3 conductors in conduit

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Re: 3 conductors in conduit

Jennifer; Be aware that line coupling effects will still have field generated net current should one parallel circuit be shut down. (Residential HVAC equipment SE cabling can run parallel to galvy pipe or metal air ducting that can promote residual current in the equipment lacking EGC grounding.) rbj, Seattle

context of ground. :D

[ December 02, 2005, 03:56 AM: Message edited by: gndrod ]
 
Re: 3 conductors in conduit

Originally posted by peter: In one, you have a red and a black wire. In the other you have the two neutrals [white wires -- I like colors]. They are both running idedtical loads -- say 100 watt light bulbs. . . . wouldn't the red and black forces cancel out?
Click to expand...
Yes they would, at least until someone turns off one of the two bulbs. This isn't allowed because the designer and installer do not control the operation of the light switches.
 
Re: 3 conductors in conduit

Hi,
Ok, heres a real hvac circuit. On heating systems we put a "emergency" switch at the top of the cellar steps for the homeowner, in case of trouble so they don't have to go in the basement.

You have a 12ga line with a 20amp breaker going to the oil heating unit. Aprox running amps is 12. The hot goes to the emergency switch and returns to the unit. No neut wire is present. Say this were run in EMT or greenfield, and could involve a 30 foot run. What effects could occure from this ?. What about when a 3-way switch is wired. Say switch A gets the hot feed, and the light to be controlled is say 6 feet away, but switch B is 15 feet from the light. You have the hot with no neut going 15 feet out, and returning to the light. Do problems occure with this ?. Do problems show when you go above a certain load current, as opposed to a light load ?.

Might have to call 1-800-NASA soon...
 
Re: 3 conductors in conduit

The issue is not weather the wire in question is being called a hot or neutral. The issue in question is how many amps are flowing in what direction in each conduit or cable.
in the case of the switch at the top of the stairs. all of the circuit amps are flowing up to the switch, then back down to the equipment. if these are in different raceways, you will have problems resulting from magnetic effects. if they are in the same conduit the effects will cancel eachother.

it is the same with the switch. as long as all of the current going to the switch returns in the same conduit, or cable that it goes to the switch in, the magnitic fields will cancel eachother.

To simplify this. the current going in one direction in the conduit or cable must be equal to the current going in the other direction.

If those currents are in different cables. then you have problems.

[ December 01, 2005, 11:28 PM: Message edited by: jbwhite ]
 
Re: 3 conductors in conduit

Jennifer,
You might want to look up Karl Riley's book on Tracing EMF's . There are clear examples both schematically and in text discourse that will explain what generates electromagnetic fields from non-code wiring. The radiated field effects are detrimental to both sensitive electronic equipment and some people. (i.e computer interferences and purported, but not medically proven, mild radiation burning.) Karl's book references both along with NEC compatible cabling practices.) rbj. Seattle

Edited double word.

[ December 02, 2005, 03:47 AM: Message edited by: gndrod ]
 
Re: 3 conductors in conduit

Originally posted by jbwhite: To simplify this. the current going in one direction in the conduit or cable must be equal to the current going in the other direction.
Click to expand...
And to complicate it just a bit, in a three phase system, the current going in one direction in one of the three wires will be returning in one or both of the other two wires. The effect is the same as in the single phase case: the total magnetic field of the three wires, at any point along the conduit, is essentially zero. The fields from the three wires cancel each other out. In a 4-wire 3-phase system, with less than perfect load balancing, there will be current in the neutral. In this case, the fields from the four wires cancel each other out.
 
Re: 3 conductors in conduit

Hi,
Well lots of more good information. All this good information will keep me out of trouble.
Last question...really !.

Say you were going to run 2- 110v 15a circuits to 2 air handlers. Both will be in one conduit, or greenfield. We have 4 conductors. My calculations are:

3/4in emt/greenfield
16- 12ga max fill for 3/4 so 40% fill = 6.4 conductors.

12 ga thhn derated 20% = 16a capacity

Nec says ground = fused max conductor size so
12ga ground. Question on this one. IS 12ga good or should you figure ground for total amps of both which would be 30a = 10ga ground. If this is the case would 2- 12ga grounds be ok, if you didn't have 10ga.

Do 2- circuits in one conduit that have motors like the air handlers present anything different, as opposed to non-motor circuit load.

How am I doing...?


Jennifer
 
Re: 3 conductors in conduit

Originally posted by Jennifer:

Say you were going to run 2- 110v 15a circuits to 2 air handlers. Both will be in one conduit, or greenfield. We have 4 conductors. My calculations are:

3/4in emt/greenfield
16- 12ga max fill for 3/4 so 40% fill = 6.4 conductors.

12 ga thhn derated 20% = 16a capacity

Nec says ground = fused max conductor size so
12ga ground. Question on this one. IS 12ga good or should you figure ground for total amps of both which would be 30a = 10ga ground. If this is the case would 2- 12ga grounds be ok, if you didn't have 10ga.

Do 2- circuits in one conduit that have motors like the air handlers present anything different, as opposed to non-motor circuit load.

How am I doing...?


Jennifer
Click to expand...
3/4" FMC will handle 16 THHN/THWN conductors. The 40% fill rating is already calculated for the tables in Annex C of the NEC.

The 12 ground is fine. See Table 250.122. Because you have increased the wire size on the 15 A circuit, you must also install a #12 ground wire, even though your circuit breaker is 15A. A single ground wire for both circuits is fine. Sounds like you're dropping off a circuit in a jbox then continuing on to next motor?

Multiple circuits in one conduit are fine as long as you pay attention to derating. See Tabble 300.15(B)(2)(a). You have 4 conductors in one conduit (2 hots and 2 neutrals). That means that you must derate the ampacity by 80%. For derating, you can use the higher ampacity values in 310.16. For example, 12 THHN, 90C column has an ampacity of 30A. Multiplied by .80 = gives you 24A, which is well above the 20A permitted for a #12 wire. #14 is rated at 25A in the 90 column and derates to 20, but you would still use a 15A breaker to protect it.

Also pay attention to 430.22. Motor conductors are sized at 125% of rated full load current. I'm assuming air handlers are continuous loads?

If I goofed up anywhere in here, I'm sure someone will correct me.

[ December 05, 2005, 08:20 PM: Message edited by: flick ]
 
Re: 3 conductors in conduit

Hi,
Looks like I "goofed". I need to get a book on understanding the nec book, so I can totally understand !. Well at least by my original misguided understanding the conduit would have been so under filled it would have made an inspector fall over from joy.

Jennifer
 
Re: 3 conductors in conduit

however, has anyone actually experienced any overheating in these situations on 2 or 3 15/20 amp circuits ?
Click to expand...
Sure have. You could fry an egg on the cover of the 4" sq j-boxes in the conduit runs where somebody had tried to pick up a neutral from a different circuit, and the hot wire was in a seperate conduit run back to the panel.
 
Re: 3 conductors in conduit

Hi,
I want to thank everyone for their replies. :)
I have a much better understanding. I'll keep reading to understand more.

As the holiday approaches, I understand why people become depressed. :(

One light goes out...they all go out !.
And theres 40 bulbs... :eek:

Jennifer
 
Re: 3 conductors in conduit

Jennifer, Look on the brighter side. May all your lights be parallel. ;)
Merry Xmas...rbj, Seattle

[ December 13, 2005, 09:51 PM: Message edited by: gndrod ]
 
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