Equipment Notes: Total Harmonic Distortion

A Real Threat to Electronic Equipment

by Ryen Van Grol, territory sales manager, Roofmaster Products Company









(Editor’s Note: Ryen Van Grol is the territory manager for San Diego County, California, and Orange County, California, for Roofmaster. Prior to his position with Roofmaster, he was with Western Process Heat, the regional master distributor for Leister USA. He is a certified Leister technician and has extensive experience and knowledge in all aspects of the construction industry, specializing in roofing equipment, tools, accessories, sales, and service. He can be reached at


Total harmonic distortion (THD) is the degree to which a voltage waveform is distorted. The greater the waveform is distorted, the greater the problems you’ll experience with your electronic equipment. As you may already know, there are many things that can be run off of a portable generator. One thing you may be familiar with is electronic equipment, Leister welders for example.

Be mindful that some of these portable generators are unsuited to be used with this type of electronic equipment. This is due to many generators in the market producing power that is outside of the THD limits. Many big box stores sell generators with THD in the range of 25-30%. The acceptable percentage for THD when running equipment with electronic components should always be less than 6-7%.

It’s critical to separate your linear and non-linear loads on your generator. If you are running an auto welder, you should never use anything on that same generator that has a linear load, such as a screw gun, sawzall, radio, battery chargers, etc. The constant on/off surge of these tools creates inconsistent power and can lead to expensive repairs to electrical boards and motors.

Generators sold at the big box stores are appropriate for these linear load tools, as well as your fridge if you lose electricity. They are not designed to handle non-linear electrical components in your industrial tools. This is due primarily to the quality of the windings within the alternators, in addition to the quality of components. It is highly important to thoroughly check all power and extension cords at every connection.

Most hot air welder manufacturers recommend that all portable generators have a maximum THD of less than 6% to avoid any likely damage to the electronics. The manufacturer of your generator should have specifications regarding the THD. If this information is not readily available, it is likely that the generator is not suitable for your sensitive equipment. My advice is that if you are willing to spend good money for top-of-the-line equipment, protect your investment by purchasing the appropriate generator.