Buying “too much” gun can lead to a variety of problems, including operator fatigue and decreased productivity. The ideal MIG gun strikes a balance between the application’s demands and the gun size and weight.

By Bill Giese
Product Manager, Bernard

When choosing a MIG gun, many consumers often make the mistake of buying a gun with capabilities that exceed their needs. ‘What’s wrong with too much gun?’ you might ask.

A gun overmatched to your application will weigh more than it needs to, will be less flexible than it could be and will cost more than it needs to. As a general rule, the lightest, most flexible gun that meets your needs is the best choice.

Picking out the right gun for your application isn’t as easy as it sounds, however. A common misconception about MIG welding is that you need a MIG gun rated to the highest amperage at which you expect to weld. For example, if you’re welding at 400 amps, you need a 400-amp MIG gun.

In fact, a gun rated at 300 amps can weld at 400 amps and higher. Moreover, in most cases the 300 amp gun will be the better choice. This is due to how guns are rated, the time operators spend welding and the benefits of using a smaller, lighter gun.

How Guns Are Rated

MIG gun rating criteria are established by NEMA (National Electrical Manufacturers Association) in the U.S. and by CE (Conformité Européenne or European Conformity) in Europe. The ratings that are assigned to a gun reflect the temperatures above which the handle or cable becomes uncomfortably warm, not the point at which the MIG gun risks damage or failure.

The ratings systems don’t specify the way a gun must be marketed, however, so there are large discrepancies between manufacturers rating systems. For example, a 300-amp gun by manufacturer “A” might be able to weld at that amperage at 100 percent duty cycle, whereas a 300-amp gun by manufacturer “B” might only be able to weld at that amperage at a 20 percent duty cycle.

Duty cycle is defined as the amount of arc-on time within a 10-minute period, so a 20 percent duty cycle would constitute two minutes of arc-on time in a 10-minute time frame. Most manufacturers do list the amperage to duty cycle ratios in their product literature, so it is a good idea to research a gun’s duty cycle ratings before making a purchase decision.

Heat Sources

This thermal image shows the reflected and resistive heat absorbed by a 300-amp Bernard Q-Gun after three consecutive welding cycles at six minutes arc-on and four minutes arc-off. The automated application is laying a bead on the surface of a weld drum at 34 volts and 374-ipm.

There are, generally, two types of heat sources that affect the handle and cable temperature: radiant heat from the arc and resistive heat from the cable.

Radiant heat, heat that is reflected back to the handle from the welding arc and the base metal, usually contributes the most heat to the handle. Radiant heat can be affected by a number of factors. Shiny aluminum or stainless steel, for example, will reflect more heat than dull “black iron” mild steel.

Additionally, a longer neck will place the handle further from the arc and thereby keep it cooler. The neck also carries heat to the handle and the heat that the neck absorbs can be affected by the consumables used.

Gas mixture and transfer process also play a significant role in radiant heat. Argon, for example, creates a “hotter” arc than pure CO2, so a MIG gun will often reach its rated temperature at a lower amperage with an Argon mix than with pure CO2. Also, a spray transfer process, in addition to requiring an 85-percent or greater Argon content, usually involves longer wire stick out and arc length, which increases the voltage and overall temperature.

Resistive heat is generated by electrical resistance within the welding cable and is responsible for most of the cable heat. Heat is generated when the electricity generated by the power source is unable to flow unimpeded through the cable and cable connections and the energy of the “backed up” electricity is lost as heat. Heat caused by electrical resistance isn’t necessarily a problem with the MIG gun, and in fact a cable large enough to completely eliminate resistance would be too heavy and unwieldy to have any practical applications.

The Operator Factor

When deciding which MIG gun is right for you, a final factor to consider is the length of time you spend welding. Many people are surprised to find that over the course of ten minutes, they usually average far less than five minutes of actual arc-on time.

A MIG gun rated to 300 amps would exceed its rated capacity if it were used at 400 amps and 100-percent duty cycle. However, that same gun should be able to weld at 400 amps and 50-percent duty cycle.

If you typically weld very thick metal at high current loads — even 500 amps or more — but you only do so for very short periods of time, you might be able to use a gun rated 250-300 amps.

A general rule of thumb is that under normal circumstances the gun will become uncomfortably hot when it exceeds its full duty cycle temperature rating. If this occurs on a regular basis, you should consider either using a lower duty cycle or switching to a higher rated gun. Exceeding a gun’s rated temperature capacity can lead to weakened connections, power cables and shorten its overall working life.

This illustration shows capabilities of a 400-amp MIG gun to weld at and above its designated rating. It also shows the de-rating properties of mixed gas compared to pure CO2, which is the shielding gas most often used to rate MIG guns.

Smaller Is Better

The primary differences between air-cooled guns that are rated to different amperages are the size of the cables, the size of the connections and the size of the handles. Thus, as guns increase in their rated capacity, they almost always correspondingly increase in mass. While gun weight and size might not make a significant difference for occasional welders, an operator who welds all day, every day will appreciate using a lighter and smaller MIG gun.

The benefits of using a more comfortable and lighter gun can result in improved productivity since it will be easier to maneuver and the operator’s arms will not become fatigued as quickly.

Larger and heavier guns can also increase an employees susceptibility to repetitive motion injuries, such as carpal tunnel syndrome, which can lead to higher insurance rates, worker compensation claims and lost productivity.

Furthermore, when it comes to the mass of a gun compared to its rated capacity, not all MIG guns are created equal. Two MIG guns rated to 300 amps could vary widely in terms of their overall size and weight. Features like a ventilated handle that permits air to flow through it can keep a gun cooler and thus permit it to rate to a higher capacity without adding any size or weight.

A Final Thought

Understandably, many operators choose a MIG gun that is overmatched to their welding application. While that gun will certainly be able to handle the current loads and heat, it also exacts an unnecessary toll on the operator. Under most circumstances, a smaller, lighter gun will also be able to handle the same heat and current loads while offering improved operator comfort and maneuverability.

Before buying your next MIG gun, be sure to fully research the differences between the available guns and your welding needs, including the actual length of time you spend welding, the processes used and the materials welded. Only then will you be able to select a gun that strikes an ideal balance between size and capacity.