rojay
Senior Member
- Location
- Chicago,IL USA
Can the optional calculation be used for sizing a residential optional standby generator if the gennie is only backing up selected loads and not the entire home?
Optional calculation NEC 220.82
- Thread starter rojay
- Start date
- Status
Good question.I presume this generator has an ATS? I'll venture a yes as long as its a single dwelling unit and there is no way to turn the excluded loads on when the generator is running.
rojay
Senior Member
- Location
- Chicago,IL USA
Yes, there is an ATS. The language in 220.82 about the “total connected load” makes me think that the standard method is the only way to go.
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
I find the actual language very confusing. But no AHJ has ever caused me a big problem after seeing we know what we are doing. Here are some of my justifications.
1. A generator is basically a feeder if done for partial or a service if done for all
2. 220.4x is for services or feeders
3. 220.82 is also stated for services or feeders
4. 702 merely says you have to use Art 220 to justify, or "another approved method", so 220.4x or 220.82 are both valid
5. Generator manufacturers have their own calcs sometimes, that can be an "approved method"
6. Engineers calcs can also be an "approved method"
7. Other approved methods may include 30 days' logging or 15 minute demand figures
1. A generator is basically a feeder if done for partial or a service if done for all
2. 220.4x is for services or feeders
3. 220.82 is also stated for services or feeders
4. 702 merely says you have to use Art 220 to justify, or "another approved method", so 220.4x or 220.82 are both valid
5. Generator manufacturers have their own calcs sometimes, that can be an "approved method"
6. Engineers calcs can also be an "approved method"
7. Other approved methods may include 30 days' logging or 15 minute demand figures
When the ATS transfers to generator then its the only power supply, there is no other power to the dwelling unit, your off grid at that point.
So I don't see an issue using the optional.
Now if you mixed things up a bit and had say a 320A service with two 200A panels to supply the dwelling, and one panel with an ATS and another panel with a manual transfer switch then that would require a general.
So I don't see an issue using the optional.
Now if you mixed things up a bit and had say a 320A service with two 200A panels to supply the dwelling, and one panel with an ATS and another panel with a manual transfer switch then that would require a general.
rojay
Senior Member
- Location
- Chicago,IL USA
I agree that the language is confusing. I saw a post on the forum some time ago where someone had asked whether the reduced service/feeder conductor sizes permitted by NEC 310.15(B)(7) could be used for a generator feeder in a partial loads backup scenario. The general consensus was that since there wouldn’t be the same load diversity in the partial backup situation, the allowances of 310.15(B)(7) couldn’t be applied.
The optional calculation in 220.82 seems to be saying the same thing.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
imo, I would use 220.83 for existing dwellings . Optional can be used if the feeder is 100 amps or more
rojay
Senior Member
- Location
- Chicago,IL USA
Can I use 220.83 if I’m not adding additional loads?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I assume so but Imo there really isn't any real help from the NEC on this. I think you need to know your loads and figure what loads you have and if they will be all on at one time. You might have to do a 100% calculation-- no percentage to de-rate ....
For example, if you had a refrigerator, microwave and a furnace you may want to add those loads up at 100%.
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
The bottom line is, from my experience, you need to do two calculations.
1. Calculate the loads as they actually are, real life, to size with headroom and not grossly over-size your generator, also considering motor starting, etc.
2. Find a compliant method in the NEC to justify to the AHJ the number you came up with in 1. above.
Generator system design is a balancing act.
1. Calculate the loads as they actually are, real life, to size with headroom and not grossly over-size your generator, also considering motor starting, etc.
2. Find a compliant method in the NEC to justify to the AHJ the number you came up with in 1. above.
Generator system design is a balancing act.
A conductors from a standby generator are always a feeder. The definition of a service says they are the conductors and equipment connecting the serving utility to the wiring system of the premises served. The conductors from a standby generator are not connected to the serving utility and are, by definition, not a service. They conductors from a standby generator are also not a branch circuit because they are not between the final overcurrent device protecting the circuit and outlet(s). Therefore, it has to be a feeder.
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
I agree, I made a mistake in #1 there. The generator is always a feeder.
What if it was not partial load but supplying the whole house, could the reduced size be used or if it was supplying the entire house and certain pieces of equipment, such as heat strips were disengaged or AC units that could be controlled through the load shedding built into the ATS?
- Status