I have an eco flow delta pro and need to run the xt60 cable to the panel which is 100ft away. I see 10 gauge in 100ft length for sale for about $150 but is 10 Gauge sufficient or do I need a lower gauge for this length.
It depends...
I'd plug some numbers into a voltage drop calculator and see what you come up with, I like this one, there are many others.
Ideally you want to be under 3%.
I suspect 10AWG will be pushing it with the lower VOC rating of the eco. Over 3% can be ok, you'll have greater losses.
How many Amps and what voltage does your panel(s) put out. If you are anywhere near minimum voltage for the inverter you may need to calculate voltage drop over that distance using the voltage drop calculator posted earlier. The wire should be sized to carry the current at minimum and if voltage drops close to minimum then increase wire size to reduce voltage drop. Voltage drop is not critical if it exceeds minimum by a good margin.
I have some panel strings that are really pushing limits. I'm around 7.5% loss which is acceptable for me since its only 30w on these low power 400w strings.
I just buy 10 gauge THHN wire in the 500' rolls from homedepot for my higher amp panel strings. 14 gauge for the lower ones. My conduit of choice is 1/2 irrigation distribution tubing in the 500' rolls. Seal the end with silicone. By some MC4s off Amazon and keep your fingers crossed.
Then again, I run low voltage/power strings and they are not as needy for special treatment. Not the best setup for a 600v string if you know what I mean. Someone might die.
What kind of current and voltage are you seeing at the panels? Buying 8 guage is a waste of money if your panels are not putting out more than 30 Amps and your voltage is safely above the minimum for your charge controller.