Generator Sizing

B

Birken Vogt

Senior Member
Location
Grass Valley, Ca
Grouch1980 said:
I have the bills for an entire year... all of 2023. The 2 values I provided in post #13 are the maximum demands... I just had them flipped.
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I find that number a hard thing to believe on a house this size. I would run a NEC load calc with all the square footage/equipment/appliances just as a sanity check. You can't allow a generator to overload even once. Realistic worst case as Larry said in #2. All air conditioning running (or heat depending on climate) plus dryer and water heat or anything of that nature all in as much as it is likely to be. Cooking equipment I allow more diversity. Sometimes those demand numbers are spread out over 15 minutes or 60 minutes and may not quite be true instantaneous demand. Or run a worst case simulation on the house with a max logging current clamp.
 
Grouch1980

Grouch1980

Senior Member
Location
New York, NY
Birken Vogt said:
I find that number a hard thing to believe on a house this size. I would run a NEC load calc with all the square footage/equipment/appliances just as a sanity check. You can't allow a generator to overload even once. Realistic worst case as Larry said in #2. All air conditioning running (or heat depending on climate) plus dryer and water heat or anything of that nature all in as much as it is likely to be. Cooking equipment I allow more diversity. Sometimes those demand numbers are spread out over 15 minutes or 60 minutes and may not quite be true instantaneous demand. Or run a worst case simulation on the house with a max logging current clamp.
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Very true... I'm taking all the large loads into account. AC's, heating, electric cooking and dryers, etc.
 
tortuga

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
Grouch1980 said:
Hi all,
I'm adding a new standby generator to an existing 3 story townhouse. The existing electrical service to this townhouse is 600 amps at 208 volts 3 phase
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Will you be installing a ATS on both service disconnects (meters)? Is the commercial one three phase and the residential single phase?
Sounds like the building was mixed use at one time and is now a single family dwelling?
I'd probably convert it to a standard 320A residential service while adding the ATS.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
tortuga said:
Will you be installing a ATS on both service disconnects (meters)? Is the commercial one three phase and the residential single phase?
Sounds like the building was mixed use at one time and is now a single family dwelling?
I'd probably convert it to a standard 320A residential service while adding the ATS.
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Yeah or how about just get rid of the self contained meter and combine everything under the 400A CT service?
 
Grouch1980

Grouch1980

Senior Member
Location
New York, NY
tortuga said:
Will you be installing a ATS on both service disconnects (meters)? Is the commercial one three phase and the residential single phase?
Sounds like the building was mixed use at one time and is now a single family dwelling?
I'd probably convert it to a standard 320A residential service while adding the ATS.
Click to expand...
I'm looking at putting in two ATS switches. one on each load side of a meter. 2 meters feeding 2 ATS switches; the ATS switches then feed the service equipment, which are several panels with main circuit breakers (the main circuit breakers are the service switches). I wasn't planning on altering the metering for the building. And the EM terminals of both ATS's would go to an emergency panel, and that panel is fed by the generator.

At least that's what I'm thinking of doing for now.
 
Grouch1980

Grouch1980

Senior Member
Location
New York, NY
tortuga said:
Will you be installing a ATS on both service disconnects (meters)? Is the commercial one three phase and the residential single phase?
Sounds like the building was mixed use at one time and is now a single family dwelling?
I'd probably convert it to a standard 320A residential service while adding the ATS.
Click to expand...
And both the residential and commercial meters are 3 phase.
 
Grouch1980

Grouch1980

Senior Member
Location
New York, NY
Another question: Is there any issue or violation by placing the ATS switches between meters and the main breaker panels (the main breakers in the panels being the service switches)? In other words, can I place the ATS's on the line side of the service switches?

If I place the ATS's on the line side, do they now have to be service entrance rated ATS's, thereby requiring to disconnect the neutral and ground terminals from each other inside the panels? Or can they be regular ATS's, and I keep the main bonding jumpers within the panels?
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Grouch1980 said:
Another question: Is there any issue or violation by placing the ATS switches between meters and the main breaker panels (the main breakers in the panels being the service switches)? In other words, can I place the ATS's on the line side of the service switches?

If I place the ATS's on the line side, do they now have to be service entrance rated ATS's, thereby requiring to disconnect the neutral and ground terminals from each other inside the panels? Or can they be regular ATS's, and I keep the main bonding jumpers within the panels?
Click to expand...
Normally, the ATS's would need to be service rated, would become your service disconnects, and the N-G would need to be separated at the existing service panels and the conductors to them changed from 3 wire to 4 wire. However, you may be able to go the much loved, much talked about, call them emergency disconnects route and skip all that. What code are you on?
 
Grouch1980

Grouch1980

Senior Member
Location
New York, NY
electrofelon said:
Normally, the ATS's would need to be service rated, would become your service disconnects, and the N-G would need to be separated at the existing service panels and the conductors to them changed from 3 wire to 4 wire. However, you may be able to go the much loved, much talked about, call them emergency disconnects route and skip all that. What code are you on?
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I’m on the 2008 code. NYC is still behind. What are these emergency disconnects?
 
Grouch1980

Grouch1980

Senior Member
Location
New York, NY
T

topgone

Senior Member
There are questionable figures there:
1) the multiplier 40 tells me the meter is getting input from a CT with CTR 200:5A (=40).
2) An energy consumption of 10,080 kWH per month will yield an average load of = 10,080 kWH/(31 days x 24 hours per day) = 13.5 kW or an amp draw of 13.5 x 1000/(208 x 1.732) = 37.5 A!
3) I am wondering why the peak demand stayed at 8.4 kW (23.3 A) when the computed "average load" was 13.5 kW (37.5 A).
4) Your demand factor is the peak demand divided by the connected loads or contracted demand. I'd be surprised if your principal opted for a 40 kW demand from the utility when your peak demand is only 8.4 kW! (larger demand schedule means more expensive energy rates). 13.5 kW (average) is far from 8.4 kW (peak)!
The highest average 15-minute period of demand over a month is known as peak demand.
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Grouch1980

Grouch1980

Senior Member
Location
New York, NY
topgone said:
3) I am wondering why the peak demand stayed at 8.4 kW (23.3 A) when the computed "average load" was 13.5 kW (37.5 A).
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Just digging back into this thread.... I don't know! You're saying the peak load was lower than the average... which would force all the other numbers to be below the peak.
 
Grouch1980

Grouch1980

Senior Member
Location
New York, NY
electrofelon said:
Normally, the ATS's would need to be service rated, would become your service disconnects, and the N-G would need to be separated at the existing service panels and the conductors to them changed from 3 wire to 4 wire. However, you may be able to go the much loved, much talked about, call them emergency disconnects route and skip all that. What code are you on?
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I attached a service rated ATS switch by Generac. I'm looking at the 200 amp, 3 phase model: RTSW200G3.

I have several questions::
1. Is the service disconnect a circuit breaker that's inside the ATS? If so, is it accessible? it seems like you have to unscrew the cover off the ATS to gain access to it.
2. For the model number I'm looking at, Is the service disconnect a 200 amp circuit breaker?
3. Is the service disconnect located on the line side of the normal terminal?
 

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electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Grouch1980 said:
I attached a service rated ATS switch by Generac. I'm looking at the 200 amp, 3 phase model: RTSW200G3.

I have several questions::
1. Is the service disconnect a circuit breaker that's inside the ATS? If so, is it accessible? it seems like you have to unscrew the cover off the ATS to gain access to it.
2. For the model number I'm looking at, Is the service disconnect a 200 amp circuit breaker?
3. Is the service disconnect located on the line side of the normal terminal?
Click to expand...
Yes it has a 200 A service disconnect breaker inside, on the line side of the contactor. it's easily accessible. Usually that cover is just held on with a thumb screw.

Although technically "service rated" could mean it is just the contactor which could be your service disconnect but not have any over current protection, I have never seen such a beast in an ATS so essentially "service rated" means it has a service disconnect with ocpd that is the first thing the line conductors hit. Think of it as an enclosed circuit breaker that happens to have some other stuff after the load terminals.
 
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