Motor Wire Size Calculator
Engineers who have successfully sized and specified electric motors to meet their application requirements will agree that the design process is quite involved. It demands a ton of iterative steps, calculations, and design considerations to arrive at a system that works as expected.
But just because the motors have been specified, it doesn’t mean engineers should just take the plunge with installing and commissioning these motors. Instead, they want to also select the right cable size for the motor and the application.
Wrongly specifying cable sizes causes several undesirable effects. For example, an oversized cable incurs unnecessary costsand power losses. In contrast, undersized cables typically burn during operation, causing risk to human life, the motor, and infrastructure.
This article presents four important factors that engineers must consider when specifying a cable wire size for their application. The article will also include some key electric motor wire sizing calculations and tables.
Electrical conductor sizing
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1. The Type of Motor
Motors typically fall under two categories:
- High Tension (HT) motors
- Low Tension (LT) motors
High Tension (also called High Voltage) motors usually supply above 1.1 kV and have a rating that is more than 150 kW. In contrast, Low Tension (or Low Voltage) motors usually supply below 440 V and have a rating of less than 150 kW.
Keep in mind that LT motors generally require more current than HT motors, so the wires are always thicker in LT motors than in HT motors.
2. Motor Full Load Current Calculator
The first step to take when specifying cable size is to calculate the full load current that the cable needs to cater for. This article will consider both single- and three-phase circuits, and the full load current for such circuits can be calculated as follows:
Single-phase full load current formula
Three-phase full load current formula
Where:
Pout=mechanical power output (kW)
PF=Power Factor
V=Voltage (V)
E=Efficiency
So consider a scenario where there is the need to calculate the cable full load current for a 125 kW motor (Low Tension motor), which has an efficiency of 93% and is operated at 415 V and power factor of 0.8. By substituting these parameters into the equation, the full load current the cable needs would be approximately 234 A.
3. Derating Factors
Keep in mind that the full load current calculated in the previous section is for an ideal situation(ambient temperat ure). In reality, there are several derating factors that increase the temperature experienced by cables.
For example, the thermal conditions of cables buried in the ground will differ from cables laid in, say, a duct or the air. Additionally, the grouping of cables causes heat buildup since cables are in close contact, and there is oftentimes poor heat dissipation. E ngineers must account for all these factors to prevent damage to the cable insulation and reduce system losses
Engineers can account for the thermal conditions for a ground cable using a ground temperature correction factor, K1. Likewise, a grouping factor, K2, allows engineers to account for cable grouping. Cable manufacturers usually provide tables that contain correction factors for different cable application requirements. For example, table 1 shows a typical cable grouping factor .
Table 1. Cable grouping factor
Still considering the previous example, the total derating factor is a multiplication of all the correction factors considered for the cable selection, as shown in the equation below:
Where:
K1=Ground temperature correction factor
K2=cable grouping correction factor.
Other factors that cable manufacturers provide include cable depth correction factor, cable distance correction factor, soil correction factor, and thermal resistance correction factor. Engineers will have to determine which of them apply to their application requirements.
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4. Total Derating Current and Motor Wire Size Chart
After obtaining the total derating factor, engineers can obtain the current capacity for their desired cable size (or application requirement) from cable wire size charts provided by cable manufacturers.
The total derating current can then be obtained as follows:
Consider a scenario where an engineer is looking to select a 1.1 kV, 3 core motor with 240 mm2 , Aluminum, XLPE cable for a single run. If the current rating (from the wire size chart) was obtained as 402 A and the total derating factor was calculated to be 1.1254, the total derating current will be 504.1 A.
As a rule, It is recommended that engineers specify cable sizes such that the total derating current is higher than the full load current.
Motor Wire Size Calculator: Engineers Should Reach Out to Cable Manufacturers
While this article presents helpful information about electric wire sizing, engineers are advised to reach out to electrical cable manufacturers to discuss their application requirements. Some cable manufacturers also have wire sizing calculators (sometimes called wiring calculators) on their websites for electrical conductor sizing.
