I have a SquareD Q08-16L100RB Panel that I want to convert to use as a temporary on a construction site but it needs a main breaker installed. It currently has no main breaker but SquareD says you can install a "back-fed" main breaker and a retaining clip. Does back-fed mean that the line power from the meter is fed back through the main breaker to the bus? Does it mean that I install the two hot cables from the meter to the lugs on the main breaker instead of the lugs mounted in the panel that connect to the hot bus?

Regards, Michael

Back feeding means that the output of a breaker is used as the input to a panel.

Many times this is done with an interlock kit so that the main and the back-fed breaker cannot be on at the same time. This is for generator applications.

If there is no main breaker, then any breaker could be used to feed a panel. Should be labeled as such.

I am still not clear on the wiring, would you please edify? Was I correct that I would place a 100 amp breaker in the first position and attach the 2 hot wires from the meter to the 2 terminals on this (main) breaker?

Yep. Technically the (main) breaker could be in any position. The first position makes the most sense.

Wow that was fast. Does this mean that current can always flow both ways through a breaker?

Well AC voltage in the US reverses polarity 60 times per second. That's one way of saying yes.

Breakers are essentially thermal (heaters) devices with a latching mechanism. Heat typically unlatches the breaker. Heaters don't care which way the current flows. A little hydraulics in the breaker changes how the breaker responds to an overload (trip time vs. overcurrent).

DC breakers don't care either.

Correct if I am wrong, I am thinking you want to backfeed a breaker, not a panel, In this case, The wires feeding your new panel Hook to a breaker(load side of breaker),
When this breaker is on, it energizes the Panel. The retaining clip(s) is so if breaker is pulled out of panel and is ON, the exposed breaker connections will not shock you if contact is made.

Yes you are correct. Thank you for your assistance as well. The whole thing keeps getting clearer :)