two parallel conductor in under ground
- Thread starter aliasghar
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chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
aliasghar said:
I'm thinking yes. Can you give us more info on the application.
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
aliasghar said:
I have discussed this here and in the field with inspectors and PE's.
Your 350 has a resistance of around .04 ohms per 1000 ft as per tables 8 and 9. If one of your conductors is 10' longer than the other you have around 4 tenths of 1 ohm difference.
I have yet to get an opinion on this matter that tells us where the difference in length comes into play.
Bob NH
Senior Member
- Location
- The "Live Free or Die" State
chris kennedy said:
At 0.04 Ohms per 1000 ft the difference in resistance of 10 ft will be 0.0004 Ohm.
The 75 C Ampacity of Table 310.16 is 310 Amps.
For this example, I will use a circuit required to deliver 600 Amps to a load, and let the length for one conductor be 100 ft and the length for the other be 110 ft.
The resistance of the 100 ft conductor will be 0.004 Ohm and the 110 ft conductor wil be 0.0044 Ohm.
The voltage drop will be the same for both conductors since they are tied together at both ends.
The resistance of the pair will be:
1/R = 1/.004 + 1/0.0044 = 477.273 so R of the pair is 0.002095238 Ohm.
600 Amps x 0.002095238 = 1.25714 Volts drop.
The shorter conductor at 0.004 Ohm will carry 1.25714/0.004 = 314.29 Amps.
The longer conductor at 0.0044 Ohm will carry 1.25714/0.0044 = 285.71 Amps.
That is the proof; now here is an easy way to figure the effect of two different wire lengths:
The shorter wire of a parallel pair will carry current that is equal to the average current times the ratio of the length of the longer wire to the average length of the two wires.
(110 ft/105 ft) x 300 Amps = 314.29 Amps.
An even simpler approximation is that the shorter conductor will carry 0.5% more than average current for each 1% difference in the length of the two conductors.
A 10% difference in wire length is enough to put one conductor out of spec.
skeffibew1
Member
Yes you can run them in two diffrent conduits as long as you run A,B,C, and the neutral in each conduit. For example you can not run 2 A phases in the same conduit, and they also have to be the same length.
phases
phases
as long as no metallic condut/enclosure is involved, I believe he can run all the phases in a seperte conduit (A in one conduit, B in a 2nd etc). I have seen this done with the objective to maintain the conductors at the same length and make a neater termination (no "crossover" of cables)
phases
skeffibew1 said:
as long as no metallic condut/enclosure is involved, I believe he can run all the phases in a seperte conduit (A in one conduit, B in a 2nd etc). I have seen this done with the objective to maintain the conductors at the same length and make a neater termination (no "crossover" of cables)
- Location
- Logan, Utah
Take a look at the exception to 300.3(B)(1). This section allows conductors installed in nonmetallic raceways underground to be permitted to be arranged as isolated phase installations.
But the OP poster stated that he wanted to use 2 PVC conduits with the "A" phase in one and the "B" phase in the other, there would be a problem with this if you were to install a grounded conductor because you would either need to have a third conduit or install the two sets with "A","B", grounded conductor, and EGC in each conduit.
Chris
But the OP poster stated that he wanted to use 2 PVC conduits with the "A" phase in one and the "B" phase in the other, there would be a problem with this if you were to install a grounded conductor because you would either need to have a third conduit or install the two sets with "A","B", grounded conductor, and EGC in each conduit.
Chris
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