Voltage drop-two loads

[seifdogg]

We have a 50A and 10A load 1900' from source currently running fine on #4/0 AL. I told the customer we could move the 50A load 1000' farther without replacing existing wire, using the logic that if it's running now, as long as we size wire correctly to the new location of the 50A load, it would continue to run fine. I'm second guessing myself now after running voltage drop formulas. Can I overcompensate for my mistake by oversizing wire the rest of the way?


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[just the cowboy]

You need actual load

You need actual load

You need the actual load. 1900' loaded at 50 amps needs 600 mcm for 3% drop. Without more info such as wire type ,conduit or direct burial, voltage and phase. Your #4s show around a 25% drop at that load.

As to continuing out with proper size it would have to be calcaulated at 0% drop to insure no change of operation.

 

[charlie b]

We have a 50A and 10A load 1900' from source currently running fine on #4/0 AL.

What do you mean "running fine"? Have you measured voltage at the end of the 1900 foot run? What are the loads? Are they single phase or three phase? What is the voltage? I can't give you an answer without more information.

 

[seifdogg]

What do you mean "running fine"? Have you measured voltage at the end of the 1900 foot run? What are the loads? Are they single phase or three phase? What is the voltage? I can't give you an answer without more information.

By running fine, I mean they have had the motors running at full load 5 months out of the year since 1983 without any problems. Did not test voltage assuming it was fine. 480V 3 phase system. Generally the power company provides approx. 492V. Assuming a 3% voltage drop there was probably somewhere around 475 volts at the motor when running. My question is more about keeping the existing #4/0 from source to 1st load, then installing something around a #350 the rest of the way. Can't wrap my mind around that. Seems like you would start with bigger wire, then downsize from there.


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[ptonsparky]

It doesn't matter if the wire increase is at the end or start of the circuit. Use the resistance for the total distance. 1+2=2+1.

 

[charlie b]

If the load is 60 amps, 1900 feet of 4/0 aluminum gives a voltage drop of about 20 volts, or 4%. If you leave the 10 amp load at that location, and move the 50 amp load another 1000 feet away, and use 350 MCM aluminum wire for the next 1000 feet, it will result in an additional voltage drop of 5.25 volts, or another 1%. That would provide a voltage at the motor of 455 volts, or about 95% of the nominal 480V. If the power company gives you an extra 12 volts to begin with, your voltage at the end of the 2900 foot run would be about 467 volts, or about 97.3% of nominal. That should be fine for operating the motor.

Now, having said that, if this system has been operating since 1983, it is nearly 34 years old. I typically think of electrical equipment, including wires, as having a useful life of about 40 years. That is by no means a hard and fast rule, and many members of this forum will tell stories about equipment lasting far longer. But what I mean to say is that if I am dealing with equipment that is approaching 40 years of age, I will start having concerns about its insulation systems and other characteristics. So before you give your customer any assurances that the scheme you are suggesting will "work fine," I would suggest to the customer that there is value in spending some time testing and evaluating the condition of both the wires and the motors.

 

[seifdogg]

If the load is 60 amps, 1900 feet of 4/0 aluminum gives a voltage drop of about 20 volts, or 4%. If you leave the 10 amp load at that location, and move the 50 amp load another 1000 feet away, and use 350 MCM aluminum wire for the next 1000 feet, it will result in an additional voltage drop of 5.25 volts, or another 1%. That would provide a voltage at the motor of 455 volts, or about 95% of the nominal 480V. If the power company gives you an extra 12 volts to begin with, your voltage at the end of the 2900 foot run would be about 467 volts, or about 97.3% of nominal. That should be fine for operating the motor.

Now, having said that, if this system has been operating since 1983, it is nearly 34 years old. I typically think of electrical equipment, including wires, as having a useful life of about 40 years. That is by no means a hard and fast rule, and many members of this forum will tell stories about equipment lasting far longer. But what I mean to say is that if I am dealing with equipment that is approaching 40 years of age, I will start having concerns about its insulation systems and other characteristics. So before you give your customer any assurances that the scheme you are suggesting will "work fine," I would suggest to the customer that there is value in spending some time testing and evaluating the condition of both the wires and the motors.

Charlie,

That was a very helpful and educated response to my question. I had never considered portions of a circuit broken out as mathematical formulas, just always started at the source and worked "downhill". Just when you think you know everything related to electrical theory. Concerning degradation of electrical equipment, I hadn't thought that wire might have a shelf life but given the constant temperature expansion and contraction it makes sense. One more thought: since motors are almost always rated 115/230/460 instead of the voltage we supply them with, 120/250/480, wouldn't they run (although certainly not as efficiently) on the lower voltages?


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[ActionDave]

Charlie,

That was a very helpful and educated response to my question. I had never considered portions of a circuit broken out as mathematical formulas, just always started at the source and worked "downhill". Just when you think you know everything related to electrical theory. Concerning degradation of electrical equipment, I hadn't thought that wire might have a shelf life but given the constant temperature expansion and contraction it makes sense.

The wire is going to last nearly forever under normal or even harsh conditions. Equipment, switches, breakers, starters, other associated parts, on the other hand, can become worn or alternately froze up if not maintained.

One more thought: since motors are almost always rated 115/230/460 instead of the voltage we supply them with, 120/250/480, wouldn't they run (although certainly not as efficiently) on the lower voltages?


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Yes, they run fine plus or minus twenty percent of the the nameplate voltage.

 

[iwire]

Yes, they run fine plus or minus twenty percent of the the nameplate voltage.

Dave, I know one of your things is ignoring voltage drop and I do sometimes as well.

But saying a motor will run fine at 20% drop is going too far in my opinion. That is more DIY than professional type work

It might run fine if it is underloaded but if it is mechanically loaded close to its HP rating 20% under voltage is not going to be 'fine' it might not even start due to the additional voltage drop due to locked rotor current.

 

[ActionDave]

Dave, I know one of your things is ignoring voltage drop and I do sometimes as well.

But saying a motor will run fine at 20% drop is going too far in my opinion. That is more DIY than professional type work

It might run fine if it is underloaded but if it is mechanically loaded close to its HP rating 20% under voltage is not going to be 'fine' it might not even start due to the additional voltage drop due to locked rotor current.

I really do hate all the hand wringing that has been going on about voltage drop the last few years, but you are correct, I should not have said twenty percent. The main thing is that motors are rated at 230V and 460V rather than 240V or 480V to account for nominal voltage variations and some voltage drop or over voltage.

Where is that chart of yours?

 

[ggunn]

...Just when you think you know everything related to electrical theory...

I would never think that. I learn something new practically every day.

 

[cpinetree]

I'll take a stab at it.
I used a segmented voltage drop calculator (I believe from someone on this site, and I'm sorry I can't make proper attribution)

I down sized the extra 1000' to 4/0 Alum. I didn't see a need to run 350mcm, the VD is under 3% and with the higher starting point voltage you are within 2 volts of your target (480v).

I get a total of 2.83% at your last point.
I used 492v starting point. The end point voltage is 478v

see the att'd pic



Found the original post that the above calculator came from: http://forums.mikeholt.com/showthread.php?t=141401