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#78432  09/24/01 12:28 AM
Conductors in Parallel


Subject: Question Date: Sun, 23 Sep 2001 16:47:20 1000 From: "Joe Wilhelm" <jrwil@attglobal.net> To: <joetede@ix.netcom.com>
Hello,
I would appreciate your help with the following:
If I understand correctly, a 14 gauge wire is rated for 15 amps and 8 gauge is rated for 40 amps. If I combine the 14 and 8 wires in parallel does that then provide 55 amp load capability?
Thank you.
This installation is contrary to common practice. Please see Sections 1108 and 3104 in the 1999 NEC.
Joe Tedesco, NEC Consultant



#78434  09/24/01 07:29 AM
Re: Conductors in Parallel


First off 3104 only permits conductors 1/0 and larger to be paralleled. It also requires they be the same diameter. Secondly, the values suggested for ampacity are ambiguous, due to lack of conductor & application detail.



#78435  09/24/01 10:01 AM
Re: Conductors in Parallel

Member

Joined: Nov 2000
Posts: 2,148
IL


While this installation is not code compliant, neither conductor would be overloaded. If you use the AC resistance from Chapter 9 Table 9 you can solve this as parallel resisitor problem. If we make the circuit 100' long and assume a 55 amp load the resistnace of #14 is 0.31 ohms and the the #8 is 0.078 ohms. The total resistance of these two conductors in parallel is:
1/0.31 + 1/0.078 = 1/Rt = 16.046
Rt= total resistance
1/16.046 = 0.0623 ohms = resistance of 100' of #14 in parallel with 100' of #8
Now if we use E=I(R) to solve for the voltage drop we get E= 55(0.0623) = 3.426 volts
We can now use this voltage drop to solve for the current in each of our conductors. Again E = I(R) 3.426 = I(0.31) = 11.05 amps for the #14.
3.426 = I(0.078) = 43.92 amps for the #8
These two values do not exactaly equal 55 amps due to rounding. They do total 54.97 amps.
Don(resqcapt19)
[This message has been edited by resqcapt19 (edited 09242001).]
Don(resqcapt19)



#78436  09/24/01 04:04 PM
Re: Conductors in Parallel


Subject: RE: Followup to Answers to your question Date: Mon, 24 Sep 2001 10:11:25 1000 From: "Joe Wilhelm" <jrwil@attglobal.net> To: "Joe Tedesco" <joetede@ix.netcom.com>
Joe, Thank you very much for your help! While I get around on the internet, I don't know how to "post" or put up a reply or followup question... so if I could ask you to please post this followup:
Thank you all for your helpful comments, especially "resqcapt19" for your calculations! In light the reply's, could you please tell me how much current 8 gauge will carry at 220V and 110V over a 50' long run? I'm thinking of using a 50 amp double pole breaker (220V) by the main panel to feed the 220V to another box 50' away with 4 number 8 wires (2 hot, 1 neutral, 1 green). The box at the end of the 50' will have at least 1 double pole 20 amp breaker for a 220V line and some 20/15 amp breakers for 110V lines. Will this work ok? Would it meet "code"? Someone told me that a number 8 wire is only good for 40 amps so that's why I was thinking of combining the 8 and 14 wires (a 3/4" conduit is already in place and I don't think I can get 4 number 6 wires through it). Thank you all for your help!
Joe Tedesco, NEC Consultant



#78438  09/24/01 05:05 PM
Re: Conductors in Parallel

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Joined: Nov 2000
Posts: 2,148
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Joe, The #8 is limited to 40 amps unless the terminations at both ends of the wire and the wire itself is listed for 75°C. Four #6 THWN will fit in a 3/4" conduit. Also Table 250122 only requires a #10 ground wire for a 40 or 50 amp circuit. You could install three #6 and one #10 to make the pull a little easier. The voltage drop on a 120 volt circuit of #8 50' long with 40 amps of load would be 3 volts of 2.5%. Don(resqcapt19)
Don(resqcapt19)



#78439  09/24/01 07:39 PM
Re: Conductors in Parallel

Junior Member

Joined: Sep 2001
Posts: 1


So the voltage drop of #8 for 50' is 3 volts... what is the resistance of 50' of #8?



#78440  09/24/01 09:21 PM
Re: Conductors in Parallel

Member

Joined: Nov 2000
Posts: 2,148
IL


jw, I used a voltage drop calculator on another site to figure the voltage drop. I didn't use the resistance, but it would 0.039 ohms. Don(resqcapt19)
Don(resqcapt19)




