I saw this in a company presentation.

Problem:
“Pair” is a singular noun being used incorrectly as a plural noun.

Explanation:
Two of anything together is a single pair. Multiples of this means multiple pairs. Note, however, that if you were talking about the size of a cable, then it would be correct to say, “a 400-pair cable“; note the hyphen, which joins “400” and “pair” to form a modifier of “cable”. Similarly, “a twenty-foot ladder” would be correct, and “twenty foot of ladder” would be incorrect.

I also believe that use of “pair” instead of “pairs” helps to prove my “Devolution toward Simpler” linguistic hypothesis. It is simpler to say “pair” than to say “pairs”.

Solution:
“400 Pairs of Copper Wiring“