cable size

I have to install a 50HP motor, Three Phase, 480 Volt, 57.7
Full load Amps, 90 Degree Temperature. The length of cable I would Need is Two Hundred Feet. What size of copper conductor would I need? I am a second year apprentice and I still have trouble with these calculations. Thanks in advance.

If this is a school exercise show us what you have done so far.
If this is on the job, look up the cable manufacturers tables to get the size.

If I was doing it I would use 35mm 3 core neutral screened cable, because it is the most economical cable size that will meet my requirements, but this info is not going to meet your requirements, as we use metric cable sizing.

mm2

02-11-2010 CAD By S.ALANEN

POSITION 50HP


(CABLE Q1-M1) MCMK 3x25+16

CABLE: 1 VALUES

- Type MCMK 3x25+16
- Lenght 61.00 m
- Max fuse (short circ.protection) 160.00 A
- Loading capacity (+25°C) in air 110.00 A
- Loading capacity (+32°C) in air 101.08 A
- Max core temperature 70.00 °C
- Core temp., nominal current 44.38 °C
- Max Current between phases 5360.31 A
- Max Earth fault current in motor 7580.62 A

MOTOR, NOMINAL VALUES

- Voltage 480.00 V
- Power out 37.00 kW
- Rotation Speed 1750.00 RPM
- Current 57.70 A
- Fuse for 5 s start 160.00 A
- Cos Fii .83
- T O R Q U E 201.91 Nm

VALUES IN NOMINAL CURRENT WITH CABLE

- Power loss in Cable .44 kW
- Power loss in Motor 2.77 kW
- Voltage loss .83 %
- Voltage in Motor 476.01 V
- T O R Q U E 200.62 Nm

START VALUES WITHOUT CABLE

- Cos Fii .45
- Is/In 6.70
- Current 386.59 A
- Ms/Mn 2.60
- T O R Q U E 524.98 Nm

START VALUES WITH CABLE

- Voltage in Motor 463.21 V
- Voltage loss 3.50 %
- Current 373.07 A
- T O R Q U E 506.62 Nm

VALUES WITHOUT LOAD

- Power loss in Cable .14 kW
- Current 32.36 A

1 x MCMK 3x25+16 MAX LOAD f(TEMPERATURE), Tmax=70°C
- MAX LOAD (+25°C) in air 110.00 A
- MAX LOAD (+30°C) in air 103.71 A
- MAX LOAD (+35°C) in air 97.01 A
- MAX LOAD (+40°C) in air 89.81 A
- MAX LOAD (+45°C) in air 81.99 A
- MAX LOAD (+50°C) in air 73.33 A
- MAX LOAD (+55°C) in air 63.51 A
- MAX LOAD (+60°C) in air 51.85 A
- MAX LOAD (+65°C) in air 36.67 A

One Core Resistance in 25°C : .73 Ohm/km
One Core Inductive Reactance: .09 Ohm/km/ 60 Hz

Second year apprentice huh?
You have the Canadian flag showing but saying Houston. I am confused are you in Canada or USA? So I first need to know what code book you are working from.

If you are confused by the numbers the hard part of the story is two fold
1. It takes practice
2. Who is going to do it when you are a journeyman? Are you really going to be able to trust some EE in the office?

Here is your assignment from me
Tell me the considerations you must make when picking the conductor - include ambient temperature, length of wire run and all of them.

Oh and while you are at it what considerations must you use for choosing and sizing conduit. I want 5 conductors in it 3 phases, neutral and a ground.

Dan Bentler

clarke2006 said:

I have to install a 50HP motor, Three Phase, 480 Volt, 57.7
Full load Amps, 90 Degree Temperature. The length of cable I would Need is Two Hundred Feet. What size of copper conductor would I need? I am a second year apprentice and I still have trouble with these calculations. Thanks in advance.

Click to expand...

In the Uk - sorry I can't comment on Canada - you would need to start by deciding what type of cable to use, based on the requirements of the installation ie is armoured cable needed because of the risk of mechanical damage.

Think of the heating effects of the current it will carry and the volt drop in the cable.

Also, remember the rating of the cable shall be greater than that of the protective device. (short circuit protective device NOT overload)

Next apply any de-rating factors such as method of installation (Like is the cable buried), grouping factors (Other warm cables which raise the temperature of your cable),ambient temperature, thermal insulation. divide these into your protective device rating.

This gives the minimum current capacity of the cable. You may find you need a cable considerably larger than you thought!

Now this tells you which table to look at.

Select the cable accordingly and look at the volt drop.
Multiply the volt drop/amp/metre and see if is within acceptable limits.

If not, go up a size.

markie said:

In the Uk - sorry I can't comment on Canada - you would need to start by deciding what type of cable to use, based on the requirements of the installation ie is armoured cable needed because of the risk of mechanical damage.

Think of the heating effects of the current it will carry and the volt drop in the cable.

Also, remember the rating of the cable shall be greater than that of the protective device. (short circuit protective device NOT overload)

Next apply any de-rating factors such as method of installation (Like is the cable buried), grouping factors (Other warm cables which raise the temperature of your cable),ambient temperature, thermal insulation. divide these into your protective device rating.

This gives the minimum current capacity of the cable. You may find you need a cable considerably larger than you thought!

Now this tells you which table to look at.

Select the cable accordingly and look at the volt drop.
Multiply the volt drop/amp/metre and see if is within acceptable limits. If not, go up a size.

Click to expand...

Dang it you just did his homework for him.

Dan Bentler

leitmotif said:

Dang it you just did his homework for him.

Dan Bentler

Click to expand...

I missed off the answer!! Notice I used the phrase "Think of..." Hopefully he will. If there is a new post asking "How do you calculate volt drop.." I have failed.

I did all this again last year - I am 47 - as the Institute of Electrical Engineers update the requirements for electrical installations quite regularly.