17 Kilowatt generator. yep, you need a big one.

And continuous load rules suggest that if 17 kW is the load, then the generator capacity should be about 20 kW.

An emergency generator would typically be sized to run a number of different appliances. For a standby generator, the actual vs. nameplate capacity depends on whether you're burning natural gas or propane as fuel.

Kohler has an online sizing tool that you may find to be helpful:

https://kohlerpower.com/en/residential/generators/generator-selector#your-home

continuous load worked out to about 14K watts. 17K was adding an 25% to 14K.

4 tons equal's approx. 19 hp times 746 watts equal's 14176 watts. I didn't add anything for the fan motor because I believe he's not going to get a generator anyway.

I agree, air conditioning isn't something I'd worry about. 20K generator would be a better choice like you said. If he's serious and wants to spend $6k to $9k on the low end he may want to go with NG or LP as the fuel and get a big LP tank. Going with gas will be nothing but problems and as we've just seen in Louisiana, no power means gas stations can't pump fuel.

My Millbank 20-kW motor generator is fed from an in-ground 500-gal propane tank (size determined by vaporization surface area at minimum local temperature; tank has to be purchased if in-ground). The tank, generator, generator pad, trenching, electrical parts including > $1000 transfer switch, >$100 load shed relay (has to be able to run continuously), and miscellaneous mostly electrical parts, excluding me doing all the electrical labor, was roughly $20k in 2010-ish dollars.

I would of never guessed it would be so much. I have a couple of smaller generators I keep tuned up for the bare essentials, refrigeration, a few lights and well pump. My wife asked me just a few weeks ago what something like your would cost. I'll be sure to tell her.

Have you needed to use it at all? If so did you have any neighbors visit wanting to run cords to their house? Just wondering, I have a smaller generator I've rolled out on three occasions in 20 years and each time I was quickly paid a visit from all the neighbors in ear shot wanting to wheel my generator home with them, it was a problem to say the least. I obliged even taking the risk of losing my own frozen foods wheeling the thing all over the place. Worst part is every time after the power came on I told every one of them they should buy a generator for emergencies. To this day none of them bought one, and none are poor. One had the gull to say why? I can use yours if I need one. Any thoughts?

What size generator do I need to ruin a 4 ton ac?

One way to calculate the is to use the EER rating of the AC. Let's assume the EER rating is 10.0. The equation for EER is:

EER (BTU/W) = cooling (BTU)/power (Watts)

Solving for power:

Power = Cooling/EER = 48,000/10 = 4800 W or 4.8 Kw

But you need a lot of more power to start up the AC. The circuit breaker for the AC has to be sized to handle the initial surge of current on start up. A 4 ton AC will typically use a 40 amp circuit breaker. To solve for the maximum required power on start up:

Power = Voltage * current - 220 V * 40 amp = 8,800 W or 8.8 Kw

A good rule of thumb is to size the generator at twice the size need for continuous operation under worst case conditions. A 9 - 10 Kw generator should be sufficient. This would be to run the AC only. If the you already have the AC connected, you could measure the start up current and then size the generator as needed.

Portable generators do not need a pad, don't need a large propane tank, and for many they need but don't have transfer switches. They are also noisier, and don't run themselves once a week to keep themselves lubed. Mine can run my electric kitchen and most other stuff, excluding pool motors and dryer. PCs and networking appliances are on UPS, which only have to cover for about 20 seconds before the generator automatically starts.

My neighbors seem more mannerly (or at least distant). There was an outage once where we were at a party across the street. Everyone came to our house to finish the cooking and eat.

When I bought my house and for the next ten years or so outages were short lived. I heard that a power company local exec lived nearby. Later they became longer, and I installed the generator as part of my kitchen renovation. More recently, outages have been short, and somewhat rarer due I think to the power company isolating power lines to smaller areas.

We bought a 22K generator last year. It runs on propane. Total cost with install was $15,000 including propane tanks and we did the site work. It will not run a whole house A/C with everything else running but it runs the entire house including artesian well pump, furnace, freezers, lights, and all kitchen appliances.

We lost power for two weeks after a tornado hit my town this Spring. I bought a dual fuel generator. I've got a couple of LP tanks for barbeque. LP is the only way to go with a portable generator. It's clean. It's easy to use. It's not dangerous like gasoline. If I only use the LP side of things, the maintenance is lessened. I can power my on-demand hot water heater, a refrigerator freezer and anything else that can be plugged into a socket. I even invited neighbors who were sweating in the sun clearing the debris over for a cold beer and hot shower. While I would like to have a whole house generator, I just can't fully justify the cost. Even in the Deep South, the house cools down most evenings.

To determine the size generator required, you need the full technical specs of the HP or AC condenser you plan on installing. In ‘Electrical Data’ locate the Unit Size - Series that will be installed. Then locate the data relating to the compressor for that unit. ‘LRA’ is the spec you need, it stands for ‘Locked Rotor Amps’ that is the maximum current measured in amps that that unit will require during start up. Multiply that number by the voltage (230v) and divide the answer by 1000 to give you the kW rating - but you’re not done! You need a generator with about ⅓ greater power output to provide the reserve that might be needed to START that compressor, particularly if it’s starting against a high pressure situation.

Make sure the generator installed can deliver that power requirement with whatever energy source is being provided (NG, propane).

My personal preference is always for a diesel generator, as much as I don’t like oil. Feed it with heating oil as this is a stationary generator and heating oil does not unnecessarily include all ‘Road Taxes’ that regular diesel would include. That way you’re not dependant on a delivery network such as for NG. Make sure you have a large enough oil storage tank - properly installed for storm conditions. Generally that would usually a ½ standard size or full size oil tank.

YMMV

IMPO

SR

FWIW, some newer ac units have an ”Intellistart” or similarly named feature that can reduced current usage by up to 60% at startup. That info would be helpful in sizing your generator.

This should be true and I had thought of that, that units with variable speed compressors ‘should’ have a ‘softer’ start. That might be another reason to purchase such a unit. The question also remains of what will your unit do after a power failure when you’re trying to re-initialize it after a major power outage event that was preceded by a voltage spike or brownout.

While installing a generator with an automatic or manual transfer box, I’d also recommend having the electrician install ‘Whole House Surge (& Lightning) Protection’ at the main load centre panel breaker box to mitigate damages to all electrical equipment - both 115v and 230v caused by these phenomena!

IMPO

SR

"But you need a lot of more power to start up the AC."

This is the biggest problem using a generator for powering and AC system.

That's why you have to significantly over size the capacity. If you are serious about powering a 4 ton AC, then it may make sense to have a dedicated generator rather than trying to size a whole house generator to power it.

Re: mike_home

“That's why you have to significantly over size the capacity. If you are serious about powering a 4 ton AC, then it may make sense to have a dedicated generator rather than trying to size a whole house generator to power it.”

That’s not accurate Mike, the reason such a large capacity generator is necessary is that the LRA rating relates to the ultra-low AC resistance during start-up of the compressor motor’s coil windings during the first few cycles of operation. It’s the virtual equivalent of almost literally throwing a crowbar across the compressor rotor, as if throwing a stick into the spokes of a wheel, causing the compressor motor to stall out and draw maximum current, “Locked Rotor Amps”. It’s short lived but enough to stall out, blow or trip the circuit breaker any undersized generator - yet this current is still required to start this SOB!

After the compressor switches from ‘Start’ mode to ‘Run’ mode and maybe even before that, with the compressor’s RLA or Running Load Amps or Rated Load Amps that is a much lower rating than LRA, there is plenty of excess generator capacity in these larger rated generators to run the entire house simultaneously and probably including all fridges, stoves, electric dryers etc of a normal household - and all at the same time.

IMPO

SR

I think we are saying the same thing. The initial start up current is much higher than that of the steady state. The generator must be sized for that current surge. Any appliance with a motor will have a start up current.

The original author asked what size generator is need to run a 4 ton AC. If the question is "what size is needed to run a 4 ton AC and a complete household of lights and appliances", then that is a different question.

What was I thinking. I deleted it.

Re: mike_home

“That's why you have to significantly over size the capacity. If you are serious about powering a 4 ton AC, then it may make sense to have a dedicated generator rather than trying to size a whole house generator to power it.”

I understood this post to mean that you’re saying to either have a dedicated HVAC generator or 2-discreet generators, 1 for HVAC & another for the rest of the house.

“The original author asked what size generator is need to run a 4 ton AC. If the question is "what size is needed to run a 4 ton AC and a complete household of lights and appliances", then that is a different question.”

I feel that’s still the same interrelated question because the ‘right size’ generator will have more than enough capacity to run the entire house with either no or modest additional energy management.

The question is not what size generator is required to run a 4-ton HP but rather what size generator is required to run this specific make/model HP or AC condenser. You have to drill deep into the specs for a particular HP or AC condenser otherwise you’re sizing by rule of thumb that is never a good idea. I’m also sure that there are many ‘professionals’ selling generators that could not define or tell you what the difference is between LRA & RLA - because they don’t know!

I would not recommend buying a stationary generator from anyone that was either unclear or vague as to the difference of these terms just as we would not want to size a system without complete Manual J and Manual D calculations as that would indicate the use of rule of thumb methods.

IMPO

SR

fsq4cw,

I do agree the electrical requirements of the AC condenser needs to be looked at carefully to determine the size of the generator. I would like learn what calculations professionals use in cases like this.

Here is the Product data Carrier 24ACB3 condenser. The electrical data is found on page 9. Click on the image to read it.

The 4 ton condenser shows a LRA of 109 amps, RLA of 19.9 amps. for the compressor. The fan FLA is 1.2 amps. The maximum circuit breaker size is 40 amps.

What would be the generator size calculation of this 4 ton condenser? Once the generator size is determined, how would you calculate the calculated generator size would be sufficient to power the AC and the all the circuits in the house at the same time?

From a theoretical point of view, this is an interesting question. One could look at the 109A RLA unit's 40A circuit breaker for the energy it would allow to pass in a transient without faulting (or for an I^2t value). But there is no information, at least for my generator (20 kW, 83A), of how much energy can be pulled from the field as a transient without collapsing the field or other ill effect. Certainly the voltage will drop, and that alone might violate the RLA assumption of voltage available when starting. (In other words, is it 109 amperes at 230 Vac, or at 197 Vac?) I look forward to the professional approach to generator sizing if it isn't the comfortable path of just adding 109A to the household current.

Soft starts are often used in applications where residential air conditioners will be powered by a generator. They have the ability to reduce LRA's by up to 75%. You might still need some power management, but it's worth considering. Micro Air is one manufacturer.

microair.net

Best wishes for a successful project.

Re: mike_home

“The 4 ton condenser shows a LRA of 109 amps, RLA of 19.9 amps. for the compressor. The fan FLA is 1.2 amps. The maximum circuit breaker size is 40 amps.

What would be the generator size calculation of this 4 ton condenser? Once the generator size is determined, how would you calculate the calculated generator size would be sufficient to power the AC and the all the circuits in the house at the same time?”

I believe the questions were answered in a previous post.

Re: kaseki

“From a theoretical point of view, this is an interesting question. One could look at the 109A RLA unit's 40A circuit breaker for the energy it would allow to pass in a transient without faulting (or for an I^2t value). But there is no information, at least for my generator (20 kW, 83A), of how much energy can be pulled from the field as a transient without collapsing the field or other ill effect. Certainly the voltage will drop, and that alone might violate the RLA assumption of voltage available when starting. (In other words, is it 109 amperes at 230 Vac, or at 197 Vac?) I look forward to the professional approach to generator sizing if it isn't the comfortable path of just adding 109A to the household current.”

Your 20kW generator ‘should be’ sufficient. I have heard of 3 or 4-ton HP’s started with much smaller generators and that might be so. I personally would not want to knowingly install an undersized generator for myself or for a client only to find out it failed to perform during a critical emergency.

Circuit breakers are designed to withstand an elevated inrush of current - momentarily, as a slow blow fuse might. The tables of Electrical Data are engineering specs and are posted for a reason. The LRA is posted for a reason. It is the inrush of current that the compressor can draw during startup when its impedance is very low as it’s creating the magnetic fields and starting to run against a high mechanical resistance. That LRA spec is posted to tell you what you might read on startup when measuring with an ammeter and how to size a generator - especially when a compressor is trying to start after a lock out condition due to high pressure, when mechanical resistance will be at its highest and its current draw will be at its maximum.

After the compressor has started (in the electrical Data Table posted by Mike) and current draw drops down to RLA there will be enough excess capacity to run the entire house as the compressor will only require about 4.58kW. A 34kW generator (with about 1/3rd more head room) will handle full load and the rest of the load with no problem - virtually guaranteed!

Keep in mind that some people require AC for medical conditions and are also running medical equipment as well that requires constant power without interruption.

Many sales people will tell you that you can get by with much smaller generators. This is in part because they want to make the sale and not price themselves out of the running. Perhaps inverter driven variable speed compressors can be ramped up with a lower jolt of current. Read the specs; be sure they’re interpreted properly. Consult a Master Electrician.

If you want an impartial second opinion, speak to an electrical engineer with naval experience, they’ll tell you what you should have.

IMPO

SR

For the record, my air conditioners are load-shed so the issue doesn't apply to me.

The breaker values (higher than RLA; but lower than MCA are obviously chosen to handle the RLA transient per the breaker time-current curve. What I wanted to know was whether there is some known (predictable or typical or successfully hypothesized rule of thumb) transient capability of motor-generators that would be applicable to sizing them against the air conditioner's locked rotor current transient allowed by the breaker. Obviously, sizing against RLA plus household load will be successful. But the OP's OP question was limited to the air conditioning only, and that was the genesis of my question about generator transient capability, if any.

Brief review of some literature, e.g., https://mart.cummins.com/imagelibrary/data/assetfiles/0058629.pdf, https://www.cummins.com/sites/default/files/2020-03/PowerHour_Generator%20Set%20Sizing%20Software_Transient%20Performance%20and%20Motor%20Load%20Capabilities%20(1PDH).pdf, and https://www.cummins.com/sites/default/files/2021-08/considerations_for_generator_set_selection_presentation.pdf suggests that when abruptly dumping an induction motor starting load onto a generator, considering both voltage and frequency stability, the margin of reserve may be less than that which the breaker provides when operating from the higher capability power company feed.

Gen set manufacturers have software programs available to size generators against motor starting requirements, given that the motor can be sufficiently characterized. I have not determined whether generator characterization is available at the few dozen kW end of the generator spectrum. Hence my present understanding is that generator transient over-current capability depends on too many factors to have a rule of thumb, beyond the intuitive but not so far clearly expressed requirement that the voltage-current-frequency-power factor conditions at the generator have to allow the motor being started to produce more torque as it comes to speed than the torque required at each intermediate speed. See image from Cummings.

Sizing the generator so it will continuously supply 109 amps at 230 V and then adding another 33% will work. But the result is a 34 KW generator. Once the compressor starts, the current drops to about 20 amps. You have purchased a generator whose size is much larger than is need most of the time.

A better way to do this is to follow Charles Ross' suggestion of using a soft start kit. Here is an interesting video which demonstrates starting a 3 ton condenser with a 6.2 KW generator when a start kit is added!

How to run central ac running on portable generator during an emergency

Thanks, Mike, for bringing the soft-start kit to our attention. Although I don't presently need one, it is always helpful to know such devices are commercially available. (Although load shed upon public service power loss, my Mitsubishi compressor units have inverter driven motors so their LRA values are relatively modest compared with some more traditional AC units.)

You have to give credit for the soft start kit to Charles Ross. I was skeptical it would work as well as advertised. But the unless the video has been altered, it seems to work well. A few hundred dollars on a soft start kit can save thousands of dollars on the purchase of a generator. These devices are solid state. I would think they would have a long life.