How do you calculate cable size for given load?

Answer 1

Generally speaking, you would find conductor ampacity (the amount of current it can handle continuously) in Table 310.16 of the NEC (National Electrical Code). The numbers in the table are good as long as you don't go over the stated number of conductors at the top of the table and the 86 degree Celsius ambient temperature. If anything changes, use the adjustment factors for conduit fill and ambient temp.

However, with all the possible other applicable sections of the NEC, it really depends on the type of load and howand where the cable is fixed because its operating temperature will depend on whether it is enclosed in a duct, is buried underground or is outside in the open air.

For a typical load, not knowing anything else about it, I would multiply the load (in amps) by 1.25 and find a wire with at least that ampacity.

<><><>

As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.

Answer 2

The wire size is based on the amperage of the load. With this amperage the electrical code book has tables that show the maximum amp capacity that each conductor can legally carry. With this wire size additional calculations are made if there is distance involved to reduce voltage drop of the wire. Also calculations are made to readjust the size of the wire if the ambient temperature that the wire passes through are higher than normal.

Answer 3

125% for continous loads.. 100% for all others factoring in ambient temp. for wire sizing

----------------------

The process of calculating service conductor sizes is quite complicated and cannot be explained in the space provided here. Many qualified electricians cannot do the calculations. I'm not being critical. Most electricians have never learned the process. Even most of us who have passed the Master Electrician exam, which requires you to be able to do this, have never had to do it in the field.

The basic premise of the calculations is based on the fact that you will not run everything electrical all at the same time. Branch circuits, those that run to each piece of equipment, are sized for that equipment being on. But you would never run everything all at once. So the more circuits you have, the larger the building, the more calculating there is to do.

Answer 4

The formula is already calculated and is found in the electrical code book for easy reference. The amperage is given for a specific insulation temperature in degrees C. Look across to find the wire size recommended for the amperage of the load.

Answer 5

Article 310.16 of the national electrical code

Answer 6

you need to know the amperage for the circuit... then use the wire size table of the NEC

Answer 7

Electricians use the tables in the electrical code book.