mkgrady
Senior Member
- Location
- Massachusetts
300 kW generator. That sounds about right. I wonder how big the natural gas pipe run to it was
Sizing Stand Alone Home Generator
- Thread starter chevyx92
- Start date
- Status
stickboy1375
Senior Member
- Location
- Litchfield, CT
Not sure, if/when I go back, I'll take some more pics...
Here is the actual generator... You can see the gas line supplying the generator itself.... I don't know where the main is.
Last edited:
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I suspect that the generator incorporates the final pressure regulator stage, so the feed pipe does not need to be as large as it otherwise would be.
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
Id imagine. :lol:
In a case like that Id probably drop a 35 or 50kw and not look back.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
The house isn't that big, but you did say it is on an island. Any chance they weren't considering possible future loads when sizing things?
Still could be considering more then necessary, but would justify it some.
Still could be considering more then necessary, but would justify it some.
stickboy1375
Senior Member
- Location
- Litchfield, CT
You would have to see the place, believe me, the service is extremely over sized.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Well it did seem pretty extreme other then if there is big future plans.
I did a home once that was at least 10000 square feet with all electric heat. Had 4- 200 amp service disconnects mostly because it was cheaper to do that then to run a single 600 amp switch then tap to same panels I was otherwise going to use, and even with all that may still have been able to put less then a 600 amp fuse in the switch, but 400 amps would probably have been asking for at least an occasional trip.
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Padmount, though surprisingly it was a 75kVA. We did have 60 kVA of electric heat plus the rest of the power for the home though.I bet the POCO hooked it up to a 50kw sow or pig without incident.
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
75x1000/240=312amps. Pole pigs generally can take a 200% overload for an hour or two, so 600amps. So it is sized correctly in theory as long as the load isn't constant all day.
But in normal homes, even those with an electric range can get away with a 40 or 50amp service 99% of the time.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I agree, and is why I was a little surprised it wasn't a 50kVA transformer. Only time you would ever have all 60 kVA of heat running for any lengthy period is going to be in most extreme cold weather conditions, and one must also remember the transformer is in the cold ambient when that does happen, but probably is free of snow and ice accumulations
mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
I agree, and is why I was a little surprised it wasn't a 50kVA transformer. Only time you would ever have all 60 kVA of heat running for any lengthy period is going to be in most extreme cold weather conditions, and one must also remember the transformer is in the cold ambient when that does happen, but probably is free of snow and ice accumulations
Exactly. 50kva would have done it. And yahhh, no bushing flash overs during ice storms
Radioman91
Member
- Location
- Swartz Creek, MI. 48473 USA
Generator Sizing
Generator Sizing
As a factory authorized service dealer for Generac we routinely find both the generator and the fuel supply system grossly undersized. As the OP stated, NEC Article 702.4(B) REQUIRES the generator to be sized as per Article 220. Generac and most other generator manufacturers use the "Optional Calculations" found in Article 220.80 Part IV for sizing generators.
Since this discussion regards automatic transfer equipment NEC Article 702.4(B)(2)(a) also applies, requiring the installed generator be capable of supplying the "Full Load" that is transferred by the automatic equipment OR per NEC Article 702.4(B)(2)(b) requiring an "Energy Management" system that will limit the connected load to a value less than or equal to the capacity of the installed generator.
Also remember that most of these residential and small commercial generators are dual fuel rated as "Standby" equipment. The Standby rating per Generac factory tech school is noted below.
•Air-Cooled generators are “STANDBY” rated and MUST NOT be operated beyond 60% of their load capacity except as noted below.
•1 hour out of every 12 hours
•2 hours out of every 24 hours
•250 hours out of any 12 month period.
Example: 20Kw Generac standby generator operated on natural gas.
Maximum Connected Load - 18Kw (Note: a 20Kw unit operated on Natural gas is de-rated to 18Kw.)
Maximum Continuous Load - 18Kw x 60% = 10.8Kw
The above examples assume that the fuel supplying the unit is adequate and meets or exceeds ALL of themanufacturers requirements. The following statements appear in all of the Generac factory installation manuals.
•“GAS PIPE SIZING IS ONE OF THE MOST COMMONLY MADE MISTAKES. A PROPERLY SIZED GAS PIPE IS CRITICAL TO THE PROPER OPERATION OF THE GENERATOR. THE GENERATOR INLET SIZE HAS NO BEARING ON THE PROPER GAS PIPE SIZE.”
•“THE GAS SUPPLY AND PIPE MUST BE SIZED AT 100% OF THE GENERATOR BTU RATING.”
For our example noted above, the 18Kw generator MUST HAVE a natural gas meter and piping system capable of supplying a minimum of 308,000 Btu/hr. This fuel must be supplied at a pressure of 7"W.C. with a pressure drop not exceeding 1/2" W.C. at the generator under full load conditions. In addition to the generator fuel requirements you must add the fuel requirements of ALL other gas appliances connected to the system.
The generator fuel piping system MUST also meet ALL of the following requirements:
•You MUST add 2.5ft for each bend, tee, elbow, or angle to the overall measured distance from the source to the generator.
•ALL fuel shut-off valves MUST be “Full Flow Rated” and match the internal dimensions of the required fuel pipe I.D.
•Piping length is measured form the “LOW PRESSURE DISCHARGE PORT” on the natural gas meter or LP regulator to the “INLET PORT” on the generator.
•This is a CRITICAL step! The generator gas supply must NEVER be tapped from the gas line serving the building. It MUST be a dedicated gas line of adequate size run directly from the meter to the generator.
•Flexible fuel line
• a) Do NOT bend!
• b) Do NOT attach directly to the generator!
• c) Check ALL connections for leaks.
•Sediment trap (Drip Leg) near generator.
•“Full Flow Rated” shut-off valve near generator.
•Verify capacity of gas meter or LP tank to supply sufficient fuel for “Full Load” generator operation plus ALL other connected gas appliances operating simultaneously, at a pressure drop NOT exceeding 1/2" W.C.
•The supplied “Flex-Line” is a vibration isolation device and is NOT to be bent or twisted in place of using the proper pipe fittings.
•Generators must NEVER be hard pipe connected without a “Flex-Line” installed!!
Problems caused by neglecting the proper sizing of the generator and the fuel supply system are install errors and are generally NOT covered by the manufacturers warranty programs. Reliability issues are almost always caused by installers NOT following the manufacturers installation recommendations/requirements and the applicable requirements for generator installations found in the NEC.
Hopefully this information will be found helpful.
Regards,
Mike
Note: References are from the 2011 NEC and applicable to "Residential" generator installations.
Generator Sizing
As a factory authorized service dealer for Generac we routinely find both the generator and the fuel supply system grossly undersized. As the OP stated, NEC Article 702.4(B) REQUIRES the generator to be sized as per Article 220. Generac and most other generator manufacturers use the "Optional Calculations" found in Article 220.80 Part IV for sizing generators.
Since this discussion regards automatic transfer equipment NEC Article 702.4(B)(2)(a) also applies, requiring the installed generator be capable of supplying the "Full Load" that is transferred by the automatic equipment OR per NEC Article 702.4(B)(2)(b) requiring an "Energy Management" system that will limit the connected load to a value less than or equal to the capacity of the installed generator.
Also remember that most of these residential and small commercial generators are dual fuel rated as "Standby" equipment. The Standby rating per Generac factory tech school is noted below.
•Air-Cooled generators are “STANDBY” rated and MUST NOT be operated beyond 60% of their load capacity except as noted below.
•1 hour out of every 12 hours
•2 hours out of every 24 hours
•250 hours out of any 12 month period.
Example: 20Kw Generac standby generator operated on natural gas.
Maximum Connected Load - 18Kw (Note: a 20Kw unit operated on Natural gas is de-rated to 18Kw.)
Maximum Continuous Load - 18Kw x 60% = 10.8Kw
The above examples assume that the fuel supplying the unit is adequate and meets or exceeds ALL of themanufacturers requirements. The following statements appear in all of the Generac factory installation manuals.
•“GAS PIPE SIZING IS ONE OF THE MOST COMMONLY MADE MISTAKES. A PROPERLY SIZED GAS PIPE IS CRITICAL TO THE PROPER OPERATION OF THE GENERATOR. THE GENERATOR INLET SIZE HAS NO BEARING ON THE PROPER GAS PIPE SIZE.”
•“THE GAS SUPPLY AND PIPE MUST BE SIZED AT 100% OF THE GENERATOR BTU RATING.”
For our example noted above, the 18Kw generator MUST HAVE a natural gas meter and piping system capable of supplying a minimum of 308,000 Btu/hr. This fuel must be supplied at a pressure of 7"W.C. with a pressure drop not exceeding 1/2" W.C. at the generator under full load conditions. In addition to the generator fuel requirements you must add the fuel requirements of ALL other gas appliances connected to the system.
The generator fuel piping system MUST also meet ALL of the following requirements:
•You MUST add 2.5ft for each bend, tee, elbow, or angle to the overall measured distance from the source to the generator.
•ALL fuel shut-off valves MUST be “Full Flow Rated” and match the internal dimensions of the required fuel pipe I.D.
•Piping length is measured form the “LOW PRESSURE DISCHARGE PORT” on the natural gas meter or LP regulator to the “INLET PORT” on the generator.
•This is a CRITICAL step! The generator gas supply must NEVER be tapped from the gas line serving the building. It MUST be a dedicated gas line of adequate size run directly from the meter to the generator.
•Flexible fuel line
• a) Do NOT bend!
• b) Do NOT attach directly to the generator!
• c) Check ALL connections for leaks.
•Sediment trap (Drip Leg) near generator.
•“Full Flow Rated” shut-off valve near generator.
•Verify capacity of gas meter or LP tank to supply sufficient fuel for “Full Load” generator operation plus ALL other connected gas appliances operating simultaneously, at a pressure drop NOT exceeding 1/2" W.C.
•The supplied “Flex-Line” is a vibration isolation device and is NOT to be bent or twisted in place of using the proper pipe fittings.
•Generators must NEVER be hard pipe connected without a “Flex-Line” installed!!
Problems caused by neglecting the proper sizing of the generator and the fuel supply system are install errors and are generally NOT covered by the manufacturers warranty programs. Reliability issues are almost always caused by installers NOT following the manufacturers installation recommendations/requirements and the applicable requirements for generator installations found in the NEC.
Hopefully this information will be found helpful.
Regards,
Mike
Note: References are from the 2011 NEC and applicable to "Residential" generator installations.
Radioman91
Member
- Location
- Swartz Creek, MI. 48473 USA
NEC 2014 Article 702.4(B)(2) DOES NOT require your standby generator be sized to match the service of the residence, however it MUST be sized greater than or equal to the connected load as determined by Article 220. This is not a new requirement as it has been in the NEC since the 2008 edition. Common sense would also indicate adding 10-20% to the connected load calculation to allow for future additions and load changes.
Also be aware that virtually all of the air-cooled standby generators sold in the USA are rated using LP fuel. When powering these units with natural gas they are generally de-rated by 10-15%, check with the manufacturer regarding the specific unit that you are installing.
This addresses the "Connected" load issue as required in the NEC. The air-cooled and many of the smaller liquid cooled generators are also "Standby" rated by their manufacturer and subject to the following "Continuous" load limitations.
•Air-Cooled generators are “STANDBY” rated and MUST NOT be operated beyond 60% of their load capacity except as noted below.
•1 hour out of every 12 hours
•2 hours out of every 24 hours
•250 hours out of any continuous 12 month period.
Example: Article 220 "Connected" load calculation = 19.2Kw. Fuel is natural gas.
Generac 22Kw air-cooled unit is selected and connected using a SE rated (Whole House) transfer switch.
Maximum "Connected" load for this unit = 19.5Kw (Generac 22Kw unit running on natural gas is rated 19.5Kw). OK
Maximum "Continuous" load for this unit is 11.7Kw (19.5kw rating x 60% = 11.7kw) ?? Maybe- See Below
Compliance with the mfg. continuous load requirements will require that you determine the nature of these loads. if the greater majority of your connected load is lighting, small cord and plug connected equipment, along with a couple of small motors and an HVAC compressor you are likely OK. Should you have high wattage resistance heating appliances (i.e. water heater, range, cooktop, oven, clothes dryer, and electric baseboard/strip heaters BE CAREFUL. These high wattage appliances will many in many cases far exceed the "Standby" time and load limits noted above requiring the addition of a load management system or the selection of a larger generator.
The other CRITICAL issue is adequate fuel supply. ALL standby generators MUST have their fuel supply (Natural Gas meter/LP Tank) and the connecting fuel piping sized to provide 100% of the full load requirements of the generator that you are installing. This is in my experience the biggest problem with standby generator installations. Nearly 75-80% of the units that we are called to service have grossly insufficient fuel supply systems. This can cause a multitude of problems including failure to start, hard starting, and has the possibility of causing severe engine damage due to "Lean Burn". Inadequate fuel supply is an INSTALLATION MISTAKE and damages caused are generally NOT covered by the manufacturers warranty programs.
In addition to the fuel supply requirements of the generator, your fuel supply (Natural Gas meter/LP Tank) MUST BE capable of supplying 100% of the load requirements of ALL other gas appliances in the residence with ALL appliances operating simultaneously.
Example: Generac 22kw (19.5Kw) air-cooled standby generator operating on natural gas.
Generator 281,000Btu/hr
Furnace 100,000Btu/hr
W.H. 50gal 50,000Btu/hr
Range 65,000Btu/hr
_________________
Total gas load 496,000Btu/hr
Your natural gas meter MUST be capable of flowing 496,000Btu/hr of gas at a pressure of 7"W.C. with a pressure drop not exceeding 1/2" W.C. with all gas appliances operating simultaneously. Many natural gas meters are dual rated, using both 2" and 1/2" pressure drop columns. When sizing a meter for use with an internal combustion engine as in a standby generator, you MUST use the 1/2" W.C. pressure drop column.
A little care and planning of your standby generator installation will go a very long way to insure that the generator is reliable and provides your customer with many years of dependable service.
Regards,
Mike
- Status