First off 310-4 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.
Re: Conductors in Parallel#78435 09/24/0109:01 AM09/24/0109:01 AM
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.
[This message has been edited by resqcapt19 (edited 09-24-2001).]
Re: Conductors in Parallel#78436 09/24/0103:04 PM09/24/0103:04 PM
Subject: RE: Follow-up to Answers to your question Date: Mon, 24 Sep 2001 10:11:25 -1000 From: "Joe Wilhelm" <email@example.com> To: "Joe Tedesco" <firstname.lastname@example.org>
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 follow-up question... so if I could ask you to please post this follow-up:
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
Re: Conductors in Parallel#78437 09/24/0103:43 PM09/24/0103:43 PM
It's quite easy to post a reply here. You just have to register first. You just fill out a simple form where you choose a "Name" and a password. Then you only have to click on "Post Reply" to respond to something already here or "Post New Topic" will start a totally new Topic.
BTW, If I'm not mistaken, according to table 250-122 you could run a #10 for your ground. That would be 3 #6 and a #10 in your 3/4" conduit for the 50A circuit.
Re: Conductors in Parallel#78438 09/24/0104:05 PM09/24/0104:05 PM
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 250-122 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)
Re: Conductors in Parallel#78439 09/24/0106:39 PM09/24/0106:39 PM