300v rated.
===========

I took what you wrote a bit too literally. The example obviously is correct but if I mistook the intent just how do you think a less than proffesionally trained and seasoned journeyman electrician would take it (just giving you a hard time)
 
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Old 02-12-07, 03:47 AM
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petey:There is nothing illegal with running a 12v control circuit in the same conduit as a 240v motor feed, provided ALL the conductors in the conduit are at least 300v rated.

============================

If this is a class 2 or 3 low voltage power source you have just made it illegal. You could re-classify it as a class 1 source, but then you would need to make sure it was in a chaper 3 wiring method throughout the entire system. You would also need to protect the transformer with a fuse the same way you would a class 1 transformer, and re-lable the transformer.

If to keep systems seperate or re-classifiy one is a daily topic in the world of hvac controls.
 
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Old 02-12-07, 04:14 AM
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jwhite: as I perused those chapters, I believe even class 1 is not allowed carte` blanche to be intermingled. Don't remember the qualifications right now but seems there were limitations.


we simply need the OP to explain his needs so we can assist him to the fullest.

Until he does that, we are getting way off the question and into our general discussions (not that I don't enjoy them but they don't answer the OP's question)
 
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Old 02-12-07, 04:40 AM
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OK, you guys are right. I think I made it a bit too general in saying it is not expressly illegal. I should have said it "can" be perfectly legal under certain circumstances.
I also agree that we are getting off on a very commercial/industrial tangent. The statement that mixing voltages is illegal is still not completely true, but in most residential settings, using typical residential wiring methods, it is true.
 
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Old 02-13-07, 08:24 PM
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Speedy,
I was using 75degC as an example because if you look at most lugs in your panels or in a distribution panel they are going to be dual rated. 60/75deg C. You will not find many 90deg C rated lugs unless you are looking at service entrances or in an industrial setting. If there is no rating marked on a lug or on any paperwork, most inspectors will assume that it is rated for the minimum of 60deg C and could ding you if they want.
Just using an example.
 
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Old 02-13-07, 08:43 PM
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I understand that for the rating of the circuit. We use the lowest temp of any item in the circuit.

I was referring to adjustment factors alone. For this we use the actual temp rating of the conductor, not the other parts of the circuit.

So for a #12 THHN you use the 90c column for adjustment which is 30 amps. Even at nine current carrying conductors in a conduit the derating factor is 70% which is still 21 amps so no further step need to be taken.
 
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