parallel feeds

[chevyx92]

If you have three sets of wires feeding each phase and you amp probe let's say "A" phase and the three wires on that phase each read 40 amp. That reading isn't additive meaning it's not really120 amps in "A" phase. Correct? Got into a discussion with coworker and he thinks you add the readings.

 

[iwire]

More or less yes.

There will typically be some meter error and of course the load may change while you are moving the meter around.

 

[roger]

If you have three sets of wires feeding each phase and you amp probe let's say "A" phase and the three wires on that phase each read 40 amp. That reading isn't additive meaning it's not really120 amps in "A" phase. Correct? Got into a discussion with coworker and he thinks you add the readings.

If each of the three conductors connected to A measured 40 amps individually you would in fact add them and you would have 120 amps flowing on A phase. The key is current division

Roger

 

[jap]

If each of the three conductors connected to A measured 40 amps individually you would in fact add them and you would have 120 amps flowing on A phase. The key is current division

Roger

And this is something I still cant wrap my head around..

JAP>

 

[ron]

And this is something I still cant wrap my head around..

JAP>

The three 'A' phase conductors are connected to the exact same spot on the upstream and downstream ends of the conductors. The current flows essentially equally on each one, dividing the total current / flow by three in this case.

Imagine if you had a water faucet and had a three way splitter. You connect three garden hoses to the three outlets on the faucet. When you turn the water on to fill your pool, the water will essentially flow the same out of each hose into the pool, similar to the paralleled conductors. This analogy is to think of it another way that might be more tangible.

 

[Sparky2791]

Think of it this way. If you have one large current draw on the load side of the paralleled feeder and have three separate paths to pull the load from the line side feeding it, electricity takes the path of least resistance so provided the paralleled conductors are all of the same length, which is required by NEC, you have equal resistance for the loads path you have equal distribution of the load on each conductor. If one of the paralleled conductors was significantly longer it would have a higher resistance so the load would favor the other conductors.

 

[GoldDigger]

An interesting analogy that also points out some of the pitfalls of the method is this:

Let's say you have an RV and you want to measure its weight, but you have only a single wheel scale to use.
You jack up one wheel at a time, putting the scale under each one, and then add up the readings.
The potential problem is that with any wheel held the thickness of the scale off the ground, that wheel will be bearing more of the vehicle's weight that it does with that wheel on level ground.
So your four wheel total will be higher than the actual weight.

In the case of the amperage measurement, either a clamp meter or a good ammeter will not have a significant effect on the current flowing in that wire. So you can add up the meter readings. And you should find that they are all equal to within the accuracy of the meter, or else there is a problem with your parallel conductors. But if you have to actually disconnect the wire to make the reading, you have a chance that the change in contact resistance, etc. causes a lower current flow in whatever wire you are measuring. This risk of error is greater for shorter wires, since you do not have as high a common resistance in all three wires to even out the measurements.

Now if you are talking about adding currents in general and not just on parallel paths, then you can have real world effects if you do not know the power factor of each load/circuit and so cannot do the required vector addition to get the total current.

 

[m sleem]

As long as R1=R2=R3 then I1=I2=I3 & I=I1+I2+I3

 

[electricalist]

Parallel feeders show exactly what you dont want neutral to do,,,,,divide itself to other paths

 

[Carultch]

If you have three sets of wires feeding each phase and you amp probe let's say "A" phase and the three wires on that phase each read 40 amp. That reading isn't additive meaning it's not really120 amps in "A" phase. Correct? Got into a discussion with coworker and he thinks you add the readings.

Let's hope that the readings are additive. Because if not, that means there is probably a ground fault.

For a healthy feeder of multiple wires per phase, the total phase current is the sum of the current in each wire of that phase. And the current is divided equally, if they are constructed properly.

 

[Carultch]

If you have three sets of wires feeding each phase and you amp probe let's say "A" phase and the three wires on that phase each read 40 amp. That reading isn't additive meaning it's not really120 amps in "A" phase. Correct? Got into a discussion with coworker and he thinks you add the readings.

Let's hope that the readings are additive. Because if not, that means there is probably a ground fault.

For a healthy feeder of multiple wires per phase, the total phase current is the sum of the current in each wire of that phase. And the current is divided equally, if they are constructed properly.


Here is an interesting problem for you, on the subject of parallel feeder conductor sets.

The NEC requires the parallel wires per phase to all be the same length, and have many other similar characteristics as well. However, in a practical sense, it isn't always possible to get them exactly the same length. Suppose you are a contractor who built a 500A feeder with two parallel sets of 250 kcmil Cu. One set of wires ran a length of 45 ft, and the other set of wires ran a length of 48 ft. The inspector notices this, and challenges you to prove whether or not it is a problem.

At the full 500A, what is the current in each set of this parallel feeder?
Will the ampacity of any of the conductors be exceeded?

Answers: 242A in the 48 ft set, 258A in the 45 ft set. YES, unfortunately.

 

[don_resqcapt19]

Let's hope that the readings are additive. Because if not, that means there is probably a ground fault.

For a healthy feeder of multiple wires per phase, the total phase current is the sum of the current in each wire of that phase. And the current is divided equally, if they are constructed properly.


Here is an interesting problem for you, on the subject of parallel feeder conductor sets.

The NEC requires the parallel wires per phase to all be the same length, and have many other similar characteristics as well. However, in a practical sense, it isn't always possible to get them exactly the same length. Suppose you are a contractor who built a 500A feeder with two parallel sets of 250 kcmil Cu. One set of wires ran a length of 45 ft, and the other set of wires ran a length of 48 ft. The inspector notices this, and challenges you to prove whether or not it is a problem.

At the full 500A, what is the current in each set of this parallel feeder?
Will the ampacity of any of the conductors be exceeded?

Answers: 242A in the 48 ft set, 258A in the 45 ft set. YES, unfortunately.

Yes, the shorter the runs are and the closer the load is to the total ampacity of the parallel conductors the more likely it is that you will exceed the ampacity of the shortest conductors. In this example there is over 6% difference in the lengths. The percent difference and not the actual footage difference is what you need to be looking at.