Paralell Conductors
- Thread starter mon305
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charlie tuna
Senior Member
- Location
- Florida
Re: Paralell Conductors
the long conductor will be lightly loaded--guessing about 40 per cent
the long conductor will be lightly loaded--guessing about 40 per cent
jimwalker
Senior Member
- Location
- TAMPA FLORIDA
Re: Paralell Conductors
May i ask just how only one ended up 4 feet too long
May i ask just how only one ended up 4 feet too long
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Paralell Conductors
Since you are apparently interested in percentages, the actual resistances (i.e., the type and size of cable) can be left out of the calculation. If all seven wires were the same length, then their resistance would each be, let us say, ?R.? Placing seven of these in parallel gives a total resistance of ?R/7? or about 0.1429R.
Since one wire is longer by 4 feet out of a 40 foot run, its resistance would be 1.1R. The parallel combination of the other six gives a subtotal resistance of ?R/6,? or about 0.1667R. So you wind up with a set of two resistors in parallel, with their values being 1.1R and 0.1667R. That parallel combination gives a new total resistance of 0.1447R (instead of the 0.1429R that equal lengths would have given us). Thus, the total resistance is high by about 1.3%, and the total current would be low by that same 1.3%.
Finally, let?s look at how the seven wires share the load. If all seven were the same length, then each would handle 1/7th (14.29%) of the load current. Instead, you have a current divider, with six wires (resistance of 0.1667R) in parallel with one wire (resistance of 1.1R). The long wire will take on current in the ratio of (0.1667) / (1.1 + 0.1667). This means that the long wire gets 13.16% of the current, instead of the 14.29% that should have been its share. Thus, it is lightly loaded by about 8% (not 40%, sorry Charlie Tuna).
Since you are apparently interested in percentages, the actual resistances (i.e., the type and size of cable) can be left out of the calculation. If all seven wires were the same length, then their resistance would each be, let us say, ?R.? Placing seven of these in parallel gives a total resistance of ?R/7? or about 0.1429R.
Since one wire is longer by 4 feet out of a 40 foot run, its resistance would be 1.1R. The parallel combination of the other six gives a subtotal resistance of ?R/6,? or about 0.1667R. So you wind up with a set of two resistors in parallel, with their values being 1.1R and 0.1667R. That parallel combination gives a new total resistance of 0.1447R (instead of the 0.1429R that equal lengths would have given us). Thus, the total resistance is high by about 1.3%, and the total current would be low by that same 1.3%.
Finally, let?s look at how the seven wires share the load. If all seven were the same length, then each would handle 1/7th (14.29%) of the load current. Instead, you have a current divider, with six wires (resistance of 0.1667R) in parallel with one wire (resistance of 1.1R). The long wire will take on current in the ratio of (0.1667) / (1.1 + 0.1667). This means that the long wire gets 13.16% of the current, instead of the 14.29% that should have been its share. Thus, it is lightly loaded by about 8% (not 40%, sorry Charlie Tuna).
charlie tuna
Senior Member
- Location
- Florida
Re: Paralell Conductors
not to be aurgumentative guys but i really never thought a few feet of cable would make a difference and to tell you the truth i looked at with the ohms law prospective:
now actual expieriance sets in:
a 4000 amp bus stub burns up on a 55 story building we service. it has three 4000 amp service mains to feed the tower. this feeder feeds the middle rise of the building and is needed for elevator operations.
i need to feed the line side of the main switch with eight 500 mcm cu per phase and i need 80 feet per leg to reach the vault bus thru the vault door--layed on the floor. the blow up occurred around 4:00pm. the supply house's cable cutter had gone for the day and someone else cut the runs and we found two legs about eight feet short or ten per cent of the length. i didn't think much of it. after energizing we found that those two runs were carrying 760 to 800 amps at peak times - the six others were carrying about 280 amps. we built a plywood "raceway" over the cables and installed fans at one end to keep the cables cool. another time we had to jump the high rise bus duct with the mid rise and again cable lengths were not the same and it made a big difference. we make a point to keep all parallel feeders the same on the same phase. we lay them out and check them from the supply house before pulling wire.... it does make a big difference.
not to be aurgumentative guys but i really never thought a few feet of cable would make a difference and to tell you the truth i looked at with the ohms law prospective:
now actual expieriance sets in:
a 4000 amp bus stub burns up on a 55 story building we service. it has three 4000 amp service mains to feed the tower. this feeder feeds the middle rise of the building and is needed for elevator operations.
i need to feed the line side of the main switch with eight 500 mcm cu per phase and i need 80 feet per leg to reach the vault bus thru the vault door--layed on the floor. the blow up occurred around 4:00pm. the supply house's cable cutter had gone for the day and someone else cut the runs and we found two legs about eight feet short or ten per cent of the length. i didn't think much of it. after energizing we found that those two runs were carrying 760 to 800 amps at peak times - the six others were carrying about 280 amps. we built a plywood "raceway" over the cables and installed fans at one end to keep the cables cool. another time we had to jump the high rise bus duct with the mid rise and again cable lengths were not the same and it made a big difference. we make a point to keep all parallel feeders the same on the same phase. we lay them out and check them from the supply house before pulling wire.... it does make a big difference.
- Location
- Illinois
- Occupation
- retired electrician
Re: Paralell Conductors
Charlie,
There may be things other than just the length that affected the current division on your temporary feeder. The cable configuration can have a big effective on the impedance of the conductors. Ohms law does apply and for that much difference in current there must have been something other than the 10% length difference involved.
Don
Charlie,
There may be things other than just the length that affected the current division on your temporary feeder. The cable configuration can have a big effective on the impedance of the conductors. Ohms law does apply and for that much difference in current there must have been something other than the 10% length difference involved.
Don
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Paralell Conductors
Don and Charlie: You are both right. My calculation accounted for resistance only. The physical arrangement of cables (that I presumed to have been symmetrical) could throw mutual capacitance and mutual inductance into the equation. To resolve this new, more complicated equation, you would need detailed information about how the cables are installed, with respect to each other.
Don and Charlie: You are both right. My calculation accounted for resistance only. The physical arrangement of cables (that I presumed to have been symmetrical) could throw mutual capacitance and mutual inductance into the equation. To resolve this new, more complicated equation, you would need detailed information about how the cables are installed, with respect to each other.
charlie tuna
Senior Member
- Location
- Florida
Re: Paralell Conductors
cable temperature enters into it also - you would think the cable that carries the most current would heat up causing a resistance to current flow and shift the load back to the other cables?
i had these cables run on the floor- 8-"a" phase then 8 "b" phase etc, but flat on the floor - i do respect "cable length after seeing these examples and since i now practice equal cable lengths in each phase i find the current very close to equally balanced between cables of that phase.
cable temperature enters into it also - you would think the cable that carries the most current would heat up causing a resistance to current flow and shift the load back to the other cables?
i had these cables run on the floor- 8-"a" phase then 8 "b" phase etc, but flat on the floor - i do respect "cable length after seeing these examples and since i now practice equal cable lengths in each phase i find the current very close to equally balanced between cables of that phase.
- Location
- Illinois
- Occupation
- retired electrician
Re: Paralell Conductors
charlie tuna.
Don
charlie tuna.
That could be a problem. I read a case study of a similar installation in aluminum cable tray where the cables were burning up. The problem was caused because the cables were run like yours and not grouped A,B and C. As I recall it was the cables near the center of the tray that were having the problem. When it was reinstalled using the A,B,C grouping the problem went away.
Don
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