YES, use:
Outdoor flexible cords (extension cord) for Damp Locations & Hard Usage- Types:
Hard Service Cords: ‘SO’, ‘SJTOO’, ‘SJTO’, or ‘SJTOO’.
Heater Cords: ‘HS’, ‘HSJ’, ‘HSO’, or ‘HSJO’.
I didn''t notice that you said two 120 Watt panels (240W total). 240W = 18.5 A @ 13 V.
To reduce voltage drop (< 3%), over a round-trip distance of (say) 50 Ft (25'' each Pos. + Neg.), you''d have to use #6 AWG cables.
Using #12AWG cable, you can expect a voltage drop of:
VD = A x R = 18.5 x (0.00162 x 50) = 18.5 x 0.081 = 1.5 Volts or about 11.5% (to about 11.5V).
Using #10 AWG, you''ll suffer a 7.25% Voltage Drop (to about 12V).
As you can see, it doesn''t pay to mount Solar Panels at great distances from the batteries.
The issue of Voltage Drop and Wire Size is addressed in my article "Ohm''s Law & You", published in "Good Old Boat Magazine" - issue 29, March/April, 2003).
A draft version of the article is on-line at:
The Cruisers Forum: http://cruisersforum.com/index.php?referredid=79
under the Yacht Maintenance & Tips Heading:
“Ohm’s Law & Boats” (March/April 2003 Issue 29
of “Good Old Boat Magazine”)
http://cruisersforum.com/showthread.php?s=&threadid=372&highlight=Voltage+D rop