solar subpanel

[solarroddog]

im installing a res. solar system and in order to meet the 120 rule of art. 690.24 (b)(2) the NEC i will need to install a subpanel/load center(125amp) with no main breaker. the load center will have three 20 amp breakers from three strings of panels. there will be a plackard on the sub panel stating, ( for solar loads only, no other loads to be added). my question is, do the conductors from the load center to the main service panel need to be sized for 125 amp or just large enough for the three 20 AMP load in the sub panel. the breaker in the main panel feeding the solar load will be 50 amp.

[cmac]

load determines wire size

It's assumed you already have a 125A rated panel for the sub-panel, since a sub-panel can be rated for whatever is appropriate for the loads. But, always good to anticipate some future expansion or inclusion of additional loads in the sub-panel. Personally, unless you already have the 50A breaker, I'd use a 60A breaker, not because of your 3 20's, but both a 50A and 60A breaker can accommodate a #6 wire. Regardless, the wire size to the sub-panel is dictated by the loads, not the panel rating. Good general rule in this case is to use largest wire capable of insertion in the breaker being used.

[Naptown]

You still have not met the 120% rule by installing a sub panel. Even with the sub panel you are feeding perhaps more into panel than it is rated for. Read the code again. It states that the sum of all current supplying circuits may not exceed 120% of the ratings of the bus bars. If you are feeding 60A via a sub panel into a 200A panel you are still over the 120%
What is the max amperage of the inverters and what main panel do you have. There are ways around this.

[cmac]

Actually, there's too little information anyway. If the sub panel is powered by a solar system, a properly isolated/dedicated sub panel, then what is the 50A breaker for? Is there bypass control present elsewhere, or is the sub panel just for back feed, not powering loads, or is the 50A breaker actually supposed to be AC IN1 to an inverter, or is this a DIY version of "...when I lose power I'll turn it off and then power the sub panel with solar...", etc