Navigating Towing Capacities:

Understanding Heavy Trailers Over 3500kg

The term “COF” frequently crops up in discussions when building out specifications for trailers. These have become more prevalent with larger pickup trucks from the US market becoming a more common sight on NZ roads. A COF, or Certificate of Fitness, is required for trailers over 3500kg, and whilst Trailparts provide components for this class of trailer, there are complexities to be aware of in the design and build stage that are not required on trailers under this weight.

Trailer with COF

In New Zealand, a heavy trailer is officially designated as one with a Gross Vehicle Mass (GVM) exceeding 3500kg, and are further categorised into two distinct classes based on their GVM:

  • Class TC:

    This category encompasses heavy trailers with a GVM ranging from 3500kg to 10,000kg. These trailers form a crucial segment of the NZ towing landscape, often seen in large boat trailer applications as well as special purpose trailers which at the lower end of the class eliminate the need for drivers with truck licenses whilst still being able to carry decent loads.
  • Class TD:

    Heavy trailers that have a GVM exceeding 10,000kg. In New Zealand these are almost exclusively truck-towed trailers.

Building a TC Class Trailer

Almost anyone can build a 3500kg trailer that operates under the Warrant of Fitness (WoF) regime where frameworks for design are remarkably ill defined and are not governed by government legislation. Engineering considerations are taken care of by the trailer builder, and this leads to a relatively low-cost way to haul loads.

However, when designing and building a TC-class trailer requiring a COF, in plain language the very opposite is true. Experts will be needed to design and spec the trailer, who will also consider the vehicles that will be towing it. During the build, qualified tradesmen will be required, and an engineer will then sign the finished trailer off. None of this is impossible, of course, but it does introduce a cost and complexity that can easily double the cost of the 3500kg trailer.

Having said that, usually the customer proposing a TC class trailer will either have a business case that justifies the extra cost, or they have the disposable income for a (for example) purpose-built boat trailer that means the extra cost doesn’t matter!

What to Know When Owning a TC Class Trailer


TC Class trailers require a new COF every 6 months, and these are conducted at workshops specially set up to do them. These checks are more extensive than for trailers with a WoF and tolerance for vehicle defects is low. Additional checks include those for structural integrity and brake efficiency, among others. In recent times, many provincial towns have lost their COF testing stations, so the COF process often requires a trip to where there is one, so it is important to have the trailer checked before going to the effort.

Vehicle towing:

COF trailers would often be considered as commercial vehicles used when looked at by compliance officers. It follows, then, that penalties for overloading are significant, so it is important to look beyond the more obvious towing capabilities of the pickup or truck and become acquainted with GVM (Gross Vehicle Mass) and GCM (Gross Combination Mass) data as well. Let’s use the Dodge Ram 1500 Limited-spec pickup as an example.

  • Has a towing capacity of 4500kg.
  • Has a GCM (vehicle + load + trailer) of 7713kg.
  • Has a kerb weight of 2749kg (full tank, no driver).

Adding the vehicle kerb weight and our 4500kg (COF) trailer weight together comes to 7249kg. This leaves 464kg for occupants and any cargo – showing that it doesn’t take much to become loaded past the vehicle GCM rating.

Your license:

Having made this outline on weights above, it is important to look at what your driver’s license allows too. You can read about this here: New Zealand driver licence classes: what you can drive | Waka Kotahi NZ Transport Agency ( As shown in the table, a Class 2 license is required for a vehicle combination in excess of 6000kg.

COF Label and Expiry Dates:

When a vehicle passes its COF inspection, the inspector applies the COF label on vehicle, which remains valid for six months. However, operators are required to have the trailer at COF standard at all times, and it is important that the next certificate is issued before the expiry date on the label.


With heavy trailers operating under the COF compliance regime, understanding the interplay between classes of trailer, GVM and GCM considerations, driver licensing requirements, and the necessity of a Certificate of Fitness is paramount.

There are many benefits in being able to tow heavier loads, but it is also important drivers know the limitations well and are informed so that their vehicles are operated safely and without voiding manufacturer’s warranties.