Calculating Load for a Generator?

[davedottcom]

Is there a demand load calculation for a back up generator in a dwelling?


*15KW Generator with a 70 Amp Breaker Feeding into a Transfer switch/load center?




Dave

 

[bigjohn67]

Re: Calculating Load for a Generator?

When figuring for a generator you figure it at 100% of the load you want to carry in case of power Interuption. We never figure a demand factor. We do generators all the time.

 

[davedottcom]

Re: Calculating Load for a Generator?

Big John,
That's what I figured to ensure the breaker would not trip at start up.

But it seems "certain installers" are only calculating the load for the HVAC units in the "Cool" mode not "Heat" which will draw much more!

They are assuming the only power failures will be in the summer "Hurricane" months.

Dave

 

[benaround]

Re: Calculating Load for a Generator?

ddc,

Wouldn't the demand factor be for the dwelling? Then size the gen. to it. Just a thought.


frank

 

[davedottcom]

Re: Calculating Load for a Generator?

Frank, yes I would agree with that. However, the generators are often not sized to feed the entire service, just certain circuits. My question is how do I calculate the load on the circuits that will be fed from the transfer switch/load center?
(Mainly the HVAC system.)

Dave

 

[George Stolz]

Re: Calculating Load for a Generator?

Is A/C really a consideration? For Pete's sake.

"Sahry hon, the food's gonna spoil, but by gar, we ain't gonna sweat 'bout it!"

If somebody can't get by with 60 amps in an emergency situation, they need to be dropped on an island (with no cameras) for a week.

 

[davedottcom]

Re: Calculating Load for a Generator?

George, I understand your disgust!
BUT, it is a huge factor here in Florida.
Try to compare it to you being without heat in January (The Discomfort level, not pipes bursting and stuff!)

The deal is these generators are being installed like wild fire down here (since last years hurricane season) and I swear I haven't seen one hooked up correctly yet!

Dave

 

[George Stolz]

Re: Calculating Load for a Generator?

Well, you can only police yourself and advise the others, ey?

 

[davedottcom]

Re: Calculating Load for a Generator?

George,
Yep, but people look at me like I have 3 eyeballs when I tell them their new $5,000.00 15kw generator can't handle their HVAC equipment.
I have also refused to hook up a bunch of these generators due to the installer wanting to use 100 amp transfer switches on 200 amp panels. (Feeding the whole 200A panel) I drove by a few weeks later, their they were, all hooked up by someone else!
Dave

 

[benaround]

Re: Calculating Load for a Generator?

ddc,

I would have to figure 100% of the A/C here in Az. because there are times when it won't shut off, at least for 3hr's or more,and that's a continous load.If it is like that in your area I would consider the same.

frank

 

[hurk27]

Re: Calculating Load for a Generator?

I've got one coming up, 65kw Generic with a 400 amp disconnect feeding a 400 amp ATS. I know the generator is a little under sized for 400 amps, but the calculated load on the house is less than 125 amps. Around here most houses have all natural gas heating and cooking and clothes drying. the high electric rates keeps it that way.

 

[mc5w]

Re: Calculating Load for a Generator?

I have noticed that the lowest bid gets the job. Most people are not interested in quality, they are interested in CHEAP!

I can agree that most people cannot resist a bargain and they do not understand that doing things the right way is in their best interest.

 

[mc5w]

Re: Calculating Load for a Generator?

I forgot to add that most 3600 RPM generators do not have enough reactive power capability to run significant refrigeration and air conditioning equipment. A 5,000 watt generator has a hard time starting a 1/4 HP freezer and if you try to start a second freezer it takes something like a minute for the second 1/4 HP freezer to start.

 

[roger]

Re: Calculating Load for a Generator?

Originally posted by mc5w:
I forgot to add that most 3600 RPM generators do not have enough reactive power capability to run significant refrigeration and air conditioning equipment. A 5,000 watt generator has a hard time starting a 1/4 HP freezer and if you try to start a second freezer it takes something like a minute for the second 1/4 HP freezer to start.

Although you have a point, nobody in this thread was talking about a generator that small.

Roger

 

[jimwalker]

Re: Calculating Load for a Generator?

I don't know about others but all i am after is being able to keep the frig going.We just bought a new 5500 watt 120/240 gen..We will be prepared this year as many others will be.It is large enough to keep water pump,frig,cook top,washer,dryer ?,and bat charger for all our 12 volt systems.Not all at one time.This is not about being comfortable,it is about being able to survive.One important thing to remember is they take gas.At 6 gallons per 12 hours at half load we are looking at maybe 10 gal per day and must figure at least 3 or 4 days before we can buy more gas.Storage is a problem.
To fellow Florida huricane prone people,we learned a neat trick to keep water.Go to walmart and buy 2 new trash cans at $5 each to hold water.Far cheaper than bottle water and if not needed nothing is lost.

 

[davedottcom]

Re: Calculating Load for a Generator?

Originally posted by davedottcom:
Frank, yes I would agree with that. However, the generators are often not sized to feed the entire service, just certain circuits. My question is how do I calculate the load on the circuits that will be fed from the transfer switch/load center?
(Mainly the HVAC system.)

Dave

Actually, I correct myself, most of the Generators are bought "Sized to meet their budget" with little or no thought about the actual load on the circuits they intend to feed.
Most seem to be 15kw or smaller. Not large enough to handle the load of most HVAC systems & the lights, tvs, refrigs, freezers, water heaters at the same time.

I haven't had the chance to mess around with one that is up & running yet. The one I installed will be getting propane this week and another one that I promised the customer I would "Inspect" the electrician's work will also be installed this week. He was assured he will have HEAT & AC in addition to almost every general lighting circuit and water heater with a 15kw Genn. I have a hard time believing it's going to work. Last time I checked 15,000/240 was only 62.5Amps.

Dave

 

[rick hart]

Re: Calculating Load for a Generator?

If sizing a generator for HVAC you either have to size by the locked rotor current of the largest motors starting as a unit or have some sort of soft starting means.

Personal generators just don't have the kick needed to start a motor that they can otherwise supply power to run. That is why a shorted generator output will not trip the output breaker but still burn out the generator. A motor starting will look like a short to an under sized generator.

 

[LarryFine]

Re: Calculating Load for a Generator?

As for what happens if AC or heat is running during a power failure, it's not difficult to use auxiliary contacts, or even an added relay, to lock out the HVAC by simply interrupting the 24-volt supply during generator use.

 

[davedottcom]

Re: Calculating Load for a Generator?

Originally posted by LarryFine:
As for what happens if AC or heat is running during a power failure, it's not difficult to use auxiliary contacts, or even an added relay, to lock out the HVAC by simply interrupting the 24-volt supply during generator use.

Larry, I do understand you're theory however, the HVAC system is something most customers are expecting to work when running thier (Expensive!) generator. So locking it out would defeat one of the main reasons for having the generator.

This would be a good idea to do for the type of transfer switch that feeds the whole service, yet can not handle the load of the HVAC system.

Thanks,

Dave

 

[davedottcom]

Re: Calculating Load for a Generator?

Originally posted by rick hart:
If sizing a generator for HVAC you either have to size by the locked rotor current of the largest motors starting as a unit or have some sort of soft starting means.

Personal generators just don't have the kick needed to start a motor that they can otherwise supply power to run. That is why a shorted generator output will not trip the output breaker but still burn out the generator. A motor starting will look like a short to an under sized generator.

Rick, I agree with your first statement but I think in addition to the locked rotor current you would also have to include the load of the heat strips incase the HVAC system was in "Heat" mode at the time of power failure.

As for the "kick" needed to start these HVAC motors, I don't know how I could tell if the Gen. was capable or not before trying it!?!?
I'm not being a smarta$$ but are the generators marked somehow for how much "Kick" they have!??

Is it safe to say using an "Undersized" generator for starting such motor & heater loads could damage the generator?

I would think so!

Thanks for the reply,

Dave