8 Factors you Should Consider When Sizing Conductors

I’ve been spending the last few weeks reading electrical handbooks that focused more on a practical side rather than the theoretical part to help me expanding my knowledge and skills.

Books truly hold the power to accelerate your personal growth and take your life to the next level.

I found that cable sizing is a very important skill that every electrical engineer has to be aware of. So today I’m sharing what I learned about conductor sizing from The electrical systems design specification handbook. In this article, I’m going to present a summary of some main points on cable sizing.

List of contents of this article:

1 — Factors in Sizing Conductors

2 — Formula for Sizing Conductors

3 — Processes of Sizing Conductors

Factors in Sizing Conductors

There are many different factors we should take into account when sizing conductors including:

• The amount of current drawn by the load we want to serve. The cable must be sized based on ampacity (current-carrying capacity). Cable ampacity is the amount of current that the conductor can carry under condition of use.
• Physical distance between the source and the load.

• The ambient conditions in the area where the conductors will be installed.
• The type of method used to mechanically protect the conductors. Popular wiring methods include conduit, cable tray, encasing in concrete and direct burial.

wiring method: Cable tray which is mechanical support used in manage & protects cables

• The resistance of the cable. Cable resistance results in electrical losses (P = I^2*R = heat), which results in a voltage drop.
• Ease of installation. Heavier conductors for large power distribution systems are harder to install or remove.

For loads requiring greater ampere, we will place additional conductors in parallel. Those conductors placed in parallel must be of the same material, be terminated in the same way, and have the same length and cross-sectioned area.

• Access for maintenance and inspection. It should be noted that stranded conductors are easier to terminate and bond.
• Future growth. It is common practice to add 25% to the installed current-carrying capacity to main feeder circuits when performing a design.

Formula for Sizing Conductors

The selection of the conductor size is based on the above considerations including, cables ampacity under actual site installation condition, voltage drop, etc. . For example, size of underground cables is different in case the cable was in air.

The minimum size available for cables can be calculated using the following formula:

Multiply by root three in case 3 Phase

Example: suppose we have the following load:

Max. demand load= 980𝐾𝑉𝐴, 60Hz and 400V (LV) 3 phase system and temperature factor = 0.87, grouping factor = 0.82.

Then, we can calculate the FLA as below:

𝐼 =980 ∗ 10³ / (√3 ∗ 400)

𝐼 = 1415 𝐴𝑚𝑝

There is no cable with current carrying capacity of 1415 Amp available, so we will place additional conductors in parallel. referring to manufacturer cables catalogue table. The available size in the market for multi-core cable is 300mm² with current carrying capacity of 638 Amp. In case four cable placed in parallel then 4 x 638 = 2552 Amp total ampere.

Current Rating

When we apply the derating factors:

2552 Amp *0.88*0.83 = 1864 Amp which fulfill the load requirement (1415 Amp)

Formula For Voltage Drop:

where VD = voltage drop, L = length, I = FLA, R = AC Resistance, cos(theta) = power factor, X =Reactance.

The voltages is dropped due to the distance even in very small distance the VD is existing we cannot avoid it but must be as minimum as possible at most 5% of the main source as per NEC.

let’s say, we have A 50m length, 300mm² cable has an AC resistance of 0.08 Ω/km and an AC reactance of 0.096 Ω/km. So, the VD across the cable is:

• 14.53/400*100=3.6325% which is lower than the maximum permissible voltage drop of 5%.

Processes of Sizing Conductors

1- Calculate the VA to identify the load (V x A x 1.73 for three-phase loads or V x A for single-phase loads).

2- Identify the system voltage and the number of phases.

3- Calculate full load ampere (FLA), considering all the derating factors.

4- Determine what temperature rating is required. See NEC table 310.15(a).

5- Specify the situation of the cable if it direct buried, or run in air, installed on raceway or some other wiring method.

6- Know what is the insulation type for the cable that will properly reject heat and protect the wiring from ambient conditions.

7- Determine the number of conductors required operate to the load.

With this, we come to the end of this article. Thanks for reading. If anyone have comments or suggestions on how to improve the post, please add them below or you want to share more information about the topic discussed above, please don’t hesitate to discuss through gmail or add me on LinkedIn.

References:

1- Efficient Electrical Systems Design Handbook by Albert Thumann.

2- The electrical systems design specification handbook for industrial facilities by DiLouie, Craig Marrano, Steven J.

3- National electric code (NEC).