There are no normal rep ranges where fat is significantly used for fuel.
It’s either the phosphagens (ATP and creatine phosphate) or glucose.
Up to a 3 minutes of continuous intense effort, you don’t really use fat for fuel (unless you are completely deprived of glycogen).
This is a rough estimate of how much fuel can each energy system provides.
Phosphagens = 4-10 seconds (up to 15-20 with creatine supplementation)
Anaerobic alactic (making fuel without oxygen and without producing lactate) = 10-40 seconds… main energy sources are ATP-CP and muscle glycogen
Anaerobic lactic (making fuel without oxygen and with lactate production) = 40-120 seconds … using mostly glycogen
Combo anaerobic lactic and aerobic = 2-4 minutes … using glycogen and recycling lactate for fuel.
After the 3-4 minutes mark is when you start to use fat for fuel to some extent.
CAVEAT: the first factor that determines which energy source is being used is the intensity of effort/muscle contraction (the body doesn’t know in advance how long the effort will last).
To make things simple, the shorter duration an energy system can last (e.g.phosphagens is really short, anaerobic lactic is mid-duration, aerobic is long duration) the slower it produces energy.
First understand that the ONLY fuel the muscles can use is ATP. If cannot use fat, glycogen or amino acids… the energy systems will take the fat or the glucose (or ketones) and make ATP out of it.
The fast/powerful energy systems make ATP rapidly but don’t last long (they are thus the dominant systems during weight training) and the more resistant systems make ATP a lot more slowly (so they are not powerful enough to fuel the intense muscle contractions needed when lifting) but they last a lot longer.
For example the phosphagens and anerobic alactic systems produce fuel (ATP) SUPER FAST whereas the aerobic system takes a long time to make ATP BUT has almost an unlimited supply of it so that the muscles can use it for fuel.
The more intensely your muscles need to contract the more rapidly they need the fuel (ATP). So when you are weight training, the default energy systems are those that provide energy rapidly.
Even moderate weight lifting (like what you can lift for 15-20 reps or more) is intense enough to make the body use one of the anaerobic systems, which means that you do not rely on fat for fuel.
If you wanted to use weight lifting in such a way that fat is used to a significant extent, the intensity would be so low that it would not do anything for muscular development or strength. And your sets would need to be at least 4-5 minutes long. Might as well just do cardio.