Generator load calc for compound

[JoeNorm]

I have a customer who wants a whole property generator. The service is 320A with meter main enclosure with dual disconnects, one at 200a and one at 150a.

The property incudes a main residence currently under remodel, pool house, carport, guest building, garage, well pump building, etc.

Is the process for sizing simply going through all the dwelling calcs and adding them up? Seems like an arduous task to gather every single nameplate in this situation.

How would you approach a situation like this? Budget is not really a concern.

 

[PD1972]

If the remodel does not include additional electrical load (mainly one-for-ones), then one method would be to size the generator off of existing utility information. Hopefully existing kW demand is available as that is the easiest way to size a generator for an existing building. If your customer isn't getting billed for demand, it is also possible to extract kW based off max. 15 minute kWh recordings, which I have had utility companies provide me with.

If you are unlucky enough to not be able to use any existing demand information, I believe you will need to either:
A. Add up all the loads (essentially an NEC 220 calculation) for the property or
B. Generator based on service size of the property

Between A & B above, option B is definitely easier, but you will likely end up with an oversized generator and potentially wet stacking issues if diesel.

 

[JoeNorm]

Thank you, These are good suggestions

 

[hillbilly1]

With that probable size generator. natural gas or propane would be better than diesel because of maintenance and fuel degradation, unless they are prepping for WWW III, or a civil war.

 

[grasfulls]

A. Add up all the loads (essentially an NEC 220 calculation) for the property or
B. Generator based on service size of the property

Would any inrush calcs be necessary? The utility can handle these better than a genset.

 

[PD1972]

Would any inrush calcs be necessary? The utility can handle these better than a genset.

If there are inrush concerns, then a manufacturer's sizing software (Generac, Kohler, etc.) that allows you to input loads and starting methods should be used instead. The software allows you to input information such as load steps, AC unit tonnages, starting methods, oversized alternators, running load %, allowable % voltage dip, etc. to get the correctly sized generator.

Sometimes it is important to use this software, such as if the generator serves fire pumps or large motor loads relative to generator size. For residential, I think it is less important as long as you are leaving a 20-30% spare capacity on the generator relative to calculated numbers.

 

[Knightryder12]

I would definitely use a generator sizing software for this. I do not recommend using the utility information to size a generator, but that's just me. Generator sizing software is usually free from the generator websites.

 

[Birken Vogt]

Many generator sizing programs are nonsense. I don't use them. Kohler makes a good one but have not used it in years. Article 220 is the best way IMO. You get kind of a worst case with 220 but not as stupid as just adding up all the numbers which would be ridiculous.

Here is how it works. All fixed in place appliaces and those with dedicated circuits such as AC, refrigerator, dryer, well pump. That usually only gives you about 10 nameplates to track down. 3 VA per square foot living space but 35% demand factor on that. You know the drill. Then just run it through the formulas. Not really that hard. I like the John Sokolik spreadsheet but there is another one around too that Dennis Alwon had something to do with.