Know the load

When it comes to portable generators, size matters. Here’s how to correctly size your unit for your loads.

Pulling a portable generator off of the pickup and onto the jobsite is one of the first steps to getting set up on the job. Unfortunately, if the generator is undersized, productivity will suffer. A generator that’s too small for the job may even damage your tools or the generator.

“Improper sizing is one of the leading causes of disappointing generator performance,” says Tim Sheehan of Yamaha Motor Corporation. It’s not that difficult to correctly size the generator for the job. Here, Yamaha shares a step-by-step way that makes understanding power needs and selecting the right generator for the job easier.

How many watts do you need?
Make a list of all the power tools that will be plugged into the generator, then look in the operator manuals or on the tools’ nameplates for their watt ratings.

If watts aren’t listed, take the amp rating and multiply it by the voltage to get its wattage. Add up each tool’s wattage to figure your highest likely load.

You can also refer to the partial list of tools in Table 1 to estimate your load. But remember, these are only averages and your power tool may consume more power. If you are using averages to figure your wattage, increase the total by 10 percent to give you some cushion. Then look at the tool list and match up the tools that will most likely be used simultaneously. Add up their wattages. In most instances, a generator with that watt rating or slightly larger will handle your power tool load.

Consider starting requirements
Some lighter-duty or inexpensive generators are not engineered to handle the heavy power draw that’s put on them when larger motors start. In these instances, you must account for that power draw and up-size the unit to make sure it can handle the load. Your generator must be built to handle not just the running load, but the total load at start as well.

Figure your worst-case scenario
Once you have identified the total watts required to start a motor, figure what your total load is with all motors running plus the watts needed to start your largest motor. If you can set your motor-starting sequence so the largest motor starts first, you may be able to use a slightly smaller generator than if you start your largest motor last. If you can’t predict or control motor-starting sequence, you may want to consider sizing the generator so that it has the power to start all motors at once.

Pick your generator
Generators are specified at maximum output and rated output. Select a unit that has a maximum power rating that meets your worst-case power demand.

“Yamaha generators also have a very high surge (starting) output rating,” says Sheehan. “This output is about three times the rated output and is available for a very short time — two or three seconds — when extra power is needed to start a motor.” If loads surpass this rating for any longer period, the units’ thermal or non-fuse breakers will cut power to protect the generator.

The maximum output rating is generally the amount of current the generator can produce over a 20-minute period before the non-fuse breakers cut power. This rating allows the unit to be used for heavier jobs of short duration.

The rated output is the amount of current the generator can supply continuously.

Although most power tools operate on 120 volts, you may want to consider a generator that produces 120 and 240 volts if you think you will ever have a need to power 240-volt tools or appliances.

Finally, some generators are equipped with a 12-volt DC output. This is provided only to allow battery charging. Connect battery-powered tools to the battery, and then the generator to the battery. Directly connecting DC-powered tools to the generator could damage the tool.

Run with the right cord
Even if you correctly size the generator for the job, using the wrong extension cord could create safety hazards and tool damage.

An extension cord that is too light or too long for the job creates resistance, which in turn creates heat that can melt the cord, causing a short or electrocution danger. A too-light cord also starves your power tool, causing it to build up heat that can damage the tool.

See Table 2 to match the proper power cord to the load and length of run.

Always consider safety
Generators produce two things that can be dangerous if not properly managed: electricity and exhaust, which contains carbon monoxide.

Make sure the generator, extension cords and power tools provide shock protection. Ground-fault circuit interrupters should be used.

Never operate a generator in an enclosed area or where the exhaust fumes may be blown or pulled into an enclosed area. The buildup of tasteless, odorless and colorless carbon monoxide can — and has — killed work crews.

Published in the January/February 2001 issue of Contractor Tools and Supplies magazine.

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