Q & A of the Day - How can I calculate the cable size to supply a motorised valve?

Question: I have to supply and control a motorised valve on one of the sites I am working on. The valve is 24VDC actuated with a power consumption of 135W, and it will be supplied via a 230VAC/24VDC power supply within the control panel. The valve is approx 330m away from the control panel.

I need to know the size of cable to supply the valve with, as I don't have access to Amtec etc. I've tried using cable calculators on the web, but they will not calculate for a 24VDC supply. Can you help?

Answer: First, it is important to establish the design current of the circuit (Ib) and select the type and size of protective device (In) as well as the type of cable to be used.

Ib can be determined by dividing the voltage into the power rating of the valve (approximately 5.6A).

Any relevent factors (normally derating) are then applied using the formula below:

It = In/(Ca x Cg x Ci x Cc)

Where:

It - is the tabulated current-carrying capacity of the cable
In - is the rated current or current setting of the protective device
Ca - is the rating factor for ambient temperature
Cg - is the rating factor for grouping
Ci - is the rating factor if the cable is surrounded with thermal insulation
Cc - is the rating factor depending upon the protective device or installation condition

A voltage drop check must then be carried out, using the formula below, to ensure that both current-carrying and voltage drop parameters are met.

Voltage drop = (mV x Ib x length)/1000

Where:

mV - is the millivolts dropped per ampere per metre for the type and csa for the selected cable (Tables 4D1A to 4J4A)
Ib - is the design current of the circuit
length - is the length of the circuit in metres

It should be noted that the voltage drop can be the overiding factor when calculating conductor csa.

You would need to establish the max permitted VD at the valve terminals, either from the nameplate or manufacturer. Having checked both current-carrying capacity and voltage drop calculations it can be confirmed that the selected cable is acceptable.

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