﻿ Calculation of the PLC Panel Power Consumption...

# Calculation of the PLC Panel Power Consumption and Heat Dissipation

There are two important things which is part of the PLC control panel design, power consumption calculation and heat dissipation calculation. The power consumption calculation will determine how much the PLC panel needs an electrical voltage feeder. The heat dissipation calculation will determine how much the cooling capacity that must be provided or how big the panel shall be. In practice, the calculation result is informed to the electrical engineer and to the HVAC engineer (if the panel is located in the air conditioned room) or to the panel enclosure manufacturer (if the panel is located in outdoor hazardous area). We will go into detail regarding this calculation but please take a note that this calculation is for preliminary only, the final calculation shall be done by the PLC panel Vendor.

Calculation of the PLC Power Consumption

In order to calculate how much the required electrical power for our PLC panel, we need to consider all components inside the panel and also all devices that are connected to the panel. The typical component inside the panel including but not limited to PLC (with all necessary module), AC to DC power supply, interposing relay, local HMI, etc. The devices that are connected to the panel are varying, but in typical it is a transmitter, solenoid, valve actuator, etc. Those devices need to be included in the power consumption calculation because they are powered from the PLC panel.

Before we calculate the power consumption of the PLC panel, we need to create a data table that specify all necessary PLC panel component and also all field devices connected to the panel. The data shall be based on published vendor catalogue in order to have a precise calculation.

Let&rsquo;s say we are having a skid gas dehydration package PLC control system as follows:

Analogue Input: 46 ea

Analogue Output: 7 ea

Digital Input: 37 ea

Digital Output: 43 ea

Based on the above input/output requirements, we can create below data table:

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Based on the above data table, the PLC power consumption is calculated as follows:

Power Consumption of the PLC Panel = (Power Consumption of PLC component) + (Power Loss of 120/24V DC Power Supply) + (Devices Power Consumption Connected to the PLC)

Power Consumption of the PLC Panel = (Total calculated current@1.2V DC x 1.2) + (Total calculated current@5V DC x 5) + (Total calculated current@24V DC x 24) + (Total calculated current@120V AC x 120*Power Factor*(100%-Efficiency))

Power Consumption of the PLC Panel = 338,6 Watt

Thus the power consumption of the PLC panel based on the above data table is 338,6 Watt.

Calculation of the PLC Heat Dissipation

The PLC heat dissipation calculation is almost the same with the above power consumption calculation except we exclude the devices that are connected to the PLC panel in our calculation. Why it is the same with the power consumption calculation? It&rsquo;s simple, because the energy can&rsquo;t be created but it can be transformed into another form. The PLC consume the electric power, but it doesn&rsquo;t do any physical work, thus we can sure that all consumed energy is dissipated as heat.

Based on the above concept, the PLC heat dissipation is calculated as follows:

Heat Dissipated inside the Panel = (Power Consumption of PLC components) + (Power Loss of 120/24V DC Power Supply) + (Heat Dissipated by the Cabling)

The heat dissipated by the cabling can be calculated as follows:

P = I^2 * (L*Rc)

Where,

P = power dissipation per conductor in an enclosure in Watts

I = current through conductor in Amperes

Rc = resistance of conductor in ohms per meter

L = total length of conductor in meter

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As per the required analogue input/output of our system example above, we calculate the cabling dissipation as follows:

 Analogue Cabling Power Dissipation     Current 20 mA Analogue Terminal Block 106 EA Terminal Block Resistance N/A   Cable Length to terminal block 0,5 m Cable Length from terminal block 0,5 m Cable Resistance 0,0181 Ohm/m Analogue Cabling Power Dissipated 0,000767 watt       Digital Cabling Power Dissipation     Current 75 mA Digital Terminal Block 86 EA Terminal Block Resistance N/A   Cable Length to terminal block 0,5 m Cable Length from terminal block 0,5 m Cable Resistance 0,0181 Ohm/m Digital Cabling Power Dissipated 0,008756 watt

From the above calculation of cabling dissipation, we can conclude that the heat dissipated by the cabling system is negligible in this case.

Thus the total PLC panel heat dissipation based on the example data above is:

Heat Dissipated inside the Panel = 234,69 Watt