It's likely that there isn't 40A load on the 40A breaker. The A/C
installer would have sized it high if they had any brains. However, the
full picture here is that a bigger generator might cost a few thousand
more, but buying it too small means buying it twice.
If you cut it too close you can have the engine have trouble starting,
stall when loads change, and wear out sooner than expected. IMO you
don't want "big enough", you want "excessively big". I know I've harped
on this point before, but if your two hot legs are unbalanced then
there's more resistance on one side of the generator than the
other....the smaller the generator the easier it is to have problems
with that. How many people are bothering to check how well balanced the
two hot legs are on their electric panel? I got educated about that
problem by an 8Kw generator with 13A on one leg and 2A on the other.
The engine ran noisy and stalled every few minutes, and we had to
hastily rewire to fix it. A bigger engine wouldn't have cared so much.
Chuck's suggestion of ((load + 50%) *2) is good sense for all of the
above reasons.
Also pick a Sunday morning sometime and shut off your main breaker for a
few hours to see how well your generator carries everything. You can
turn the breaker back on if there's a problem, but if there's a problem
during an outage you just have to suffer until it's over.
-Adam
On 2/10/2020 9:59 AM, Josh Luthman wrote:
Is that 40A HVAC an electric heater or something? That's massive.
I have a 30A two ton mini split that only pulls about 9 amps (110) at
full throttle heat.
Josh Luthman
Office: 937-552-2340
Direct: 937-552-2343
1100 Wayne St
Suite 1337
Troy, OH 45373
On Sun, Feb 9, 2020 at 9:04 AM <ch...@wbmfg.com
<mailto:ch...@wbmfg.com>> wrote:
If your HVAC is 240 then you have more HVAC load than that
generator can pull on just that one load.
40 amps x 240 Volts is 9600 Watts.
7.5 kW generac will only deliver 31 amps at 240 VAC And you
really don’t want to load a generator up that much. 50% is a good
derating factor. Also, generators publish their power output at
sea level.
Knock off 3% for every 1000’ above sea level and 2% for every 10
degrees above 70 degrees F
Rounding up, you are close to 5000’ so that is 15% lost for
altitude and in the summer at 110 degrees that is another 8%.
So you lose as much as 23% of capacity on a hot summer day.
Worse if you power it from natural gas.
I would add up all your loads, add 50% for growth then double that
figure for not loading the generator too much and to account for
derating.
The installation is the expensive part. Don’t scrimp on kW at
this stage of the game. Sounds to me like you need a 20 kW or
more depending on your true HVAC loads.
*From:* Sterling Jacobson
*Sent:* Saturday, February 8, 2020 6:04 PM
*To:* AnimalFarm Microwave Users Group
*Subject:* [AFMUG] Generator for office
I need a smaller footprint generator for my 1300 sqr foot office.
Was thinking of getting the 50A 7.5KW Generac 6998 for $2050.00
that includes the 50A transfer switch.
I think my breakers has several 20A switches and 40A for the HVAC.
Not sure if this would cover the power needed for the whole unit,
or just the fiber cabinet/rack I have that has max 20A circuit to it?
The next highest power draw is probably the HVAC and/or water heater.
The lights only draw about 500W since I switched them all to LED.
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