Washington State only requires them over 3,000lbs.
RCW 46.37.340
Or if the trailer weighs 40% of the tow vehicle.
(3) Brakes on all wheels. Every vehicle shall be equipped with brakes acting on all wheels except:
(a) Trailers, cargo extensions, semitrailers, or pole trailers of a gross weight not exceeding three thousand pounds, provided that:
(i) The total weight on and including the wheels of the trailer shall not exceed forty percent of the gross weight of the towing vehicle.
My truck weighs 4000 lbs, so I would need trailer brakes on a 1600 lb trailer