Generator Sizing

[Eddie702]

I am sizing a generator for a decent sized house. It has a 200 amp service with a 125 am sub panel next to the main service. All the generator loads are in the 125 amp panel.

I calculated the load on the 125 amp panel at 98 amps using the optional calculation AC larger than heating. The loads are as follows:

General lighting 2774 sq ft @ 3va=8322va First 10000@ 100%= 10000
cook top 5800 Remainder (total-AC load-10kw) = 18888 @ 40%= 7555.20
dryer 5000 AC load @ 100%= 5904
well pump 1656 Total= 23459.20/240 volt=98 amps
disposal 850
dishwasher 1320
Boiler (runs for DHW) 1440
laundry 1500
2 small appliance 3000
AC 5904
Total 34792

I used the square footage @3va for the entire house which gives me a little fudge factor as not all the general use circuits are on the sub panel.

They are using a portable generator now.

My plan is to put the new ATS between the Main Panel and the sub panel and feed the ATS with the 125 amp breaker

What size would you select for the new generator?

Am I missing something?

 

[LarryFine]

I would stop at 20 to 24kw. Do they really have to do laundry during the day in the summer during outages?

They haven't tripped the 125a sub-panel's feeder breaker, have they? That's the most you'd need to provide.

 

[Eddie702]

I would stop at 20 to 24kw. Do they really have to do laundry during the day in the summer during outages?

They haven't tripped the 125a sub-panel's feeder breaker, have they? That's the most you'd need to provide.

No, never tripped the 125, I laughed when she insisted the garbage disposal be on the emergency panel, kind of rediculous everything she wanted. Right now they have a 7kw portable gen hence the sub panel with the breaker interlock on the sub, old gen feeds into the sub. I put the sub panel in and moved the circuits over a few years back. and did the power inlet for the portable. Now she wants a bigger gen.

I would rather not rip the main panel out to get the ATS in there. What do you think about feeding the ATS with the 125 from the main panel, then feeding the sub panel from the ATS (naturally) and picking up the gen input.

With auto transfer does the gen have to be capable of supplying the 98 amps calculated load on the sub panel????

 

[LarryFine]

What do you think about feeding the ATS with the 125 from the main panel, then feeding the sub panel from the ATS (naturally) and picking up the gen input.

That's what I'd do. Don't forget to remove the bonding jumper in the ATS, as it will be inserted in a sub-panel feeder.

With auto transfer does the gen have to be capable of supplying the 98 amps calculated load on the sub panel????

No, because you only have to include loads that will automatically start. A dryer is an example of a load that will not.

 

[ron]

With auto transfer does the gen have to be capable of supplying the 98 amps calculated load on the sub panel????

Due to NEC Section 702.4(B)(2), you either need to size the optional standby system (which I include the gen and ATS in the definition since the NEC definition does not describe) for full load or include load management which would manage the connected load in the event large loads like a dryer was running when the gen started for an outage.

 

[LarryFine]

Due to NEC Section 702.4(B)(2), you either need to size the optional standby system (which I include the gen and ATS in the definition since the NEC definition does not describe) for full load or include load management which would manage the connected load in the event large loads like a dryer was running when the gen started for an outage.

I repeat that a dryer need not be included, as it will stop at power loss, and not re-start with the generator.

 

[Eddie702]

Due to NEC Section 702.4(B)(2), you either need to size the optional standby system (which I include the gen and ATS in the definition since the NEC definition does not describe) for full load or include load management which would manage the connected load in the event large loads like a dryer was running when the gen started for an outage.

Ron, thank you. That is what I read as well.

 

[ron]

I repeat that a dryer need not be included, as it will stop at power loss, and not re-start with the generator.

Larry, I agree that it wouldn't auto restart, but the code language doesn't cover that.
It says either says to size based on full load or provide load management.
I guess that leaves the definition of load management up for defining, that maybe since it doesn't auto restart, it is providing load management unto itself.....