<p>My voltage at the controller at max load is ~11.4&nbsp;V 
(heated bed + motors + hotend). Is this normal? </p>

<p>I'm measuring 11.8&nbsp;V at the PSU, so 0.4&nbsp;V -> 5&nbsp;W lost in the wires.</p>

<p>I have a pretty beefy ~2&nbsp;mm diameter copper wire that's ~1&nbsp;m long. Its area is 2.5&nbsp;mm<sup>2</sup>. The diameter with shielding is 3.5&nbsp;mm.</p>

<p>Could there be a bad connection somewhere?</p>

<p>Checked the wire is warm to touch, so looks like it's actually the cause.
Is this normal? Should I go for even bigger wires?</p>

Per
this website
it matches the expectations.
Using 13AWG =~ 2.5 mm^2, 18amps, 12volts, 6feet = 2meters (1m back and forth).
Results in 0.4v drop.