Parrallel

[ryan_618]

I'm sure this has been brought up before, and if so I apologize. At what point does it REALLY cause a concern when paralleled conductors are not the same length? 3% 5%??? Also, is there a fairly simple method of measuring length of a conductor when it is in a conduit? Thanks in advance for your replies.

 

[bennie]

Re: Parrallel

The larger the conductor, the length is less of a factor.

 

[ryan_618]

Re: Parrallel

Bennie, as always thanks for your reply. Would you or someone else mind taking this a little further though? Thanks

 

[iwire]

Re: Parrallel

I have seen Don put this info up before (I think on this forum) with a little searching using his member number (4) it should show up.

Small variations in length did not make much difference in current and would not be an issue unless the conductors where maxed out already.

Bob

 

[bennie]

Re: Parrallel

ryan: The practical method for assuring the cable is the same length is to measure them and pull them in the conduit.

When pulling off of reels, there will be a little difference in length, due to the radius of bends and position of the cable in the conduit.

 

[ryan_618]

Re: Parrallel

Bennie: thanks again. What I wonder is, as an inspector, if you greatly doubt the even length of the conductors is there an easy way for the electrician to show evidence?

 

[roger]

Re: Parrallel

Ryan, one easy way to check this is, apply a known load and measure current on all (each individual) conductors connected to one phase, take any difference and then do the math. Do this for each phase and neutral.

Roger

[ July 25, 2003, 10:32 AM: Message edited by: roger ]

 

[bennie]

Re: Parrallel

Roger is correct. Parallel conductors are shorted together at each end. If all the conductors are the same impedance, the current will be the same in each one. Phase balance is not a factor.
A clamp on ampmeter is all that is necessary to check the impedance variation. You will be surprised at some of the readings, mainly on aluminum.

A preventive maintenance procedure that is never done, is to measure the current balance on feeders every 6 months or so. This can detect an oxidized connection.

 

[ryan_618]

Re: Parrallel

Hmmm....I can't believe I didn't think of that! Thanks a lot. Tell me what you think...If either of the conductors exceed the ampacity spelled out by 310.16 it is a problem, otherwise fine? (I would do this mathematically, not by applying current to the maximum of 310.16)

 

[charlie b]

Re: Parrallel

Originally posted by ryan_618:If either of the conductors exceed the ampacity spelled out by 310.16 it is a problem, otherwise fine? (I would do this mathematically, not by applying current to the maximum of 310.16)

Math will not help you here, for it would throw you into circular reasoning, as follows:
(1) The mission is to discover whether differences in the lengths of two paralleled conductors will cause a problem.
(2) If you have not measured length (or equivalently, measured resistance), then you can only discern the difference in length by first determining the difference in current.
(3) If you do not measure the actual current, you can still determine the split in current by using the current divider rule: I1 = I-total * (R2/R-total), and I2 = I-total * (R1/R-total).
(4) In order to do this calculation, you first need to know the resistance of each of the two parallel conductors.
(5) That brings you back to step (2).

 

[ryan_618]

Re: Parrallel

Charlie: Thanks for your post, you obviously know a great deal more about this than I. What I meant was to take a known load and energize it via the parralleled conductors. Measure the current with an amp clamp and see the difference. This being done, you could look at the percentage of difference. With that done, I would assume the full rating of the OCPD going through the conductors and see if the difference in percentage would allow current in excess of the conductors rating to pass through it. Would that work? Thanks again

[ July 25, 2003, 05:29 PM: Message edited by: ryan_618 ]

 

[charlie b]

Re: Parrallel

Your concept of measuring one current and calculating the impact of a higher current is perfectly valid. Regrettably, it would prove to be of no real value. Again, it boils down to circular reasoning.

Presuming your overcurrent device is set at the combined ampacity of your set of parallel cables, and presuming that the cables are of ?slightly? different lengths, then when you calculate the current through each of the cables (based on your earlier measurements at a lower current), your result will be that one cable has less than the 310.16 limit going through it, and the other will have more than the 310.16 limit going through it. You can calculate how much more than the limit, and then ask whether that will be a significant amount. But that takes you back to your original question, ?At what point does it REALLY cause a concern??

The bottom line is that, however you measure, calculate, or otherwise infer the amount of difference between the two cables, the question of ?Is that a significant difference?? will still remain open to debate. The code does not say ?within 5% of the same length,? nor does it provide any other guidance to the installer or to the inspector.

 

[bennie]

Re: Parrallel

Possible difference in impedance to become a hazard, is why 1/0 is the minimum size. The impedance per wire size increases exponentially with a decrease in size.