Looks like the magic number is 3000 lbs in Ontario.
What's interesting is the variation across the provinces.
Alberta
If gross laden weight of trailer is 909 kg / 2,004 Ibs or over, or if gross trailer weight is over half that of the unit, independent braking system is required.
Not required for trailers with a gross laden weight of not more than 910 kg (2006 Ibs) or trailers with a gross laden weight of less than half of the weight of the towing unit.
Breakaway brakes are required on trailers over 907 kg (2,000 Ibs).
Sask
Any trailer towed behind a motor vehicle with a Gross Vehicle Weight Rating (GVWR) of 1,360 kg (2,999 Ibs) and greater, requires brakes on all axles with a break-away system that will apply the brakes of the trailer in the event it breaks away from the towed vehicle.
Man
Every semi-trailer that forms part of, or is attached to, a semi-trailer truck, and every trailer that is attached to a motor vehicle and that has a gross weight in excess of 910 kg (2,006 Ibs), when upon a highway shall be equipped with brakes adequate, when operated in combination with the brakes of the motor vehicle or semi-trailer to which it is attached, to stop and hold the semi-trailer truck, or the semi-trailer truck and trailer, or the motor vehicle and trailer, as the case may be, within the distances prescribed by the law (subsection
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. Breakaway brakes are required on trailers 907 kg (2,000 Ibs) and over.
On
Trailer or semi-trailer - Every trailer or semi-trailer having a gross weight of 1,360 kilograms (3,000 Ibs) or more shall be equipped with brakes adequate to stop and to hold the vehicle.
Qc
Independent braking system is required in each weight bearing wheel where gross weight exceeds 1,300 kg (2,867 Ibs). Breakaway brakes are required on all trailers 1,360 kg (3,000 Ibs) and over.
NB
Independent braking system required where gross weight exceeds 1.5 tons or 1,500 kg (3,308 Ibs).
NS
Independent braking system required where gross weight of load and vehicle exceeds 1,800 kg (3,969 Ibs). Breakaway brakes are required on all trailers with a registered weight of over 1,350 kg (2,977 Ibs).
NFLD
Independent braking system needed where gross weight exceeds 4,500 kg (9,923 lbs) or two or more axles are used on the trailer.