# Conductor size and Voltage Drop for Design

• Dwa
In summary, if you want to use a 300 horsepower electric motor, you need a 3-phase, 480 volt power supply, and a cable with a minimum conductor size of 400MCM.
Dwa
If specs on a 300Hp electric motor;
480 Volts
3 Phase
60Hz

The full load Amps of the motor is 361A, which I found in the NEC table. My distance is 700ft to a control center and I can't seem to figure out what copper conductor size to use. In order to find the Voltage drop I have to know the CM, I assume. Can anyone help me with my design?

Time for an electrician - the conductor (cable) selection depends on, location, temp rise, how is it run (Conduit, Raceway, underground - etc). This is in the NEC - but available from a number of places online ( http://sparkyjohn.com/ampacity/ampacity.pdf )

The cable is selected to ensure it does not overheat.

Then once the cable is selected you can find the V drop along A length V = Res ( Ohm/foot * 700Ft) * I current. Noting that the voltage at the load has 2 x the voltage drop of on length ( in 3 wire this is not totally accurate but will suffice)

I would also say - the Generator should have a Circut Breaker - at the generator. The cable really needs to be sized based on the breaker rating and or setting - not the generator.

If the load is known, for example a 200A panel, you may also be able to economized by using a smaller breaker at the generator and running a smaller cable.

Since an error in this system can cause a fire (that is why the NEC in part of the Nat Fire Protection Assn) - I refer back to my first line.

Ok, from some Voltage Drop online calculators said 400kcmil would be the minimum conductor size to use in a conduit. Would that be the right ideal? The environment near saltwater, so I'm using Aluminum conduit for no corrosion.

Thanks

Umm - Aluminum and Salt water not a good combo, PVC would be better if you can get it to withstand the cable pull -

Per the NEC - 3 conductors in a conduit, 400MCM is an odd size, you may have better luck with the 500MCM (kcmil) - if you can afford the wire a size larger is often a good idea, espically if you will be starting large motors / high load.

Pulling 4 x 500MCM - in anything - is HARD - again, I refer back to a pro. I am EE with 15 years hands on service of electrical switchgear (480VAC up) - and I would PAY for a union electrician to do this out of my own pocket before I would try to tackle the job.

should be plenty of Ampacity charts from wire suppliers.

On a run that long check voltage drop it may be more limiting than thermal.

just a tip -
Beware of iron conduit fittings around high current AC conductors.
If you use any be sure all 3 phases go through same fitting.
Magnetic field around a single phase carrying hundreds of amps can heat a closed iron loop that surrounds the conductor. It makes a 60 hz induction heater.
If all 3 phases go through, currents add to zero of course.
We had trouble with iron conduit elbows in a big run buried under concrete... contractor had run out of PVC ells.

Yes, I do agree that Al is not a good combo, put it's at least can withstand the cable pull. Or maybe steel could be better, but corrosion is worse in that matter. I'm thinking 500MCM could be a better choice. Do you think it wouldn’t make any much difference though between the two sizes? I mean you have lesser Voltage load at the end of the circuit or wire. Heck, will the voltage drop for 400MCM-500MCM or above still meet NEC? You're right though; regardless to pull any wire size like that is unimaginable lol...

Thanks for pdf Jim H, helped me get a better idea.

http://www.pearen.ca/Reference/wire.htm

shows 0.00002074 ohms/ft for 500mcm

which X 700 ft X 360 amps = 5.24 volts drop, just over 1%

Last edited by a moderator:

## 1. What is conductor size and why is it important for design?

Conductor size refers to the diameter or cross-sectional area of a wire used for electrical conduction. It is important for design because it affects the amount of current that can flow through the wire and the amount of voltage drop that occurs.

## 2. How is conductor size determined for a specific electrical circuit?

The conductor size is determined by considering the amount of current that will flow through the circuit, the type of material used for the wire, and the acceptable voltage drop for the circuit. These factors are used to calculate the appropriate size of wire needed to safely and efficiently conduct the electricity.

## 3. How does conductor size affect voltage drop?

The larger the conductor size, the lower the resistance of the wire and the lower the voltage drop will be. This is because a larger wire can carry more current without overheating, which reduces the amount of voltage lost as heat. Therefore, using a larger conductor size can help to minimize voltage drop in a circuit.

## 4. What is the acceptable voltage drop for a circuit?

The acceptable voltage drop for a circuit varies depending on the application. For most residential and commercial circuits, a voltage drop of no more than 3% is recommended. However, for sensitive equipment or long-distance power transmission, a lower voltage drop may be necessary.

## 5. How does conductor size affect the cost of an electrical project?

The cost of conductor size is directly related to the amount of material used. Therefore, using a larger conductor size for a project will typically increase the cost of materials. However, this cost may be offset by the benefits of lower voltage drop and more efficient electricity transmission. Ultimately, the appropriate conductor size for a project should be determined by considering both the cost and performance factors.

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