3 Phase Pump fed by different size wire.

[Springer250]

I had a question asked to me and could not think of an answer.

If you are feeding a 3 phase 120/208V pump at 400' from feed. Can you use different size wire to feed it?

i.e use a 4/0 4/0 2/0 with a 250 MCM single

What is the problem with doing this? any voltage/amperage issues?

 

[jumper]

I will allow this as it is just a basic code/theory question and not a direct DIY question.

I will ask members to please stick the question asked and not get into installation advice or query OP about other areas.

 

[GoldDigger]

I had a question asked to me and could not think of an answer.

If you are feeding a 3 phase 120/208V pump at 400' from feed. Can you use different size wire to feed it?

i.e use a 4/0 4/0 2/0 with a 250 MCM single

What is the problem with doing this? any voltage/amperage issues?

There is probably not a code problem with using two different size ungrounded conductors to a three phase pump as long as the smallest wire is allowed by the code.

If the smaller wire size is permitted by code and you are using larger wire to reduce the voltage drop a different theoretical problem comes up which may or may not have any practical consequences:

Three phase motors are designed to run with an input voltage within a specified range, but it is assumed that the three phase (line to line) voltages are equal, whatever that value may be.
If you have a significantly (and here is where the issue becomes a practical one. What is the threshold?) higher voltage drop on one of the three wires it will produce unequal voltages on the three motor windings. That will lead to a larger percentage difference in current between the three windings and could potentially lead to overloading one or more of the windings without the motor's overloads detecting the condition. It could also cause the motor to run less efficiently.

There are very specific guidelines in the code that allow you to use a smaller EGC compared the ungrounded conductors, but even this gets complicated when you oversize the ungrounded conductors to decrease a problematic voltage drop.

 

[ptonsparky]

There are a lot of other questions....

I say, No, there is not a significant problem with the larger wire or even the smallest as long as it is adequate for the load served. I assume you are using the three larger conductors for phase wires, but it could be the three smaller or one of each size. Answer is still the same, even though I cringe at the thought.

 

[GoldDigger]

There are a lot of other questions....

I say, No, there is not a significant problem with the larger wire or even the smallest as long as it is adequate for the load served. I assume you are using the three larger conductors for phase wires, but it could be the three smaller or one of each size. Answer is still the same, even though I cringe at the thought.

... i.e use a 4/0 4/0 2/0 with a 250 MCM single....

I agree that it is not obvious from that which are the phase wires, which the neutral (if any, since it would not be used by the pump), and which the EGC.

But there is no way that all three phase wires could be the same size based on that.
The "4/0 4/0 2/0" sounds to me like a reference to a three wire cable intended originally for 120/240 with a reduced neutral.

I would have more of a problem, now that the point has been raised, with three of the circuit conductors being part of a cable and the fourth being run outside the cable.

 

[ptonsparky]

I agree that it is not obvious from that which are the phase wires, which the neutral (if any, since it would not be used by the pump), and which the EGC.

But there is no way that all three phase wires could be the same size based on that.
The "4/0 4/0 2/0" sounds to me like a reference to a three wire cable intended originally for 120/240 with a reduced neutral.

I would have more of a problem, now that the point has been raised, with three of the circuit conductors being part of a cable and the fourth being run outside the cable.

Not being allowed to ask questions raises more. I see the 250 being trenched in some feet away from the original 1PH system. Someone else will have to explain how the added impedance to the Ground Fault path, if that is how the added conductor is being used, will affect the clearing time of SC protection.

Will the added impedance due to the increase in distance from the other conductors be offset by the increase in size?

 

[Jraef]

I had a question asked to me and could not think of an answer.

If you are feeding a 3 phase 120/208V pump at 400' from feed. Can you use different size wire to feed it?

i.e use a 4/0 4/0 2/0 with a 250 MCM single

What is the problem with doing this? any voltage/amperage issues?

I'm not grasping what the heck you are talking about here in the bold text.
I'm also not sure you understand what you have. A 3 phase motor is NOT "120/208", it is 208V. The 120V is for phase to NEUTRAL loads, the motor does not use the Neutral, so you don't run a neutral wire to it.

...That will lead to a larger percentage difference in current between the three windings and could potentially lead to overloading one or more of the windings without the motor's overloads detecting the condition. It could also cause the motor to run less efficiently.

I wholeheartedly agree. Even a slight phase voltage imbalance leads to a lot more current imbalance and the creation of negative sequence currents flowing in the motor, which leads to negative torque in the rotor, resulting in 2 of the phases "fighting" the third to keep the motor spinning. More of the current that is flowing in the circuit is going toward heating than it should, but the OL relay will only see the total and the trip point is based on that being EVEN across the phases. So what can happen is that even if the current is below the trip threshold of the OL relay, the THERMAL EFFECT of that unbalanced current on the motor is as if the motor WAS overloaded, continuously, and yet the OL will not protect it.

I've seen if happen where there was just a significant difference in the length of one cable, which changed the circuit resistance in that one phase (someone didn't want to cut the cable for fear they would "waste" it) and burned up a 500HP motor, yet the OL relay never tripped because the current never exceeded the FLA.

In this case you would have (presumably) 2 phases with 4/0 cable that has a resistance of .049ohms/1000', so a total resistance of .0196ohms, and one phase of 2/0 cable with a resistance of 0.078ohms/1000' so a total resistance of .0312ohms, over 1-1/2 times as much resistance. It will make a difference in the current balance. If the motor is never loaded to more than 50% of it's rating you may never know the difference, but if the motor is loaded to 98% of it's rating you may lose the motor. Where the line is going to end up is difficult to predict, so the safe bet is to just use the same size cable for all 3 phases and not worry about it.

 

[don_resqcapt19]

It would be a 300.3(B) violation....not all of the circuit conductors are in the same cable. (assuming that the 4/0, 4/0, 2/0 is a cable assembly)

 

[iwire]

I'm not grasping what the heck you are talking about here in the bold text.

I assume one of them is an EGC.