How to determine HVAC wattage load

[Denkikun]

Howdy all, I had been bugging the guys over in the HVAC forum about gas furnace venting, and I'm now thinking that wiring a 240V circuit to my garage and going electric heat might be best... I'm trying to figure out if I have capacity in my main 150 Amp elec panel. I would plan to do my own electric work but pull a permit and have it inspected. However, the first step is the load calculation... I have an automated worksheet that the inspection folks gave me, but I am a bit stumped on one section...

--- could someone offer some support in determining the "HVAC LOAD".... it's asking me for the wattage of my air conditioning and furnace respectively...... I have a natural gas furnace and AC.. there is nothing I can find on the units that suggests wattage load... I suppose that the AC wattage is to the outdoor compressor/condenser unit? Any help here would be GREATLY appreciated

 

[Stuart in MN]

Yes, the AC load will be for the condenser unit. Somewhere in your manual, or on the company's website, there should be information on how many amps it draws, and from that you can calculate the watts (voltage x amps). A natural gas furnace AC load is basically just the blower motor, so it's pretty minimal. Again, the size of that blower motor and/or its amp draw should be in the documentation somewhere, but depending on the size of your furnace it's probably 1/4 or 1/3 hp - something in that area.

 

[Denkikun]

Yes, the AC load will be for the condenser unit. Somewhere in your manual, or on the company's website, there should be information on how many amps it draws, and from that you can calculate the watts (voltage x amps). A natural gas furnace AC load is basically just the blower motor, so it's pretty minimal. Again, the size of that blower motor and/or its amp draw should be in the documentation somewhere, but depending on the size of your furnace it's probably 1/4 or 1/3 hp - something in that area.

Thanks so much, Stuart! I will check for those manuals!

 

[PoorUB]

There should be a nameplate on the condensing unit with min and max breaker size.

 

[Denkikun]

In researching the furnace, online specifications state that it uses only 110W for the furnace (it states that most other furnaces are around 360W) - both of these numbers seem low to me, haha
The outdoor unit is a little trickier.... looked it up too and cannot find any stated wattage, but see that it is 208/230V and has a minimum circuit ampacity of 19 Amps with recommended breaker size of 30 Amps... So..... 230V x 19A= 4370W or.. 230V x30A = 6900W It sure seems that my furnace is 110W and the AC unit is between 4370 and 6900W - does that sound rightish?

 

[Denkikun]

There should be a nameplate on the condensing unit with min and max breaker size.

Just saw this PoorUB - you're all over these forums!! haha thanks - found some info, what do you think?

 

[PoorUB]

Just saw this PoorUB - you're all over these forums!! haha thanks - found some info, what do you think?

Nothing gets by me!

I am retired and some days not much is going on so I hang out here and help those in need!

 

[Stuart in MN]

In researching the furnace, online specifications state that it uses only 110W for the furnace (it states that most other furnaces are around 360W) - both of these numbers seem low to me, haha
The outdoor unit is a little trickier.... looked it up too and cannot find any stated wattage, but see that it is 208/230V and has a minimum circuit ampacity of 19 Amps with recommended breaker size of 30 Amps... So..... 230V x 19A= 4370W or.. 230V x30A = 6900W It sure seems that my furnace is 110W and the AC unit is between 4370 and 6900W - does that sound rightish?

The number for the furnace sounds suspiciously low. For the A/C condensing unit, it draws something less than the minimum circuit capacity; for estimating purposes 19 amps would be a worst case value, but it wouldn't be accurate. If you can post the brand and model numbers, maybe someone could help find the correct data.

 

[Terry D]

What size electric furnace is it.

 

[alfredeneuman]

There should be a nameplate on the condensing unit with min and max breaker size.

That has nothing to do with the wattage of the unit.
The "FLA" (full load amps) or "RLA" (running load amps) figure should be multiplied by the voltage to determine wattage.

 

[Denkikun]

The number for the furnace sounds suspiciously low. For the A/C condensing unit, it draws something less than the minimum circuit capacity; for estimating purposes 19 amps would be a worst case value, but it wouldn't be accurate. If you can post the brand and model numbers, maybe someone could help find the correct data.

Thanks Stuart, The furnace is a standard gas forced air 80K BTU Goodman unit. model GMVC960804CNAA
here is the link to the specific unit. I Actually saw on this link, that minimum circuit ampacity is 10.6 Amps and recommended breaker is 15 Amps @115V... so I guess that puts me between 1219W and 1725W

Goodman GMVC960804CN GMVC96 - 80k BTU - Two-Stage Gas Furnace - 96% AFUE

Buy Goodman GMVC960804CN Today. Free Shipping. Check the Goodman GMVC96 - 80k BTU - Two-Stage Gas Furnace - 96% AFUE ratings before checking out.

www.ecomfort.com

The AC condenser unit is a Goodman GSX130361EF. It's a 3 ton 13 seer unit. Here is the link:

Goodman GSX130361 GSX13 - 3 Ton - Air Conditioner - 13 Nominal SEER - Single-Stage - R-410a Refrigerant

The Goodman GSX13 - 3 Ton - Air Conditioner - 13 Nominal SEER - Single-Stage - R-410a Refrigerant GSX130361 has been discontinued. Check out Expert's recommended alternatives for another top central air conditioner.

www.ecomfort.com

My situation is that I have 150 Amp service in a GE panel. When I count breakers in there I come up with 470AMPS. Included in that is a double pole 30 Amp breaker for the AC condenser and a double pole 60 Amp breaker for a sub panel mounted next to the main panel. In that sub panel I counted up 145 Amps worth of breakers.... seem like quit a bit to me, but I suppose that you'd never use everything at once ! @-@ here is a pic of my situation... For the electric garage heater circuit I'd plan to add a 30 amp double pole breaker and run 240V through 10/2 wire about 60 feet to a junction box with a plug (for welding or maybe an EV charger some day) and then branch off of that to power a 5000 W electric heater. Maybe looking at a 6000W or 7500W heater, in which case I guess I'd up the breaker and go 8/2 wire. BTW, 電気子学科 means Electrical Engineering Department (found this sign in the trash at a university in Japan years ago, haha)

 

[jeepxj]

Nothing gets by me!

I am retired and some days not much is going on so I hang out here and help those in need!

checks location


yup its winter

 

[Stuart in MN]

Here's the data sheet for the furnace: https://www.goodmanmfg.com/pdfviewe...52f460022fa6258827eff0a00754798.pdf?view=true

Here's the data sheet for the a/c unit: https://www.goodmanmfg.com/pdfviewe...6bc4c0022fa6258827eff0a00754798.pdf?view=true

Minimum circuit ampacity for the furnace is 10.6 amps, minimum overcurrent device size is 15 amps at 120vac
Minimum circuit ampacity for the a/c unit is 19 amps, minimum overcurrent device size is 30 amps at 230vac

For the purposes of filling in the blanks in your worksheet I'd use those minimum circuit ampacity numbers. Volts x Amps = Watts, so

10.6 x 120 = 1272 watts for the furnace
19 x 230 = 4370 watts for the a/c

The total number of breakers in the panel doesn't necessarily mean anything when it comes to determining your total load - as you mentioned there are 470 amps' worth in your main panel, and obviously every circuit isn't running at full capacity all at the same time. For example, a bunch of them are 15 or maybe 20 amp circuits for lighting or wall receptacles, and the actual load on those circuits will be much less than 15 or 20 amps. For lights, just add up how many bulbs are on a circuit. For receptacle circuits you can typically figure 180 watts per receptacle when estimating loads. Alternately, those kinds of automated worksheets will sometimes have you enter a load number for general circuits (like lights and receptacles) that's based on the square footage of the house - I think they typically use 3 watts per square foot. I don't know which way your worksheet works, but just follow what it asks for.

 

[wyliesdiesels]

You dont add up all the breakers in the panel to determine load....

 

[Denkikun]

Here's the data sheet for the furnace: https://www.goodmanmfg.com/pdfviewe...52f460022fa6258827eff0a00754798.pdf?view=true

Here's the data sheet for the a/c unit: https://www.goodmanmfg.com/pdfviewe...6bc4c0022fa6258827eff0a00754798.pdf?view=true

Minimum circuit ampacity for the furnace is 10.6 amps, minimum overcurrent device size is 15 amps at 120vac
Minimum circuit ampacity for the a/c unit is 19 amps, minimum overcurrent device size is 30 amps at 230vac

For the purposes of filling in the blanks in your worksheet I'd use those minimum circuit ampacity numbers. Volts x Amps = Watts, so

10.6 x 120 = 1272 watts for the furnace
19 x 230 = 4370 watts for the a/c

The total number of breakers in the panel doesn't necessarily mean anything when it comes to determining your total load - as you mentioned there are 470 amps' worth in your main panel, and obviously every circuit isn't running at full capacity all at the same time. For example, a bunch of them are 15 or maybe 20 amp circuits for lighting or wall receptacles, and the actual load on those circuits will be much less than 15 or 20 amps. For lights, just add up how many bulbs are on a circuit. For receptacle circuits you can typically figure 180 watts per receptacle when estimating loads. Alternately, those kinds of automated worksheets will sometimes have you enter a load number for general circuits (like lights and receptacles) that's based on the square footage of the house - I think they typically use 3 watts per square foot. I don't know which way your worksheet works, but just follow what it asks for.

Thank so very much, Stuart! this gets me really close to wrapping the load calculation up. Yes, the sheet that I was provided by the inspector does calculate 3w per sq ft of the home. There is one last question that I was stumped on... it's asking for the number of small appliance circuits (minimum 2) and laundry circuit (minimum 1) ... I'm uncertain how to tell which circuits are appliance circuits... I'm assuming that the laundry is to the laundry room... thanks again for your time and help!

 

[Stuart in MN]

Small appliance circuits will be the ones above your kitchen counters, where you plug in toasters, mixers, etc.

 

[Denkikun]

Small appliance circuits will be the ones above your kitchen counters, where you plug in toasters, mixers, etc.

Gotcha, thanks! okay, I'm all out of questions!! hahaha guys on this forum and site have been amazing! I'm now a huge fan and am telling all of my car buddies about the cool community here!

 

[alfredeneuman]

Minimum circuit ampacity for the furnace is 10.6 amps, minimum overcurrent device size is 15 amps at 120vac
Minimum circuit ampacity for the a/c unit is 19 amps, minimum overcurrent device size is 30 amps at 230vac

For the purposes of filling in the blanks in your worksheet I'd use those minimum circuit ampacity numbers.

The minimum circuit is 125% of the unit load. The actual load is on the nameplate (RLA or FLA) of the unit.

it's asking for the number of small appliance circuits (minimum 2) and laundry circuit (minimum 1) ... I'm uncertain how to tell which circuits are appliance circuits... I'm assuming that the laundry is to the laundry room...

Laundry circuit is 1500 watts. Each small appliance circuit is 1500 watts.

 

[Stuart in MN]

The minimum circuit is 125% of the unit load. The actual load is on the nameplate (RLA or FLA) of the unit.

That's true, but for his purposes I think those numbers are okay - the point is to come up with numbers to plug into a worksheet, to figure out if he has capacity to add an electric heater in the garage, and I suspect in the end it will probably be more than adequate. There's usually a bunch of Kentucky windage in those worksheets anyway.

 

[alfredeneuman]

The nameplates include all of this info. It's just as easy to look at one as the other.
Using the actual figures cut some of that windage.

 

[Denkikun]

The nameplates include all of this info. It's just as easy to look at one as the other.
Using the actual figures cut some of that windage.

Thanks Alfred E I actually never found a nameplate on the furnace. Found only a small sticker inside unit with model number. Like Stuart said, I think I’m set enough with those numbers. Thanks!

 

[Terry D]

Maybe I not understanding, didn't you say in post #1 that you were wanting to install an electric furnace. That's why you needed to do a load calculation. But you linked a gas furnace.

 

[Denkikun]

Maybe I not understanding, didn't you say in post #1 that you were wanting to install an electric furnace. That's why you needed to do a load calculation. But you linked a gas furnace.

Thanks Terry, Sorry if it was confusing.... I'm pretty confused, haha I am planning to run a new 240V circuit to my garage and the local inspection guy gave me a load calculation sheet to fill out to see if I have ample capacity in my home's 150 AMP electric panel to add the new 240V circuit. on that sheet they ask for the wattage of my home's current furnace and AC units.... so that's what I was trying to hunt down.

 

[MBfreak]

AC Volts * AC amps single phase equals VA ( Volt Amps)
AC Volts*AC Amps * phase angle ( cos fi) equals W. This is actual work, and what your meter debits you.
The difference between VA and W for a typical HVAC unit ( no electronic speed controls) is around 15 %
Yes , I agree I am a factbased nerd.
After 55 years with electronic and electrical calcs, I admit I am seriously obsessed with facts,
Fewer sparks that way.

Ola