Generator Load Calculations

vernon_15ss

Member
Location
Goldston, NC
Occupation
Electrical Contractor
Do you figure load calculations for a 22kw generator with a 200A automatic transfer switch the same way that you figure load calculations for a house? It's a four hundred amp service but the generator will only be serving one of the two hundred amp panels.
 
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powerpete69

Senior Member
Location
Northeast, Ohio
Occupation
Professional Electrical Engineer
Do you figure load calculations for a 22kw generator with a 200A automatic transfer switch the same way that you figure load calculations for a house? It's a four hundred amp service but the generator will only be serving one of the two hundred amp panels.
If you have a 22KW, three phase generator at 480 volt, it is capable of 33.1 amps with assuming a .8 power factor. Not sure how that applies to a 200 amp panel?

If its single phase 240 volt genset, then capable of 114 amps assuming a .8 power factor. If this is the case, 114 amps might be enough....?

Perhaps these things are obvious.....explain your question in more detail please?
 

kingpb

Senior Member
Load on a generator you will have to understand not only the continuous load, which will go against the rated nameplate, as explained by powerpete69 above. But you also have to account for starting loads and how that effects the machines capability. Cat and Cummins both have on-line sizing programs that are free if you just sign-up. IMHO, I think the CAT program is better.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
The op is talking about a house. I assume he means 22000/240= 92 amps. This cannot control a load of 200 amps. What is the load of the panel? If there is an automatic transfer switch then unless there is load shedding this may not work
 

powerpete69

Senior Member
Location
Northeast, Ohio
Occupation
Professional Electrical Engineer
I don't think he is talking about a house. I believe he is talking about a commercial or industrial situation....can you figure it the same way as a house.
A 22KW generator would likely power a house just dandy - especially with gas oven and dryer.

For a commercial or industrial situation - it would only power a few smaller loads in my opinion which my be just fine if that is the case.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Engineer
I'm thinking the OP is talking about a house, since there are two 200A panels -- typical house 400A service implementation. But does it really matter? You do the same regardless of commercial -vs- residential. Calculate the load on the panel. If it is higher than what the generator can support, you need automatic load shedding installed. Pick the items you want to shed and eliminate them from the load calc. You'll find that subtracting load doesn't lower the load calc as quickly as you might think, especially with the residential Optional calculation.
 

Npstewart

Senior Member
In summary, the answer to your question is you do your dwelling unit calculation based on the standard or optional method and size your generator for that kW. Its very common to have a 22kW generator (92A @ 240) feeding into a 200A SER ATS. Youll have a 100A/2P breaker with 100A conductors feeding in one side of the ATS, and 200A conductors feeding into the other. If the load exceeds the rating of the generator then you need load shedding. However, even if it doesent exceed it, its always a good idea to use load shedding anyway because there are some delayed starting capabilities so you dont run into issues with start AC equipment with a loaded generator.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
You can calculate based on load shedding so that you can use a 20kw generator for a 20 amp service as long as the shedding keeps the load under the 20 kw. I am assuming auto transfer switch as mentioned
 

Seekthetruth

Member
Location
Florida
Occupation
Electrical
Let's say the appliances are gas and there are outlets behind them to convert to electric at a later date. Can you just use the nameplate rating of the gas appliances for your generator load calcs?
 

brantmacga

Senior Member
Location
Georgia
Occupation
Electrical contractor
Do you figure load calculations for a 22kw generator with a 200A automatic transfer switch the same way that you figure load calculations for a house? It's a four hundred amp service but the generator will only be serving one of the two hundred amp panels.
Yes, you’ll need to do a load calc. I often use the optional method for simplicity when sizing for generators.

If you’re looking at a new Generac 22KW, the transfer switch has built in load shedding capability for 4x HVAC units, and you can buy more wireless load shed modules; you may already be aware of this idk. I’ve got four 22kw units on order now for houses that have 400A services, so it’s not uncommon. These customers only wanted to run one or two of their HVAC units and all of the lighting/receps. They all have gas appliances. If the customer tells me they want to run everything with no load shedding, I can usually get by with a 36kw on 200A services and a 45kw on 400A services. Some need more, some need less.

Just FYI, generac just released a 24kw air cooled unit.


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Seekthetruth

Member
Location
Florida
Occupation
Electrical
Yes, you’ll need to do a load calc. I often use the optional method for simplicity when sizing for generators.

If you’re looking at a new Generac 22KW, the transfer switch has built in load shedding capability for 4x HVAC units, and you can buy more wireless load shed modules; you may already be aware of this idk. I’ve got four 22kw units on order now for houses that have 400A services, so it’s not uncommon. These customers only wanted to run one or two of their HVAC units and all of the lighting/receps. They all have gas appliances. If the customer tells me they want to run everything with no load shedding, I can usually get by with a 36kw on 200A services and a 45kw on 400A services. Some need more, some need less.

Just FYI, generac just released a 24kw air cooled unit.


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Thanks Have a great day
 

Chamuit

Senior Member
Location
Texas
Yes, you’ll need to do a load calc. I often use the optional method for simplicity when sizing for generators.

If you’re looking at a new Generac 22KW, the transfer switch has built in load shedding capability for 4x HVAC units, and you can buy more wireless load shed modules; you may already be aware of this idk. I’ve got four 22kw units on order now for houses that have 400A services, so it’s not uncommon. These customers only wanted to run one or two of their HVAC units and all of the lighting/receps. They all have gas appliances. If the customer tells me they want to run everything with no load shedding, I can usually get by with a 36kw on 200A services and a 45kw on 400A services. Some need more, some need less.

Just FYI, generac just released a 24kw air cooled unit.


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Agreed. KISS. Biggest concern would be, in our area, how many A/C's are you trying to run. I've seen 400 ATS with 20kW genset with LC on all A/C condensers.
 

mannyb

Senior Member
I am new to residential so I find this topic interesting. My boss uses the APP provided by Generac to size generator and transfer switch for homes The 22kw generator for a 200a service is pretty common but I dont understand how the load shedding works., he says its built in but I dont ever see any modules or anything other than transfer switch engaging on and off. How does the load shedding happen? I dont see where if sheds unnecessary loads and power only critical loads
 

brantmacga

Senior Member
Location
Georgia
Occupation
Electrical contractor
I am new to residential so I find this topic interesting. My boss uses the APP provided by Generac to size generator and transfer switch for homes The 22kw generator for a 200a service is pretty common but I dont understand how the load shedding works., he says its built in but I dont ever see any modules or anything other than transfer switch engaging on and off. How does the load shedding happen? I dont see where if sheds unnecessary loads and power only critical loads
The transfer switch has 4 dry contacts that you can loop t-stat power for HVAC through to turn them off. Outside of that, you need Smart Management Modules for load shedding; it’s a 2-pole contactor in 50A or 100A sizes, and install it at each appliance.


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Chamuit

Senior Member
Location
Texas
The Generac Smart Modules detect the frequency of the power coming in. When it drops below the acceptable parameter (can't remember what it is right now), the module opens and removes the load from the generator. They can also lock out larger loads while the generator is running.
 

InverterGuy

Member
Location
Cincinnati
Occupation
Regional Sales Manager
What about load shedding with a lighting inverter? Is that possible to take some constant lighting loads per se and use the ATS so you still have light?
 

Chamuit

Senior Member
Location
Texas
What about load shedding with a lighting inverter? Is that possible to take some constant lighting loads per se and use the ATS, so you still have light?
Not sure what you are asking.

The four contacts inside a Generac ATS can control any four loads, not just HVAC units. As long as you can use 24VAC or less for your controls, that (SACM) module will work. You can also add the Smart Management Modules to control (open or lockout) 4 additional loads.
 
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