Solar setup input?

Yes your wire is small assuming it is a single feed for 400 watts. I would go #8 for the single feed. If each panel has #12 to the combiner box that is fine.

MPPT will only really boost the amps during the short bulk phase of charging. Adjustable charge set points and other programming features may still make the swap worthwhile.

Wire size depends on how things are connected. 12awg is too small to handle 400w.

Somebody spent all that money on solar and cheaped out on wire. Silly. Regarding a MPPT controller, yes, it only helps during bulk charging but the prices are not that far apart.

2oldman wrote:
Wire size depends on how things are connected. 12awg is too small to handle 400w.

all depends on the amperage.. we need to know the Vmpp of the panels to be sure.

I suspect he has "12 Volt" panels. the Voc of these is about 22 volts and Vmpp of 17 to 18 Volts.
4 of these in series would be approx 72 V at full load
so a Victron Blue 150/35 would handle this with ease
400 Watts / 72 Volts is 5.5 Amps
12 AWG is rated for 20 Amps so no problem at all.
voltage drop at max power with say 50 ft 12 AWG = 0.43 Volts

would I chose 12 AWG.. probably not if I intended to expand the array at some point in time.

It might be better to re-wire to 10 AWG for future expansion, then you could use the renogy 100/40 and connect the panels in a 2S2P string..
then the Vmpp = 36 V, Imax = 11.1 Amps
Vdrop over say 50 ft of 10 AWG = 0.55 Volts
loss approx 1.6 %

wiring from panels to MPPT would be 10 AWG, on the 12 V side you now have a max of 33 Amps, so you would need a minimum of 6 AWG for a length of no more than 20 ft inclusive or 4 AWG if you get serious and add two more panels. if you add two more of the same panels then you would connect 2 strings of 3 panels.
the renogy 100/40 will be maxed out as it can only charge at 40 Amps. if you think you will expand at a later date then you might want to consider the Victron blue 100/50

pm me if you have questions.

20A, 12awg, voltage drop is .326v per 5' (1-way circuit length)
20A, 10awg, voltage drop is .205v per 5'

PWM < 1v drop between the panels and controller
nill between the controller and batteries or battery voltage sensor.

mppt would let you series all 4 or sets of 2, reducing current/voltage drop.
mppt, voltage drop from panels to controller = loss power.
nill between the controller and batteries or battery voltage sensor.

controller with bluetooth and phone app would let you monitor system and adjust set pts/time from the campfire!
http://www.homedepot.com/p/Grape-Solar-40-Amp-PWM-Solar-Charge-Controller-with-Bluetooth-GS-PWM-40BT/207100856

"mppt would let you series all 4 or sets of 2"

Yes, Red, but if there's any shading...

OldSmokey wrote:
4 of these in series would be approx 72 V at full load
so a Victron Blue 150/35 would handle this with ease

Yes, I should have specifically said '12awg is too small for 400w @ 12 volts, which, in the absence of any other specifics from the OP, is usually what people do. Time2roll assumed the same.

With a PWM controller I would hope it was parallel...

time2roll wrote:
With a PWM controller I would hope it was parallel...

if it wasn't then the whole system would just be a waste of time..

unfortunately the OP has inherited a bad system, but it's fixable...