How to keep looping until the user input is valid ?

[Yoriz]

You have the same variable storing the text that is displayed when asking for input as the one that is storing the actual cable size.
as 'Please enter the cable size...:' will always be used inside of get_valid_cable_size you could remove the need to pass this in as a parameter.
It would also be good to let the user know what choices of cable size are available

CABLES = (25, 35, 50, 70, 95, 120, 150, 195, 250, 300, 500)
...
...
def get_valid_cable_size():
    print('Valid cable sizes are as follows:')
    print(', '.join(map(str, CABLES)))
    while True:
        cableSize = get_valid_input('Please enter the cable size...:')
        if cableSize in CABLES:
            return cableSize
        print('Please enter a valid cable size')
...
...
cableSize = get_valid_cable_size()
print(cableSize)

Output:
Valid cable sizes are as follows:
25, 35, 50, 70, 95, 120, 150, 195, 250, 300, 500
Please enter the cable size...:21
Please enter a valid cable size
Please enter the cable size...:25
25.0