Sizing A Generator

[Little Bill]

When sizing a backup generator, do you need to include both heating and AC? Usually they don't run at the same time but some compressors run with Aux heat strips are on.

Would you calculate it (HVAC) just as you would for a service?
This is not a whole house backup but pretty close. It is a two-story house and they want all lower level lights, some designated receptacles throughout the lower level, the garage circuit, including garage door openers, the cook top and the lower level HVAC.

Here's a couple pictures of the HVAC nameplate.
ACs MCA=31.5 Max Brkr=50A
Heat MCA=52.5 Max Brkr=60A

 

[GoldDigger]

Would you calculate it (HVAC) just as you would for a service?
This is not a whole house backup but pretty close. It is a two-story house and they want all lower level lights, some designated receptacles throughout the lower level, the garage circuit, including garage door openers, the cook top and the lower level HVAC.

Will the generator be autostart or manual start? With ATS or manual transfer?
If it is manual transfer you do not have to be as careful about load segregation to a sub-panel and automatic load shedding.
If there is no way to lock out the heat strips on the heat pump, then I think you have to include both in sizing the generator, although you probably will not have both motor starting surge and heat strips active at the same time. If there is a separate disconnect for the heat strips you may be able to get away with not counting that load when doing a manual transfer, but it would limit the heat available in some weather.
The cook top (and oven?) can be an enormous single load.

 

[Little Bill]

Will the generator be autostart or manual start? With ATS or manual transfer?
If it is manual transfer you do not have to be as careful about load segregation to a sub-panel and automatic load shedding.
If there is no way to lock out the heat strips on the heat pump, then I think you have to include both in sizing the generator, although you probably will not have both motor starting surge and heat strips active at the same time. If there is a separate disconnect for the heat strips you may be able to get away with not counting that load when doing a manual transfer, but it would limit the heat available in some weather.
The cook top (and oven?) can be an enormous single load.

It will have an ATS and auto start. Cook top, no oven.

 

[augie47]

Interesting topic..... confronted it more than once

A couple of side notes:
Did you notice that there is a different MCA than the 31.5 for USA installations.
With hermetic compressors (or any significant motors) involved it it is often a good idea to consult the generator manufacturer also.

More specifically to your point.
Is this a heat pump ? If not, it would be the larger of the two loads.
I have found with the heat pumps installed in this area, some allow both the compressor and strip heating to run simultaneously,
some allow compressor with partial heat strips and some or an either/or situation. Step one would be to determine which way
your unit is set up.

 

[Little Bill]

Interesting topic..... confronted it more than once

A couple of side notes:
Did you notice that there is a different MCA than the 31.5 for USA installations.
With hermetic compressors (or any significant motors) involved it it is often a good idea to consult the generator manufacturer also.

More specifically to your point.
Is this a heat pump ? If not, it would be the larger of the two loads.
I have found with the heat pumps installed in this area, some allow both the compressor and strip heating to run simultaneously,
some allow compressor with partial heat strips and some or an either/or situation. Step one would be to determine which way
your unit is set up.

I'm not sure if it's a heat pump or not. I will say it doesn't look like most heat pumps that I've seen. But it's a packaged unit with all of it outside.
There is a "typical" looking heat pump sitting next to this unit, but I'm pretty sure it is for the upper level of the house and would not be included in the back up.

Should have taken a picture of the whole unit and not just the name plate!:slaphead:

 

[augie47]

From the nameplate, I would say conventional heat/air in which case the larger of the two loads, although, as noted, if the generator is gojng to supply the air conditioning part, you need to keep inrush in mind.

 

[Little Bill]

From the nameplate, I would say conventional heat/air in which case the larger of the two loads, although, as noted, if the generator is gojng to supply the air conditioning part, you need to keep inrush in mind.

As in sizing it for Max instead of MCA?

 

[augie47]

As in sizing it for Max instead of MCA?

When in doubt, I relied on the generator manufacturer. They always asked about LRA when it was an issue.
Others here with more knowledge and experience can give you a detailed answer, I just know from bitter experience, if you only take MCA into account and the generator si sized "close" you can encounter a problem on a/c start up.
(I have seen the problem solved by adding "softstart" to the compressor.)

I don't mean to complicate your install as it seems on a majority of the installs I see it's not an issue, but I think it's important to keep in mind especially on smaller generators.

 

[Little Bill]

When in doubt, I relied on the generator manufacturer. They always asked about LRA when it was an issue.
Others here with more knowledge and experience can give you a detailed answer, I just know from bitter experience, if you only take MCA into account and the generator si sized "close" you can encounter a problem on a/c start up.
(I have seen the problem solved by adding "softstart" to the compressor.)

I don't mean to complicate your install as it seems on a majority of the installs I see it's not an issue, but I think it's important to keep in mind especially on smaller generators.

I'm thinking he will probably need a 20KW or higher genny. Haven't done all the load calcs. yet but pretty sure 20KW would be the starting point but probably will need larger than that.

 

[RLyons]

Cool little tool here:http://kohlerpower.com/residential/...onNumber=13561&nodeNumber=1&contentNumber=103#

 

[Little Bill]

Cool little tool here:http://kohlerpower.com/residential/...onNumber=13561&nodeNumber=1&contentNumber=103#

Thanks, that should help!