No, 200-amp service does not mean you need a 200-amp generator. Your generator is sized to what will actually run during an outage. For most 2,000–3,500 sq ft DFW homes on 200-amp service, that is a 17 or 20 kW air-cooled Cummins that runs the whole house, central AC included, often with load management.
No, you don't need a 200-amp generator. The 200-amp rating is the most your service can carry, not what the house uses at once, and a generator is sized to what will actually run during an outage. For most 2,000–3,500 sq ft DFW homes on 200-amp service, that is a 17 or 20 kW air-cooled Cummins (RS17A or RS20A), which runs the whole house including central AC, often with load management. Essential-circuit setups use a 13 kW RS13A. Homes over about 4,000 sq ft or with several HVAC zones move up to the liquid-cooled RS25 and larger.
Why doesn't 200-amp service need a 200-amp generator?
Your service is sized with headroom for everything in the house, and nobody runs the range, the dryer, both AC systems, the water heater and every light at once. Generators are rated in kilowatts, not amps. Converting shows the gap. A Cummins RS20A is rated 20 kW on propane and 18 kW on natural gas. At 240 volts, 20,000 watts is about 83 amps and 18,000 watts about 75 amps.
That is well under 200, and for most homes it is enough. What decides the size is what runs at the same time during an outage, plus the starting surge of the biggest motor. In Texas that motor is almost always the AC compressor. The RS20A is rated for a 40 kVA motor start, which is why a unit that size can start and run central air.
Which Cummins size fits a 200-amp DFW home?
| Cummins model | Cooling | Typical fit | Installed |
|---|---|---|---|
| RS13A, 13 kW | Air | Essential circuits | Quoted after survey |
| RS17A, 17 kW | Air | Whole home, 2,000–3,500 sq ft | Quoted after survey |
| RS20A, 20 kW LP / 18 kW NG | Air | Whole home, 2,000–3,500 sq ft | Typical: $13,500–$17,000 all in |
| RS25 and larger | Liquid | 4,000+ sq ft or multiple HVAC zones | Quoted after survey |
Warranty differs by cooling type: 5 years / 2,000 hours limited on the air-cooled range, and a 2-year standby base warranty on the liquid-cooled range. Clements Electric installs Cummins only, as the electrical contracting arm of UTS ElectricPros, LLC, an authorized Cummins dealer. The full model catalog with equipment prices is at utselectricpros.com.
How is the load calculation done?
- List what has to run during an outage: AC systems, refrigerator and freezer, lighting, internet, a well pump, medical equipment, and any electric water heater, range or dryer you want to keep.
- Add up the running load of everything that could run at the same time.
- Check the starting surge of the largest motor, usually the AC compressor, against the generator's motor-start rating.
- Account for fuel. The same Cummins unit puts out less on natural gas than on propane.
- Decide whether to back up the whole house or a set of essential circuits. That choice also decides the transfer switch.
The full method, with the essential-load alternative, is in our standby generator sizing guide. We do the calculation on site before recommending a size.
What does load management do?
The code sets the rule. With automatic transfer, an optional standby system has to be able to supply the full load the switch connects. Where an energy management system controls the load, the generator must supply the most load that system will allow (EC&M, NEC requirements for emergency and standby systems).
In practice, load management holds off big loads until there is room for them. It can stagger a second AC compressor so both don't start at once, or keep an EV charger, electric dryer or pool pump off while the AC runs. That is how a 20 kW unit can back up a whole 200-amp house that has two AC systems.
When is a 200-amp service-rated transfer switch used?
In a whole-house setup, the transfer switch goes between the meter and the main panel, so everything in the house moves to the generator. A service-entrance-rated switch has a built-in breaker that can serve as the home's main disconnect (Current Connected, Cummins RA200 SE). Cummins packages the 20 kW RS20AC with a 200 A service-entrance-rated transfer switch, and the catalog at utselectricpros.com lists switches from 100 A to 600 A.
Because that switch transfers the whole 200-amp service, the load rule above applies. The generator has to carry everything that could run at once, or load management has to cap it. The alternative is an essential-circuit setup: a smaller switch feeds a panel holding only the circuits you choose, which is the usual pairing with a 13 kW unit.
How much fuel does a 20 kW generator use?
Per Cummins' published specifications, the RS20A uses about 268 cubic feet of natural gas per hour at full load, roughly 2.6 therms. It uses less at partial load and nothing while it sits idle waiting for an outage. Natural gas is the usual fuel in DFW neighborhoods with gas service. Propane gives the full 20 kW rating but needs a tank.
For a full breakdown of the installed price, including the transfer switch, gas line, pad, permit and startup, see our whole-house generator cost guide.
Want the load calculation done properly?
On-site sizing and a written quote for a Cummins standby generator. 7 AM to 7 PM Monday through Friday. Licensed Texas Electrical Contractor TECL #40628.
Generator sizing for 200-amp service
Yes, for most 2,000–3,500 sq ft homes, central AC included. A 200-amp rating is the service's maximum capacity, not what the house draws at once. A Cummins RS20A puts out 20 kW on propane and 18 kW on natural gas, about 83 and 75 amps at 240 volts. Homes with two AC systems usually add load management so the compressors don't start together.
For a whole-house setup on 200-amp service, usually yes. The switch goes between the meter and the main panel, and a service-entrance-rated model includes the breaker that acts as the main disconnect. Because it transfers the whole service, the generator has to carry everything that could run at once, or a load management system has to limit it.
Yes, as an essential-circuit system. A 13 kW Cummins RS13A backs up a chosen set of circuits, such as the refrigerator, lights, internet and furnace, and with load management it can run one smaller AC. It is not sized to back up a whole 200-amp house with central air. Its installed price is quoted in writing after an on-site survey.
About 83 amps at 240 volts on its 20 kW propane rating, and about 75 amps on its 18 kW natural gas rating (watts divided by volts). That is why a generator is sized in kilowatts against the load that will actually run, not against the amp rating of the service.
A control that keeps the connected load within what the generator can supply. It sheds or delays large loads, such as a second AC compressor, an EV charger or an electric dryer, until there is capacity. Under the NEC, a generator on an automatic transfer switch must either carry the full transferred load or the most load the management system will allow.
