I am having a problem with 3 x 1c500MCM per phase feeders from transformer to PDC. Phase B and C are the same with cable #1, #2 at 550A, cable #3 at 350A. Phase A wires are not as bad but still 450A, 450A and 350A. The wire came off the same reel and the cable is buried in the same trench and appears to be the same length. Any suggestions?
Current Unbalance in parallel feeders
- Thread starter Silvermfr
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bphgravity
Senior Member
- Location
- Florida
There must either be some variation in the conductors lengths or manner in which they have been terminated. I also feel your conductors appear to be undersized based on your reported loads.
- Location
- Illinois
- Occupation
- retired electrician
Silvermfr,
It is unlikely that the length or terminations could cause this much difference in current bewteen the phases. If the conductors are not in a raceway and were just layed in the trench, it is likely that the variation in seperation between the conductors is causing this problem. The conductors need to be installed in groups with each group containing each phase and the grounded conductor, if any. Also what is the calculated load? All of the conductors except the two running at 350 amps are overloaded.
Don
It is unlikely that the length or terminations could cause this much difference in current bewteen the phases. If the conductors are not in a raceway and were just layed in the trench, it is likely that the variation in seperation between the conductors is causing this problem. The conductors need to be installed in groups with each group containing each phase and the grounded conductor, if any. Also what is the calculated load? All of the conductors except the two running at 350 amps are overloaded.
Don
ramsy
NoFixNoPay Electric
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
Before Don posted, I was about to mention a few references. Apparantly, conductor proximities in parallel loads involves more EMF, induction, skin effect, and somthing called migration where current moves to one side of the conductor.
I also see Table 310-17 works for your 500kcmil, but only if its in free air. Not shure if loose conductors in trenches qualify for free air.
I also see Table 310-17 works for your 500kcmil, but only if its in free air. Not shure if loose conductors in trenches qualify for free air.
rcwilson
Senior Member
- Location
- Redmond, WA
Can the old Table B310.10 apply for single conductors direct buried in earth? 500 kcmil is good for 572 A (75C insulation) according to my 2002 NEC Appendix B.
The unequal load sharing is probably caused by the random spacing in the ground creating different impedances in each cable.
I have had problems getting good amp probe measurements on paralleled cables. If the clamp-on ammeter is not positioned so the conductor is directly in the middle of the jaws or if other conductors are too close, inaccurate readings can result. (I could get changing readings on a steady load, depending on where and how I clamped on the same cable or pushed other cables away.) Maybe the imbalance is not as great as you think.
The unequal load sharing is probably caused by the random spacing in the ground creating different impedances in each cable.
I have had problems getting good amp probe measurements on paralleled cables. If the clamp-on ammeter is not positioned so the conductor is directly in the middle of the jaws or if other conductors are too close, inaccurate readings can result. (I could get changing readings on a steady load, depending on where and how I clamped on the same cable or pushed other cables away.) Maybe the imbalance is not as great as you think.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Table B310.10 is only good for one or two conductors per phase (i.e., detail 9 or detail 10).
My money would be on a bad connection at one or more of the terminations on Phase C. I don't think a spacing problem would show up as a lower current on one phase in each of the three conductors associated with that phase. That statenent presumes that the cables were not intentionally laid with all Phase A conductors together, all Phase B conductors together, and all Phase C conductors together.
My money would be on a bad connection at one or more of the terminations on Phase C. I don't think a spacing problem would show up as a lower current on one phase in each of the three conductors associated with that phase. That statenent presumes that the cables were not intentionally laid with all Phase A conductors together, all Phase B conductors together, and all Phase C conductors together.
- Location
- Illinois
- Occupation
- retired electrician
Charlie,
Don
I can't imagine that much difference in current as a result of a poor termination without letting the smoke out. Also it is one cable in each set that has the low current value.
Don
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Don, these are very large cables, with very low resistance. It would not take much in the way of a poor connection, to add enough to the total resistance such that the current drops by 40%. This might cause overheating without the release of smoke. The Phase C cables all terminate at the same point. My first step in troubleshooting would be to look for a bad connection at that point.
My next step would be to try to learn the exact manner in which the cables were laid. I would seek out the persons who installed the cables, and ask them what they remember about the installation. If that cannot be done (e.g., they can't be found or they don't remember), and if there were no photos taken before the trench was back-filled, I might wind up recommending that they break out the shovels.
I would not apply "Occam's Razor" on this one. You don't have to look first at the simplest of possible causes. Rather, you should first attempt to verify or disprove the causes that are the simplest ones to investigate. A thermal gun is all you need to test the connections. Checking the layout of the conductors in the trench is less easy.
My next step would be to try to learn the exact manner in which the cables were laid. I would seek out the persons who installed the cables, and ask them what they remember about the installation. If that cannot be done (e.g., they can't be found or they don't remember), and if there were no photos taken before the trench was back-filled, I might wind up recommending that they break out the shovels.
I would not apply "Occam's Razor" on this one. You don't have to look first at the simplest of possible causes. Rather, you should first attempt to verify or disprove the causes that are the simplest ones to investigate. A thermal gun is all you need to test the connections. Checking the layout of the conductors in the trench is less easy.
- Location
- Illinois
- Occupation
- retired electrician
Charlie,
It is not just C phase...it is one cable from each phase that has the low current.
Ok...I can see that point now given that the resistance of 500kcmil is 0.0000258 ohms per foot.
Don
It is not just C phase...it is one cable from each phase that has the low current.
Ok...I can see that point now given that the resistance of 500kcmil is 0.0000258 ohms per foot.
Don
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
Silvermfr said:
Just to confirm, you have 3 parallel conductors per phase, 500KCMIL each and they are direct buried?
Also, what is the length of the run?
Do you know the spacing of the cable in the trench and the cable configuration, how deep are they buried, and what is the backfill material?
Where is the location (what part of country) and what are the average ambient temperatures of the air and soil?
When the current measurements were taken, what were the line-to-neutral voltages?
This is the info that would be needed to try and determine installation problems without digging them up.
Silvermfr,
1450 amps on three 500klim is a big problem, if there was a loose connection
on this it would have burnt off by now.
You best get an EC to this location ASAP. I have a feeling that this service
has been added on to time and time agian without consideration to the total
load being served!
This problem will only get worse,the conductors are 200-300 amps overloaded, get on the phone now!!!
1450 amps on three 500klim is a big problem, if there was a loose connection
on this it would have burnt off by now.
You best get an EC to this location ASAP. I have a feeling that this service
has been added on to time and time agian without consideration to the total
load being served!
This problem will only get worse,the conductors are 200-300 amps overloaded, get on the phone now!!!
I am going to try and add some information from everyones replies. First of all thank you all for your responses.
I made a small mistake in the information, phase B and C cable #1 within each phase was at 550A and the other two cables within each phase was 350A. Phase A was correct. All 3 phases were balanced at approx. 1250A total of each phase. A Dranetz 4300 was used on all the phases. This is a delta system and the voltages are balanced. I do not know the confiquration of how the cables were installed or to what depth. I have come apon this, years after the installation. The cable length is approximately 200'. I was told the cable came off the same reel and is installed underground in PVC conduit. The area is Princeton, BC Canada. The summer temperature in this area is at least 30C (90F). The winter temperature could be around -20C at the coldest. Please forward any additional questions and I will reply. Thank you all.
I made a small mistake in the information, phase B and C cable #1 within each phase was at 550A and the other two cables within each phase was 350A. Phase A was correct. All 3 phases were balanced at approx. 1250A total of each phase. A Dranetz 4300 was used on all the phases. This is a delta system and the voltages are balanced. I do not know the confiquration of how the cables were installed or to what depth. I have come apon this, years after the installation. The cable length is approximately 200'. I was told the cable came off the same reel and is installed underground in PVC conduit. The area is Princeton, BC Canada. The summer temperature in this area is at least 30C (90F). The winter temperature could be around -20C at the coldest. Please forward any additional questions and I will reply. Thank you all.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I don't think we ever determined whether phases are shared among the cables, so I'll ask it directly:
Silver, the right way to wire parallel wires and/or cables in multiple conduits is thus:
Conduit #1 should contain one conductor from each phase, a neutral, and an EGC if present. The same for conduit #2, and for conduit #3, etc. The use of multi-conductor cables doesn't change this.
Silver, the right way to wire parallel wires and/or cables in multiple conduits is thus:
Conduit #1 should contain one conductor from each phase, a neutral, and an EGC if present. The same for conduit #2, and for conduit #3, etc. The use of multi-conductor cables doesn't change this.
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