2 section of bus duct went faulty and was jumped out with 4 parallel conductors per phase, 1 conductor/jumper of each of 2 phases has a load 1/3 more than the other 3 conductors. I am getting mixed explanations. The jumpers are less than 20'. there are 16 jumpers. 4 each phase an a neutral. To me each phase group should be all equal if of equal length. But to be 1/3 higher? This is an apartment bldg. What's your theory?
Parallel conducrors
- Thread starter hwhj
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ron
Senior Member
- Location
- New York, 40.7514,-73.9925
Whether it is length or terminations, there is something causing more impedance in certain sets versus the others. 20' should be easy to visually and IR inspect.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
It may be simply a case of load imbalance. For instance all the water heaters, or ranges are on phases A and B. Are the hvac units single phase or 3 phase
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Dennis, I believe the the OP said 1 conductor of a parallel set was running "hot", not the whole phase. From point to point I think parallel conductors in a single phase are supposed to be equal in amperage draw.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
I think Ron is on the right track. But the description is not completely clear to me. Let me suggest we name the 16 jumper conductors as follows:
? Phase A conductors are called A1, A2, A3, and A4.
? Phase B conductors are called B1, B2, B3, and B4.
? Phase C conductors are called C1, C2, C3, and C4.
? Neutral conductors are called N1, N2, N3, and N4.
You appear to be saying that only two phases are affected. So I infer that it is something like this:
? Conductors A2, A3, and A4 have the same current, but A1 is one third higher than the others.
? Conductors B1, B2, and B4 have the same current, but B3 is one third higher than the others.
? All four of the C conductor have the same current.
? All four of the N conductors have the same current.
? The possibility that the A, B, C, and N sets might have different currents (i.e., unbalanced loading) is not relevant to the question.
Do I have the situation correctly described? If not, please explain.
If this is the situation, my initial guess is that the two “odd men out” (A1 and B3 in my example) are a size or two larger than all other conductors. Are you certain that all conductors are the same size?
? Phase A conductors are called A1, A2, A3, and A4.
? Phase B conductors are called B1, B2, B3, and B4.
? Phase C conductors are called C1, C2, C3, and C4.
? Neutral conductors are called N1, N2, N3, and N4.
You appear to be saying that only two phases are affected. So I infer that it is something like this:
? Conductors A2, A3, and A4 have the same current, but A1 is one third higher than the others.
? Conductors B1, B2, and B4 have the same current, but B3 is one third higher than the others.
? All four of the C conductor have the same current.
? All four of the N conductors have the same current.
? The possibility that the A, B, C, and N sets might have different currents (i.e., unbalanced loading) is not relevant to the question.
Do I have the situation correctly described? If not, please explain.
If this is the situation, my initial guess is that the two “odd men out” (A1 and B3 in my example) are a size or two larger than all other conductors. Are you certain that all conductors are the same size?
- Location
- Massachusetts
I will be shocked 
if the conductors are different sizes.
Considering that this is temp ......
How 'temp' is it?
Just a bunch of open air conductors bridging the gap where the bad bus was removed?
If so I imagine the routing of each cable is a bit different and that could make the difference.
if the conductors are different sizes.Considering that this is temp ......
How 'temp' is it?
Just a bunch of open air conductors bridging the gap where the bad bus was removed?
If so I imagine the routing of each cable is a bit different and that could make the difference.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
My problem with the situation (if I have correctly understood the description) is the following:
? First, it is one conductor out of the set of four that has a higher current. That means a lower impedance.
? Secondly, the other three conductors in the set have the same current. That means they have the same impedance, and they have a higher impedance then the odd one.
? But if the cause is poor terminations, are we to expect that three conductors have identically poor connections, and that the same situation happened in two phases?
? If the situation had been that one conductor in each set had a lower current, then we could expect to find a bad connection on that one wire. But an identical bad connection on three wires is not to be believed.
As to the possibility that the jumpers are different lengths, with a run as short as 20 feet, the one ?odd man out? single conductor would have to be much shorter (lower resistance, higher current) than the others. Here again, three identical long conductors and one much shorter conductors is not something I would expect to find.
? First, it is one conductor out of the set of four that has a higher current. That means a lower impedance.
? Secondly, the other three conductors in the set have the same current. That means they have the same impedance, and they have a higher impedance then the odd one.
? But if the cause is poor terminations, are we to expect that three conductors have identically poor connections, and that the same situation happened in two phases?
? If the situation had been that one conductor in each set had a lower current, then we could expect to find a bad connection on that one wire. But an identical bad connection on three wires is not to be believed.
As to the possibility that the jumpers are different lengths, with a run as short as 20 feet, the one ?odd man out? single conductor would have to be much shorter (lower resistance, higher current) than the others. Here again, three identical long conductors and one much shorter conductors is not something I would expect to find.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
OK. I did some quick math. Once again presuming I correctly understand the statement of the problem (OP, please confirm or refute), the one conductor (from a set of four serving the same phase) that is carrying a current one third higher than the other three (from the same set of four) will have an impedance that it 75% of that of the other conductors (i.e., lower impedance, higher current). So how do you get this situation? Here are some possibilities:
? If the length of three of the conductors is about 27 feet, and the length of the ?odd man out? conductor is 20 feet, you would get this situation. But I also think you would notice, by simply looking at the installation.
? If three of the conductors are 250MCM copper, and the fourth is 400 MCM copper, you would get this situation. Here again, I think you would notice, by simply looking at the installation, but it might not be quite as easy to spot.
? If three of the conductors had termination lugs made of one material, and the fourth had termination lugs made of a different (lower resistance) material, you might possibly get this situation. I also think you would notice this, but only if you looked at the point at which the conductors are terminated onto the bus.
? If three of the conductors were made of aluminum, and the fourth was made of copper, you might possibly get this situation. Once again I think you would notice this, but only if you looked at label on the conductor jacket, or looked at the point at which the conductors are terminated onto the bus.
And with that, I am out of suggestions.
? If the length of three of the conductors is about 27 feet, and the length of the ?odd man out? conductor is 20 feet, you would get this situation. But I also think you would notice, by simply looking at the installation.
? If three of the conductors are 250MCM copper, and the fourth is 400 MCM copper, you would get this situation. Here again, I think you would notice, by simply looking at the installation, but it might not be quite as easy to spot.
? If three of the conductors had termination lugs made of one material, and the fourth had termination lugs made of a different (lower resistance) material, you might possibly get this situation. I also think you would notice this, but only if you looked at the point at which the conductors are terminated onto the bus.
? If three of the conductors were made of aluminum, and the fourth was made of copper, you might possibly get this situation. Once again I think you would notice this, but only if you looked at label on the conductor jacket, or looked at the point at which the conductors are terminated onto the bus.
And with that, I am out of suggestions.
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