Home Generators and Starting Current

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infinity

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Location
New Jersey
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Journeyman Electrician
This is hypothetical, if I'm using a portable generator to run my central AC during a power outage will the AC starting current affect the size of the generator? Say I have a 30 amp, 240 volt running current and a 8KW generator, will the starting current of the AC equipment overload the generator?
 

ohmhead

Senior Member
Location
ORLANDO FLA
This is hypothetical, if I'm using a portable generator to run my central AC during a power outage will the AC starting current affect the size of the generator? Say I have a 30 amp, 240 volt running current and a 8KW generator, will the starting current of the AC equipment overload the generator?

Well read this http://www.dieselserviceandsupply.com/Sizing_A_Generator.aspx


Ive got a 17.5 kw genset on my home homebru old ATS off a job it runs all heat/ air and water heater with all the lites on in house everything on that includes range all burners on full and oven at one time tested no issue i think a 8kw justs for air is no problem but use that calculator see if its ok .
 
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ty

Senior Member
This is hypothetical, if I'm using a portable generator to run my central AC during a power outage will the AC starting current affect the size of the generator? Say I have a 30 amp, 240 volt running current and a 8KW generator, will the starting current of the AC equipment overload the generator?

hypothetically, it might work.

What is the starting current of the AC?
What is the Actual running watts of the Generator, and the Peak watts of the Generator?

Alot of Generators are sold with the Peak Watts as the rating.
Your AC might possibly bog down the generator too much.
 

One-eyed Jack

Senior Member
This is hypothetical, if I'm using a portable generator to run my central AC during a power outage will the AC starting current affect the size of the generator? Say I have a 30 amp, 240 volt running current and a 8KW generator, will the starting current of the AC equipment overload the generator?

With todays HE ac units I seriously doubt that 8kw will start it. I am using a 8.5 Winpower with a 16 hp two cylinder engine. It won't start my ac with less running amps than that. I can run approx 26 amps worth of load. Any more than that the ocpd on the genset will open.
 

ohmhead

Senior Member
Location
ORLANDO FLA
Well if you have a load at 30 amps and at 240 volts single phase whats the issue its 7200 watts and generator thats 8 kw .

Generator must be loaded at 50% to 70% normally to just run you do not under load them its no good . Running a large genset at a lower kva load is not good . He is not fine with the 8kw you pick your load and size it about 80 % of the gensets kw or 20 % or 25% gen/ kw to load kw .

So its too small and the calculator is wrong .

6400 watts would be for a 8kw genset
 
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ty

Senior Member
Well if you have a load at 30 amps and at 240 volts single phase whats the issue its 7200 watts and generator thats 8 kw .

Generator must be loaded at 50% to 70% normally to just run you do not under load them its no good . Running a large genset at a lower kva load is not good . He is not fine with the 8kw you pick your load and size it about 80 % of the gensets kw or 20 % or 25% gen/ kw to load kw .

So its too small and the calculator is wrong .

6400 watts would be for a 8kw genset
ohmhead,
theoretically you Might be correct. It might work.




BUT,
there is just not enough info given. On the err of caution (and being a certified Cutler Hammer installer), I would not like to say 'you're fine, go ahead' with this proposed hypothetical design.

We are talking about an AC.
Can we take 240V times the 30A given and come up with an accurate 7200W?
Is that going to be Running Watts, or Starting Watts?

And what is the Generator Rating?
Is this 8000W of running output power, or of surge output power?

edit:
Ahh, you edited your last post while I was typing.
 

ohmhead

Senior Member
Location
ORLANDO FLA
Well if you look a the web page i posted you input your volts and load amps and they give you the size generator .

But if you do it on paper they say that the load must be 80 percent of gensets total kw ?

So take my genset its 17.5 kw but it can take 18 kw under a increase of load for a time in overload .
 

ohmhead

Senior Member
Location
ORLANDO FLA
ohmhead,
theoretically you Might be correct. It might work.




BUT,
there is just not enough info given. On the err of caution (and being a certified Cutler Hammer installer), I would not like to say 'you're fine, go ahead' with this proposed hypothetical design.

We are talking about an AC.
Can we take 240V times the 30A given and come up with an accurate 7200W?
Is that going to be Running Watts, or Starting Watts?

And what is the Generator Rating?
Is this 8000W of running output power, or of surge output power?

edit:
Ahh, you edited your last post while I was typing.

Well sorry but i got the book out on my generator and this is what it says 17 .5 kw running cont but can give you 22.5 kw max so the calculator is correct 8kw genset will work but we need to know its specs .
 

iwire

Moderator
Staff member
Location
Massachusetts
I have used 15kw generator sets for 5 ton heat pump units 240v 1ph with no start up or running problems, used generac set, I know of 1 with a 12.5kw generator set that seems to struggle just a bit on start up but this heat pump unit is a very old unit, i know about this one as we had a service call concerning it, do not know who installed it
 

SEO

Senior Member
Location
Michigan
This is hypothetical, if I'm using a portable generator to run my central AC during a power outage will the AC starting current affect the size of the generator? Say I have a 30 amp, 240 volt running current and a 8KW generator, will the starting current of the AC equipment overload the generator?

Don't forget the blower fan on the furnace. The blower fan usually starts the same time as the condencing unit. Now you have another motor and lock rotor to plug into your calculations.
 

ty

Senior Member
5 Tons rating is meaningless.

I have seen 5 ton units that require 40, 50, and 60 amp ocp.
What is its rating?
And if it is a 15Kw generac from Home Depot, chances are that is a skewed rating. That is probably starting Kw, its running Kw is probably lower at 12.5kW.

The 12.5kW is probably then 10kW running.
And that is LP hookup. NG will be less.

My bet is there was absolutely nothing else running when the heat pump kicked on.
If a well pump or freezer compressor was running, that 15kW would have had trouble.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Don't forget the blower fan on the furnace. The blower fan usually starts the same time as the condensing unit. Now you have another motor and lock rotor to plug into your calculations.

I had that figured in the original 30 amp calculation. Someone asked me if an 8KW generator would work on a 30 amp running A/C load and my response was no due to the required starting current for the compression/fan and the blower unit. So the follow up question is always then what size generator would be required to allow this unit to both start and run. I will forward the links provided here and let them figure it out using the actual ampacities since the 30 amps was a ballpark number.
 
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