|Which is correct?
Because the power wiring system was designed for _____ its ability to carry higher frequencies is limited.
Explanation: You need a comma because the sentence starts with because. I like John’s short answer on Snippets: Essentially, if you can write a sentence starting with because without a comma (“Because I said so,” for example.), you know what you’ve got on your hands is a sentence fragment rather than a sentence.
If you turn the sentence around, the comma is optional — depending on what style you follow.
AP style is to omit:
The power wiring system has a limited ability to carry higher frequencies because it was designed for AC.