To reduce resistance for motor circut 2

[toolmantwo]

I have a customer who wants to run motors at 1-1/2 HP 230 Volt 60 Hertz single phase. The problem is that he has run 10 gage wire to a distance of 700 feet from the source.

The resistance is so high we lose voltage to the point of under 180 volts.

The nearest motors will start and run OK, the far motors will start to turn very slow and never reach full RPM.

These motors drive large ventilation fans.

Do we have options other than running 8 gage wire?

Thanks

 

[wareagle]

NEC lists a 1 1/2 HP at 230 volts 6.6 amps. Voltage drop
over 700 ft #10 is only 9 volts. What else do you have on this circuit? HP and Amps load? Distance? What is the voltage at the begining of this long circuit?

 

[waross]

Hello toolmantwo;
One 10 amp motor at 700 feet will require #6 AWG to comply with the 3% voltage drop rule.
If you feel you can justify using 5% voltage drop then each 10 amp motor will require #8 AWG. If your talking about feeders for three motors I will assume that the motors are evenly spaced.
233 feet to the first motor at 1.06% voltage drop.
Use #2 AWG.
233 feet to the second motor at 1.12% voltage drop.
Use #2 AWG.
233 feet to the third motor at 0.89% voltage drop.
Use #6 AWG.
At this point you are actually 0.07% over the allowable 3% voltage drop, but I doubt that you would be called on it.

You can use auto transformers to boost the voltage at each motor only if the motors are on individual conductors.

If you size auto transformers to provide the correct voltage to the farthest motor with everything running and two motors shut down, the voltage on the last motor will rise too high. In addition I don't think that an autotransformer scheme would be acceptable to satisfy the 3% voltage drop rule.

If you ran a seperate branch circuit to each motor, you would still have excessive voltage drop when starting, and still possibly have code compliance issues.

Go conventional transformers and 600 volts on the feeder.
#8 AWG will handle the load easily to the end of the 700 feet at 2.56% voltage drop.
If your fans are spaced out as your information suggests, you may be able to use a combination of # 10 AWG and #12 AWG
wire.
yours

 

[waross]

Hi wareagle
I guess i was writing as you were posting.
My code books are a little out of date.
Both my Canadian and American code books were printed when a 1.5 Hp. motor required 10 amps at 230 volts (even though the standard utilization voltage is 240 volts).
Your figure of 6.6 amps looks more reasonable.
However, in regards to the voltage drop it appears you have calculated the voltage drop due to resistance. I used a voltage drop table that is based on the impedance of a typical circuit rather than the resistance. This results in a higher voltage drop. I susspect the correct answer is somewhere in between.
respectfully

 

[toolmantwo]

We think he has them evenly spaced inside the 700 feet with 20 fans in total. The nearest fans are working OK.

The motor draws 7.8 amps on each fan.

I should get more exact information soon.

 

[waross]

I think your best bet may be 480 volts or 600 volts.
You can put small transformers with a small breaker panel at each transformer to feed 1 or more motors.
Calculate the transformer size for 1 motor per transformer, 2 motors, 3 motors, etc.
Then do a series of estimates to get the most cost efffective sollution.
Estimate #1 20 panels, 20 small transformers.
Estimate #2 10 panels, 10 double size transformers plus extra wire and raceway.
Estimate #3 7 panels, 7 triple size transformers plus extra wire and raceway.
Etc.
respectfully

 

[jraef]

Toolmantwo,
As painful as it sounds, I think waross' first statement is the only viable option, although at 7.8FLA (instead of 10FLA as he calculated on), you may indeed get away with #8 wire. Given that you say there are 20 fans, there must be separate runs for each fan.

waross, don't forget that these are 1 phase. To change to 480 or 600V he would need to change out the motors and power source! Unless you meant for him to boost to 600V 1 phase for the run and transform down to 230V at each motor site? That may be an option, but I would think it would be cheaper and easier to just run new cable. Then again, if the conduit is buried and too small for the #8 wires, it may indeed be the only viable option.

Side note to wareagle:
I think you flipped your chart open to the DC motor section. 1-1/2HP 240VDC motor FLC is 6.6A

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Hello jraef;
Yes I did mean to use step down transformers at each motor or group of motors and use the existing motors. With ingenuity in the layout, most of the existing wire could be reused. I used a similar arrangement once with one small motor at about 1000 feet and a second at about 1800 feet.
When you compared the price of cable to the price of transformers it was overwhelmingly in favor of the transformers.
respectfully

 

[wareagle]

Guys I looked at the 3 phase table. 10 amps is the correct answer from table 430.248

 

[toolmantwo]

Thanks guys

This is what our group have for conclusions:

The maximum theoretical length calculation shows that the longest #10 wire length should be no greater than 450’ with our current motor We have tested a M742 off of the shelf with 700’ of #10 conductors with start voltage of 230 volts and have 94 volts at the end of the 700’. The customer had stated to our serviceman that they were experiencing 180 volts at their installation.

After checking into the National Electric Code for acceptable voltage drop levels, the NEC (Art. 210-19,a,1,FPN # 4) states that 5% voltage drop is acceptable. After calculating the voltage drop for a branch circuit of that length and to stay within the 5% of voltage drop a #8 conductor should be used.

They will not enjoy running more wire, yet you pay the price if you take shortcuts.

 

[jraef]

Careful. Although technically acceptable, a 5% voltage drop will increase current, which will increase the motor temperature for the same given load without the voltage drop, shortening the comparable life of the motor.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Hello toolmantwo;
In regards to the 5% figure; The maximum voltage drop for branch circuits is 3%. The maximum voltage drop for feeders is 3%. The maximum total is 5%.
If you already have 3% drop on the feeders you would be limited to a 2% voltage drop on your motor circuit conductors.
Assuming that the feeder voltage drop is under 2%; my voltage drop tables show #8 AWG as suitable for up to 616 feet. I suggest that you give serious consideration to using #6 AWG for the last few motors.
This will also address jraefs concerns.
respectfully