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Guidelines for generator use with inverter welders:

By following these guidelines, the operator will minimize the risk of damaging voltage spikes and help the welding machine to perform to its full capacity.

Generator Size

Determining the exact generator size required to safely run a welding machine is not always a straightforward process.  Some of the factors to be considered are the current draw ratings of the welder, the rated output of the generator and whether this is a genuine rating (unfortunately some generators are over-rated), whether the generator will or will not be used to run other power equipment at the same time, etc.

For recommended generator sizes to run specific machines, refer to the instruction manual.

Following is a Suggested minimum generator size ‘rule of thumb’ guide*;

Max. Output
Generator Size**
Generator Size***
  Up to 160A 7kva 8+kva
  180–200A 8kva 10+kva
  250A 13kva 15+kva

*Note that if it is intended that the generator will be used to run additional equipment at the same time as the welder, the size of the generator should be increased accordingly. These figures are an approximate guide only and should not replace manufacturers recommendations.
** ‘Minimum’ size is the smallest that we suggest to minimize risk if voltage spikes etc, however it may not be enough to achieve full output from the welder.
*** ‘Ideal’ size will further minimize risk of power supply issues and will allow a higher output from the welder.

Generator Quality

As we have already mentioned, a good quality generator suitable for running an inverter should have a low THD (Total Harmonic Distortion) output.  All reputable suppliers or manufacturers of portable generators will be able to specify what the THD ratings are on their product.

Generators with a low THD rating (6% or less) will have ‘relatively’ clean power and will thus be suitable for running inverter welders.

A generator with a high THD rating (more than 6%) is likely to be a low-quality unit and should not be used to run inverter welders.

The Do’s and Don’ts of using generators with inverter welders:

  • Start up & Shutdown: Always disconnect the welder’s power lead from the generator before starting or stopping the engine on the generator.
  • Fuel Supply: Always ensure that the generator engine has a safe supply of fuel and that the fuel line is in good condition. The engine should not be allowed to run out of fuel while the welder is connected to the generator.
  • Maintenance: Always keep the generator in good condition. Do not use a generator that is in need of service or replacement.
  • Power Extension Leads: Do not use power leads that are of poor quality or in need of repair or replacement. Always use heavy duty leads – see table below.

Suggested Guide for cable size of extension leads used with Inverter Welding Machines*:

Welding Machine
Max. Output
Power Supply Suggested Minimum
Cable Size*
  Up to 200A   240V 10A/15A   Lengths up to 10m: 2.0mm2

Lengths over 10m: 2.5mm2

  250A   240V 15A   2.5mm2
  240V 20/25/32A   4.0mm2

*These figures are an approximate guide only and should not replace manufacturers recommendations.

Can I use a small(er) generator to run my welder?

As shown in the table 5a above, the recommended minimum generator size is not less than 7kva (for up to 160A welders). We often get asked questions like “why can’t I use my 5kva generator to run this welder?”.

We also sometimes see other welding machine suppliers suggesting that their machines can be powered by generators as small as 4-5 kva.

Sure the welder might actually operate to some level, but here’s why using smaller under-sized generators is NOT a good idea;

  • As explained above, the generator will have to work much harder and this will greatly increase the likelihood of severe power spikes that could damage the welder, or cause it to frequently shut down.
  • The output from the welder will be significantly reduced, possibly to the point where the machine is of little use… or becomes very frustrating to the operator.
  • Often the generator will be used at some point to operate other equipment (eg power tools) at the same time and this will further compound the above problems.

Although a larger generator will cost more initially, a correctly sized (or over-sized) generator will allow you to get the job done right the first time… and greatly reduce the risk of costly damage to your welder.  And who ever regrets having ‘too much’ performance?

Here’s another way to look at it; let’s say you’re buying a motor vehicle to tow a trailer. Would you purchase a vehicle that only just has enough power, and must constantly operate close to “red line” rpm to do the job?  Probably not!  In the same way, buying a generator that has more power/output than what you actually need makes a lot of sense.


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