When it comes to welding, too much of a good thing can often add up to unnecessary costs, potential downtime and lost productivity — especially if you have a MIG gun that’s too large for your application. Unfortunately, many people believe a common misconception: that you need a MIG gun rated to the highest amperage you expect to weld (e.g., a 400-amp gun for a 400-amp application). That is simply not true. In fact, a MIG gun that provides a higher amperage capacity than you need typically weighs more and may be less flexible, making it less comfortable to manuever around weld joints. Higher-amperage MIG guns also cost more.

The truth is, because you spend time moving parts, tacking them and performing other pre- and post-weld activities, you rarely weld continuously enough to reach the maximimum duty cycle for that MIG gun.

Instead, it’s often better to choose the lightest, most flexible gun that meets your needs. For example, a MIG gun rated at 300 amps typically can weld at 400 amps and higher — for a limited amount of time — and perform just as well.

In the United States, the National Electrical Manufacturers Association, or NEMA, establishes the MIG gun rating criteria. In Europe, similar standards are the responsibility of Conformité Européenne or European Conformity, also called CE.

Under both agencies, MIG guns receive a rating that reflects the temperatures above which the handle or cable becomes uncomfortably warm. These ratings, however, do not identify the point at which the MIG gun risks damage or failure, nor do they specify the way that it needs to be labeled or marketed. For that reason, there can sometimes be significant differences in the way different MIG gun manufacturers rate their products.

Much of the difference lies in the duty cycle of the gun. Duty cycle is the amount of arc-on time within a 10-minute period. One MIG gun manufacturer may produce a 400-amp MIG gun that is capable of welding at 100 percent duty cycle, while another manufactures the same amperage MIG gun that can weld at only 60 percent duty cycle. In this example, the first MIG gun would be able to weld consistently at full amperage for a 10-minute time frame, whereas the latter would only be able to weld for 6 minutes.

Before deciding which MIG gun to purchase, it is important to review the duty cycle ratios for the product. You can typically find this information in the product literature or on the manufacturer’s website.

Based on the gun rating explanation above, it is also essential for you to consider the length of time you spend welding before you make your MIG gun selection. Look at how much time you actually spend welding over the course of 10 minutes. You may be surprised to discover that the average arc-on time is usually less than 5 minutes.

Keep in mind that welding with a MIG gun rated to 300 amps would exceed its rated capacity if you were to use it at 400 amps and 100-percent duty cycle. However, if you used that same gun to weld at 400 amps and 50-percent duty cycle, it should work just fine.

Similarly, if you had an application that required welding very thick metal at high current loads (even 500 amps or more) for a very short period of time, you might be able to use a gun rated at only 300 amps.

As a general rule, a MIG gun becomes uncomfortably hot when it exceeds its full duty cycle temperature rating. If you find yourself welding for longer on a regular basis, you should consider either welding at a lower duty cycle or switching to a higher rated gun. Exceeding a MIG gun’s rated temperature capacity can lead to weakened connections and power cables, and shorten its working life.