How would reactance change if all conductors (3 per) of the same phase were ran together in seperate PVC conduits. A phase in one, B in next etc. I assume reactance would increase. No metal enclosures are involved, basically vault to vault. Would this be a viable way to reduce fault current availability would it have minimal results in the final imperdance calc? This service is 120/208 with (6) 500 kcm conductors per phase with 70 feet of conductor. Just thinking of other ideas to reduce AIC without line reactors or fuses.
Reactance question?
- Thread starter ddavi280
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I just ran some quick numbers....in PVC you would actually decrease the reactance and decrease the total impeadence...... raising the available fault current
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That does not sound right to me considering that the inductance of the triplets of phase wires in a pipe will be less than the open air inductance of those same wires.
But if you were to look at a single phase to neutral fault rather than a phase to phase fault the results might well be different.
Tapatalk!
But if you were to look at a single phase to neutral fault rather than a phase to phase fault the results might well be different.
Tapatalk!
Phil Corso
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Canton...
Reactance increases with physical separation!
Regards, Phil Corso
Reactance increases with physical separation!
Regards, Phil Corso
winnie
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Steel conduit increases reactance. But to run the phases in separate conduits you want non-metallic conduit, or you have to jump through a bunch of hoops to prevent induced current in the conduit.
So, assuming non-metallic conduit, the answer is as Phil mentioned: reactance will increase with increased spacing.
I'd need to dig into texts to tell you how much reactance you get per unit spacing.
-Jon
So, assuming non-metallic conduit, the answer is as Phil mentioned: reactance will increase with increased spacing.
I'd need to dig into texts to tell you how much reactance you get per unit spacing.
-Jon
Closer spacing reduces inductance and wider spacing increases it. This is apparent from fourth bullet point in
http://my.safaribooksonline.com/boo...9332503410/9-underground-cables/head9_4_xhtml
http://my.safaribooksonline.com/boo...9332503410/9-underground-cables/head9_4_xhtml
kingpb
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This is a code violation unless it is underground and the raceway has to be non-metallic, including elbows, sounds like that's covered.
However, you should check with the cable manufacturer; I have some recollection that there is a derating of the cable to do it this way because the cable heating increases. If the cable has to be increased in size or quantity, than the available fault current is going to go up, which defeats the intent what your trying to accomplish.
However, you should check with the cable manufacturer; I have some recollection that there is a derating of the cable to do it this way because the cable heating increases. If the cable has to be increased in size or quantity, than the available fault current is going to go up, which defeats the intent what your trying to accomplish.
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rcwilson
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The Okonite Cable website has a nomograph for estimating cable reactance based on phase-to-phase spacing.
Using rough numbers:
1" spacing = 0.028 ohms'/ 1000 ft. This is about the spacing of three phases in the same conduit.
4" spacing = 0.06 ohms/kft
10" spacing = 0.082 ohms/kft
Note that this is only the reactance, not resistance.
Spacing does have an effect but it is not easy to calcuate the effect for mulitple cables per phase.
The data above is for triangular configurations with all phases the same distance apart. If the cables are in a flat configuration, multiply the spacing by 1.26.
(Example: flat A-B-C, A-B and B-C are 7.94" and A-C is 15.8", the equivalent average 3-phase impedance is the same as 7.94" x 1.26 = 10" triangular spacing).
With 6 per phase, we have to calcuate the impedance of each cable to all of the other 17 cables and the ground and neutral wire. The impedance will be different for 3 phase faults and voltage drop than for a single phase fault. Some of my engineering books like Blackburn's "Symmetrical Components" have the calculations to do this, but they are not trivial.
With only 70 feet of run, it might not make that much difference in short circuit.
The wider UG duct spacing will also affect the ampacity, but I would have to run my Ampcalc program to see if it increases or decreases.
Using rough numbers:
1" spacing = 0.028 ohms'/ 1000 ft. This is about the spacing of three phases in the same conduit.
4" spacing = 0.06 ohms/kft
10" spacing = 0.082 ohms/kft
Note that this is only the reactance, not resistance.
Spacing does have an effect but it is not easy to calcuate the effect for mulitple cables per phase.
The data above is for triangular configurations with all phases the same distance apart. If the cables are in a flat configuration, multiply the spacing by 1.26.
(Example: flat A-B-C, A-B and B-C are 7.94" and A-C is 15.8", the equivalent average 3-phase impedance is the same as 7.94" x 1.26 = 10" triangular spacing).
With 6 per phase, we have to calcuate the impedance of each cable to all of the other 17 cables and the ground and neutral wire. The impedance will be different for 3 phase faults and voltage drop than for a single phase fault. Some of my engineering books like Blackburn's "Symmetrical Components" have the calculations to do this, but they are not trivial.
With only 70 feet of run, it might not make that much difference in short circuit.
The wider UG duct spacing will also affect the ampacity, but I would have to run my Ampcalc program to see if it increases or decreases.
Correct, my comment was for going from steel conduit to PVC conduit.
Legally you would not be able to group all the phases together in steel conduit.
Closer spacing with different phases or the same phase...? Having the same phase grouped togather and in PVC would not the inductive reactenance be minimal? ....there by decreasing the impedance and increasing the available fault current?.....regardless of the minimal in pact of the spacing.
trying to wrap my head around this one....please Phil or any other PE's share a little..I'm getting rusty...
winnie
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- Springfield, MA, USA
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- Electric motor research
Inductance can be thought of as caused by the energy stored in the magnetic field caused by a current.
When you have all of the conductors of a circuit grouped together, then the current flowing on one conductor is exactly balanced by the current flowing on the other conductor(s). (As long as there are no faults, or parallel paths, that is!). In this case, the _net_ current is zero, and the magnetic field external outside of the conductors is zero. The only magnetic field left is that _between_ the conductors.
The more widely apart you space the phases, the greater the 'loop area' for magnetic flux, and the greater the inductance.
-Jon
When you have all of the conductors of a circuit grouped together, then the current flowing on one conductor is exactly balanced by the current flowing on the other conductor(s). (As long as there are no faults, or parallel paths, that is!). In this case, the _net_ current is zero, and the magnetic field external outside of the conductors is zero. The only magnetic field left is that _between_ the conductors.
The more widely apart you space the phases, the greater the 'loop area' for magnetic flux, and the greater the inductance.
-Jon
rcwilson
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- Redmond, WA
I was curious about the spacing effect so I ran some numbers using Okonite's table.
Three single 500 kcmil in a conduit are about 1" spacing. Z= .03496 ohms/1000". (Z= .0222+j.027)
With flat horizontal spacing: 4" Z= .05839, 6" Z=.06774, 12" Z= .08495. That's about a 2.5 times increase.
Divide that by 6 for 6 wires per phase and muliply by 70'/1000' to get the circuit impedance of your 70' run.
With 6-500's per phase at 208V, I'm guessing there is a 750 kVA transformer. Assume it is 5.75% impedance for a short circuit of 36.21 kA.
When I use the MVA method and combine the transformer and the cable impedance , I get these 3-phase short circuits:
1" spacing: 32.24 kA,
4" spacing: 30.04 kA,
6" spacing: 29.24 kA,
12" spacing: 27.88 kA.
The reduction would be more for a smaller transformer and less for a larger transformer or one with a lower impedance.
Conclusion: Spacing ducts would drop the three phase fault current about 9% with 6" spacing and 14% with 12" duct spacing.
6 x 3 phases x 12" spacing = 19 foot wide duct bank. Or a 4 wide by 6 deep duct bank would only be 8' deep trench. Sounds easy. Don't forget to derate for depth.
Note I didn't calculate the single phase-ground fault which could be higher.
Three single 500 kcmil in a conduit are about 1" spacing. Z= .03496 ohms/1000". (Z= .0222+j.027)
With flat horizontal spacing: 4" Z= .05839, 6" Z=.06774, 12" Z= .08495. That's about a 2.5 times increase.
Divide that by 6 for 6 wires per phase and muliply by 70'/1000' to get the circuit impedance of your 70' run.
With 6-500's per phase at 208V, I'm guessing there is a 750 kVA transformer. Assume it is 5.75% impedance for a short circuit of 36.21 kA.
When I use the MVA method and combine the transformer and the cable impedance , I get these 3-phase short circuits:
1" spacing: 32.24 kA,
4" spacing: 30.04 kA,
6" spacing: 29.24 kA,
12" spacing: 27.88 kA.
The reduction would be more for a smaller transformer and less for a larger transformer or one with a lower impedance.
Conclusion: Spacing ducts would drop the three phase fault current about 9% with 6" spacing and 14% with 12" duct spacing.
6 x 3 phases x 12" spacing = 19 foot wide duct bank. Or a 4 wide by 6 deep duct bank would only be 8' deep trench. Sounds easy. Don't forget to derate for depth.
Note I didn't calculate the single phase-ground fault which could be higher.
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