# Cable Sizing Calculations

1. Oct 26, 2010

### sportynumair

Can someone please guide me (with an example if possible) on Cable Sizing Calculations. I want to learn the step by step procedure alongwith calculations that are performed during the preparation of Cable Sizing Documents. Thanks in advance!

2. Oct 26, 2010

### Staff: Mentor

Welcome to the PF. What is the context of your question? How much current and how high a voltage do you have in mind?

One place to start is with the standard wire tables:

http://www.powerstream.com/Wire_Size.htm

.

3. Oct 26, 2010

### sportynumair

Thanks Berkeman....But i want to learn the complete step by step calculation done in (for example oil & gas industries) to prepare cable sizing documents.... majorly with a 3phase voltage of 415V....

4. Oct 26, 2010

### sparkey

your cable sizing is entirely dependent on your peak amp draw and the type of wire you are using (copper or aluminum and what insulation class). for this example use this chart.

http://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

say you are installing a system with a 150A motor (72000Watt ~100HP motor based on 480V) using standard copper wire.

if all of your devices (motor starters, contact blocks, disconnect switches) are rated at 75C then you would use a 75C insulation class, ie:THW. as you look down the chart for the THW insulation class you come to 1/0 being equal to 150A

if your devices are rated at 90C you could follow a 90C insulation class, ie: THHN which would bring you to a 1AWG copper wire.

then you would need to consider your amp draw of your control devices (motor starter coils and other loads ie: transformers, relays, sensors). for the ease of this example we will say you have 3KVA (3000Watts) of control device draw which brings you to a grand total of 75000watt draw.

75000watt/480v=156.25A

6.25A does not look like much but you cannot just say eh close enough, 150A is just the Full Load Amps and shouldn't draw that much anyways. (keep in mind motors can draw upwards of 600% their Full Load Amps (FLA) on start up.

for the extra 6.25 amps you will have to use either 1/0 or 2/0 depending on your insulation class.

5. Oct 26, 2010

### sportynumair

Thank you sparkey......I will study the chart you provided.

6. Oct 26, 2010

### sparkey

just note that the chart provided is from the NEC (National Electric Code) and off had am not certain if that is the exact up-to-date chart. I'm not sure where you are from but different countries may have different figures to use (i know mexico for example has no NEC, just industry standards). if you in\going into the EE field in the states, it would be a good idea to have a copy of the NEC on hand.