Generator Sizing

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For a residential application, you could use Cummins Power Suite and use the peak usage on the bill (in kW), and then using kW-hr for your average loading. I would set up a base load of the size of your kW-hr value, and then subtract it from the peak load and use that value to set up a motor starting load of that size. Then put the motor to start AFTER your average base load is running and tell the program to size the loads for a standby generator.

For an industrial, critical or even commercial applications, this approach obviously doesn't apply.
 
A friend of mine who is an electrical contractor is asking for advice. He has a customer with a 200A service on a 5000 sq ft residence who wants him to install a generator. He has suggested a genny that has a 150A breaker ( I don't know the kW rating), but the customer is balking at the price and wants to save money by getting one with a 125A breaker (likewise). My friend is asking me how he can decide if it will be big enough not to nuisance trip out during peak demand, and there is no demand information available from the utility. This sort of thing is not in my wheelhouse; has anyone have any advice to lend?
 
You can load shed certain appliances and equipment and that would allow you to go with a smaller genny.
 
ggunn said:
A friend of mine who is an electrical contractor is asking for advice. He has a customer with a 200A service on a 5000 sq ft residence who wants him to install a generator. He has suggested a genny that has a 150A breaker ( I don't know the kW rating), but the customer is balking at the price and wants to save money by getting one with a 125A breaker (likewise). My friend is asking me how he can decide if it will be big enough not to nuisance trip out during peak demand, and there is no demand information available from the utility. This sort of thing is not in my wheelhouse; has anyone have any advice to lend?
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Turn everything they want to run off generator and amp clamp it. Take into consideration of motor loads and can use load shedding.
 
If it has a ATS, doesn't the generator need to supply the total calculated load unless you load shed? A 200A panel most likely won't have a 200A calculated load. Do a load calc. If it is larger than 125A, see what the customer is willing to load shed. If it has a manual transfer switch, the user can manually load shed.
 
Hello. What are the risks/ramifications of starting/attempting to start an HVAC system condensing unit pump with an LRA of 109 Amps with a portable genset with a "surge" amps rating of 15KW (6.5A @ 240V)? Instant windings damage? 50 Amp circuit breaker trip? And can "hard start kits" reduce LRA by 50%?
 
ggunn said:
A friend of mine who is an electrical contractor is asking for advice. He has a customer with a 200A service on a 5000 sq ft residence who wants him to install a generator. He has suggested a genny that has a 150A breaker ( I don't know the kW rating), but the customer is balking at the price and wants to save money by getting one with a 125A breaker (likewise). My friend is asking me how he can decide if it will be big enough not to nuisance trip out during peak demand, and there is no demand information available from the utility. This sort of thing is not in my wheelhouse; has anyone have any advice to lend?
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It would be best to itemize the loads to be served. Sizing the smallest generator that can supply power to those loads is done by assuming that "no load is put ON at the same time".
A short example:
You have two pumps, 1 is 50 HP and another rated at 10 HP. Say further that both have a PF is 0.8 each when running and 0.28PF each when starting.
When starting, the starting kVA draw of the 50HP motor will be around 177 kVA and then reduces to 46.63 kVA. The next unit to run will be the 10 HP pump which will likely draw about 35.4 kVA. Add that to the kVA of the running 50 HP (46.63 kVA) and you will get a total of 82 kVA! The rule of thumb I use is 2.5 X the total load, (50 + 10)/0.8 X 2.5 = 187.5 kVA ~ 200 kVA. With that "no simultaneous starting" limitation, you can survive with just 100 kVA.
 
For a residence, there are LOTS of things you don't really need in a power outage-- Water heater / stove / any electric heating- baseboard, floor, etc. / outdoor 'for pretty' lighting.
 
sajesak said:
Hello. What are the risks/ramifications of starting/attempting to start an HVAC system condensing unit pump with an LRA of 109 Amps with a portable genset with a "surge" amps rating of 15KW (6.5A @ 240V)? Instant windings damage? 50 Amp circuit breaker trip? And can "hard start kits" reduce LRA by 50%?
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didn't get any response to this
 
sajesak said:
didn't get any response to this
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Now that I reread your post, it becomes clear that you should not have placed it within an existing thread with its own topic that has nothing to do with your question.

I suggest posting a new thread and asking your question there.
 
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