Learning how to calculate for a generator

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Klein Products
Journeyman Electrician
Hi Guys, trying to expand my knowledge on generators and I thought I'd ask someone for help. I've been looking at the online calculators and when they talk about determining start-up wattage, do I use LRA for amperage or use RLA to get the wattage and then times the total by three? It seems they suggest normal wattage x3.. I may be missing something. Please be nice, I'm just trying to learn new things. :) Thanks for any assistance.


Code Historian
Electrical Design
Logan when sizing a generator my first step is to determine if its an optional generator or a 'legally required' one.
For the typical optional
702.4(B) [of the 2020 NEC] states that if you have 1) manual transfer equipment the generator only needs to be sized to carry the load intended to be run at any one time.
This the typical homewoner 'manual' inlet generator set up.

702.4(B)(2)gives two options for sizing the load supplied by automatic transfer switches.

Now if its a 'legally required' system i go to article 701, 701.4(B) requires an article 220 load calculations or 'another approved method'.

Keep in mind The 'Authority Having Jurisdiction' (AHJ) for these may not be just a local electrical inspector, as you might be on a base or government or quisi public/private managed facility.
To delve into 'legally required' systems its good to be familiar with article 220 load calculations.

Lastly Ill add that 110.3(B) includes any installation instructions that come with a generator into the code.
So if the manufacturer has a prescribed way for sizing a generator that becomes a requirement.
Hope this helps
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