Load calculation using 220.83(b) calculating service size when adding an A/C

Status
Not open for further replies.

GUNNING

Senior Member
Using 220.83(b) do you need to add both the condenser and the heat strips at 100% when figuring out service size for adding a heat pump to a house.
I have
12 amp heat pump @ 240v = 2880 VA
5kw strips
Other loads
754 sq foot house at 3 VA/sq ft
small appliance loads per 210.11(C) of 4.5Kw
fridge at 540 VA
water heater 4.5 KW
stove @ 8KW
Dryer @ 5KW
washer @ 1.4 KW

100% of the a/c is 7880 VA
100% of the Other loads is 26182 VA
A/C + First 8K + 40% of the rest of the other loads =

7.9 KW + 8KW +40% of 18.2KW = 22.5 KW

Does the heat pump need to be rated higher because its a hermetically sealed motor or take the name plate max fuse value? Do I add the heat strips to the heat pump to get my A/C load?
Does the math look ok? I already messed this up once, don't need to publish my mistakes twice down there.
 

jumper

Senior Member
I believe the heat pump should be the nameplate MCA ,and not the MOCD, + the heat strips for the HVAC load.

Remember also the you have a condenser unit and an air handler for a heat pump.
 
Last edited:

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Using 220.83(b) do you need to add both the condenser and the heat strips at 100% when figuring out service size for adding a heat pump to a house.
I have
12 amp heat pump @ 240v = 2880 VA
5kw strips

Other loads
754 sq foot house at 3 VA/sq ft
small appliance loads per 210.11(C) of 4.5Kw
fridge at 540 VA
water heater 4.5 KW
stove @ 8KW
Dryer @ 5KW
washer @ 1.4 KW

100% of the a/c is 7880 VA
100% of the Other loads is 26182 VA
A/C + First 8K + 40% of the rest of the other loads =

7.9 KW + 8KW +40% of 18.2KW = 22.5 KW

Does the heat pump need to be rated higher because its a hermetically sealed motor or take the name plate max fuse value? Do I add the heat strips to the heat pump to get my A/C load?
Does the math look ok? I already messed this up once, don't need to publish my mistakes twice down there.

On a heat pump you only need the VA rating of the pump and add 65% of the heat strips.
2880 VA + (5000 x .65) 3250 VA = 6130 VA
This takes the A/C in consideration because the compressor is used for heat and A/C
add 6130 VA to your general load.
Also, if the heat strips (supplementary heat) is not on at the same time as heat pump, the heat pump KVA doesn't have to be included.
 
Last edited:

jumper

Senior Member
On a heat pump you only need the VA rating of the pump and add 65% of the heat strips.
2880 VA + (5000 x .65) 3250 VA = 6130 VA
This takes the A/C in consideration because the compressor is used for heat and A/C
add 6130 VA to your general load.
Also, if the heat strips (supplementary heat) is not on at the same time as heat pump, the heat pump KVA doesn't have to be included.

Are you using 220.82 or 220.83(B)?

I believe that the OP needs to use 220.83(B). I do not see a supplemental heat demand factor in 220.83(B)
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Are you using 220.82 or 220.83(B)?

I believe that the OP needs to use 220.83(B). I do not see a supplemental heat demand factor in 220.83(B)

IMO, you can still figure a heat pump load the same way you would in 220.82. Besides, 220.83 says ...."shall be permitted". If you do use 220.83 you still use the larger load of A/C or heat. A heat pump (his) incorporates both, so you would have to figure it the same way you would in 220.82. That is the way I see it.
 

jumper

Senior Member
IMO, you can still figure a heat pump load the same way you would in 220.82. Besides, 220.83 says ...."shall be permitted". If you do use 220.83 you still use the larger load of A/C or heat. A heat pump (his) incorporates both, so you would have to figure it the same way you would in 220.82. That is the way I see it.

220.82 is permissive also. The overall demand factors are different between the sections. I know about the larger of the heat vs A/C loads.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
220.82 is permissive also. The overall demand factors are different between the sections. I know about the larger of the heat vs A/C loads.

The heat pump would be the same load whether it is installed on a new build or added to an existing load. The compressor will run on both heat and a/c so how do you differentiate between the two? And if the heat strips kick in while the compressor is running that is the same as having the heat and a/c on at the same time. Maybe you can explain your thinking on this further and help me if I'm in error.
 

jumper

Senior Member
Okay, I will try.

Overall and individual HVAC demand factors:

220.82(B)

(B) General Loads. The general calculated load shall be
not less than 100 percent of the first 10 kVA plus 40 percent
of the remainder of the following loads:


(3) 100 percent of the nameplate rating(s) of the heat pump
compressor and 65 percent of the supplemental electric
heating for central electric space-heating systems
.
-----------------------------------------------------------------------------
220.83(B) Where Additional Air-Conditioning Equipment or
Electric Space-Heating Equipment Is to Be Installed.
The following percentages shall be used for existing and
additional new loads. The larger connected load of air conditioning
or space-heating, but not both, shall be used.

T220.83(B)
Load Percent of Load
Air-conditioning equipment 100
Central electric space heating 100

Less than four separately
controlled space-heating units
100
First 8 kVA of all other loads 100
Remainder of all other loads 40

----------------------------------------------------------------

Individual HVAC and overall demand factors are different for each application.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Okay, I will try.

Overall and individual HVAC demand factors:

220.82(B)

(B) General Loads. The general calculated load shall be
not less than 100 percent of the first 10 kVA plus 40 percent
of the remainder of the following loads:


(3) 100 percent of the nameplate rating(s) of the heat pump
compressor and 65 percent of the supplemental electric
heating for central electric space-heating systems
.
-----------------------------------------------------------------------------
220.83(B) Where Additional Air-Conditioning Equipment or
Electric Space-Heating Equipment Is to Be Installed.
The following percentages shall be used for existing and
additional new loads. The larger connected load of air conditioning
or space-heating, but not both, shall be used.

T220.83(B)
Load Percent of Load
Air-conditioning equipment 100
Central electric space heating 100

Less than four separately
controlled space-heating units
100
First 8 kVA of all other loads 100
Remainder of all other loads 40

----------------------------------------------------------------

Individual HVAC and overall demand factors are different for each application.

Well, if you use the larger of the heat/cool load, the 5 kva heat strips would be larger than the 2.88 kva of the heat pump. So you need only add 5 kva to the load, that's if you just use the larger of the 2 as stated in 220.83(B). Is that what you are saying? You mentioned in your first post that you needed to add in the air handler and condenser. I believe using the calculation for a heat pump incorporates that in. In the example in Annex D, there are no separate listings for an air handler or condenser.
 

gndrod

Senior Member
Location
Ca and Wa
220.82 vs. 220.83

220.82 vs. 220.83

Okay, I will try.

Overall and individual HVAC demand factors:

220.82(B)

(B) General Loads. The general calculated load shall be
not less than 100 percent of the first 10 kVA plus 40 percent
of the remainder of the following loads:


(3) 100 percent of the nameplate rating(s) of the heat pump
compressor and 65 percent of the supplemental electric
heating for central electric space-heating systems
.
-----------------------------------------------------------------------------
220.83(B) Where Additional Air-Conditioning Equipment or
Electric Space-Heating Equipment Is to Be Installed.
The following percentages shall be used for existing and
additional new loads. The larger connected load of air conditioning
or space-heating, but not both, shall be used.

T220.83(B)
Load Percent of Load
Air-conditioning equipment 100
Central electric space heating 100

Less than four separately
controlled space-heating units
100
First 8 kVA of all other loads 100
Remainder of all other loads 40

----------------------------------------------------------------

Individual HVAC and overall demand factors are different for each application.

The calculations appear okay, but 220.82 is for optional single family whereas 220.83 is optional for extended single family. The difference...220.82 specifies the largest of heat or A/C load from one of six selections and 220.83 calcs are identical except for 100% of the larger of added connected heat or A/C load.

Some heat pumps are single package scroll type that use a single motor for compressor/air handler function with heat strip backup, so the total mca depends on the strip quantity loading if multiple staged. Inspectors may require installing the max feeder needed regardless of heatstrips installed.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Jumper, You can't be through "schooling" me yet. For some reason I had forgotten you were a HVAC tech.:mad: I apologize for trying to explain to you how a heat pump works. I just can't see how the NEC would think the load would be any different for a new service or adding to an existing one.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Jumper, You can't be through "schooling" me yet. For some reason I had forgotten you were a HVAC tech.:mad: I apologize for trying to explain to you how a heat pump works. I just can't see how the NEC would think the load would be any different for a new service or adding to an existing one.


Bump!!
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Remember all this is based on the fact that a heat pump is wired, thru the T-stat controls, so that the heat pump cannot run at the same time as the heat strips. Otherwise you need to calculate both units into the equation. You need to check with the HVAC installer to make sure it is wired that way.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Remember all this is based on the fact that a heat pump is wired, thru the T-stat controls, so that the heat pump cannot run at the same time as the heat strips. Otherwise you need to calculate both units into the equation. You need to check with the HVAC installer to make sure it is wired that way.

I asked the installer for my unit that was just installed a few months ago and he said not all units have the capability of preventing the compressor from running when the heat strips are on. Mine does not have that capability, or at least that's what he told me. He said it was designed for the supplementary heat to run for a short while to aid in heating when it was below a certain temperature. I know that the strips are supposed to kick in at a certain temp. but he said mine was designed to have them run together more than in an older unit.
 

gndrod

Senior Member
Location
Ca and Wa
kick in temp

kick in temp

It's been a while, if I remember, the certain temperature mentioned is below 40 F (adjustable factory setting) when the crankcase coil activates to heat up the condenser oil before the strip elements are energized. The compressor will come on before the strips are turned on in most units. The heat-cool thermostat is equipped with a system HEAT-COOL switch which provides prevention of simultaneous operation of heating and cooling functions.
 
Status
Not open for further replies.
Top