Parallel Conductors

[jcook980]

According to 310.4 parallel conductors shall "Be the same length". Obviously you wouldn't want gross differences in length but practically speaking, especially with parallel conductors in parallel conduits, how close to the same length is close enough?

We have a case where the inspector is insisting that a 2' difference in length in a 50' run of parallel conductors is a Code violation. The run has a couple of turns and the difference in length is a result of the layout of the conduits. Short of crossing conduits over each other to equalize the length we can find no effective way to equalize the length. The termination compartments are certainly not big enough to loop a couple of extra feet and the difference in impedance in these two sets of conductors is minimal.

Any ideas?

Thanks!

Jim

 

[don_resqcapt19]

Re: Parallel Conductors

That will result in about a 4% difference in current between the two set of conductors. In most cases this should not cause any problems, but if you are loaded to the maximum permitted by Table 310.16, you could exceed the permitted ampacity. For example, parallel 3/0s with a 400 amp load, you would have 204 amps on the shorter cable and 196 on the longer one. Should not be enough to cause any real problems.
Don

[ March 12, 2003, 04:21 PM: Message edited by: don_resqcapt19 ]

 

[charlie b]

Re: Parallel Conductors

Originally posted by don_resqcapt19: That will result in about a 4% difference in current between the two set of conductors.

I think the impact is far less than that. A 400 amp load (assuming for the moment we are talking about 480V, but the reasoning works at any common voltage) would represent a total resistance of 1.2 ohms. The total resistance includes both lengths of parallel cables and the resistance of the load. A 50 foot length of 3/0 copper would have a resistance of 0.094 x (50/1000) or 0.0047 ohms (from Table 9, effective Z at 0.8pf). Thus, two parallel lengths would have a resistance of 0.00235 ohms. A 52 foot length would have a resistance of 0.094 x (52/1000) or 0.0049 ohms. Thus, two parallel lengths would have a resistance of 0.00245 ohms. The difference is 0.0001 ohms out of a total circuit resistance of 1.2 ohms, or about 0.01% (not 4%). So I agree that this is insignificant.

Charlie B.

 

[mkbuck]

Re: Parallel Conductors

Charlie,

Don's calculations are correct. You calculated to the total impedance of the circuit.

A two foot difference in lenght at 50' will result in a 4% difference in impedance. In a parallel circuit, the path with the lower resistance carries the higher current. In this case 4%.

While the Code does say that the lenghts shall be equal, this should not cause a problem.

To what extent do parallel conductors regulate themselves?

A higher current due to a slighty shorter conductor will result in more heat, thus a higher resistance, thus a lower current??????

MKB

 

[Nick]

Re: Parallel Conductors

Unfortunately the literal text of this section doesn?t offer any relief. An inspector could hold his ground and be correct. However, in the real world I doubt many installations fully comply. Are there any reports of failures to substantiate the rule?

 

[iwire]

Re: Parallel Conductors

This is exactly why I try to use isolated phase installations when allowed. It is much easer to keep conductors of the same phase equal in length.

I would add that the conductors of one phase do not need to be the same length as another phase.

Is the inspector trying to enforce this?

 

[don_resqcapt19]

Re: Parallel Conductors

Charlie,
I used a resistance of 0.26 ohms per 1000' and 49' and 51' lengths of wire. The 49' has a resistance of .01274 and the 51' has a resistance of 0.01326. These 2 in parallel give a circuit resistance of .0065 ohms. Putting the circuit resistance into E=I(R) and (assuming a 400 amp total load) solving for E gives a voltage drop of 2.6 volts. Again using E =I(R) but this time solving for I based on the individual conductor resistance I get 204 amps on the 49' length and 196 amps on the 51' length.
Using your impedance numbers to solve in the same way, I get a total voltage drop of 0.98 volts and a current of 208 amps on the shorter conductor and 200 amps on the longer one.
Don

 

[charlie b]

Re: Parallel Conductors

Don: I was looking at the impact on the circuit as a whole, whereas you were looking at the two parallel cables as being a “current divider.” You correctly concluded that the difference in current between the two cables is insignificant. I correctly concluded that the impact on the circuit as a whole was even less significant. But I must admit that your approach is more relevant in addressing the original question.

 

[hurk27]

Re: Parallel Conductors

Don did you noticed that you swaped the numbers from your first post? but the second post is correct

 

[don_resqcapt19]

Re: Parallel Conductors

Hurk,
I didn't notice and I guess no one else did either. I've made the correction. Thanks.
Don