Electric Resistance Heating in HVAC units.
- Thread starter Ace18
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Some of the newer units with 20 kw require 2-- 60 amp circuits to the unit.
If this unit only has one feed then you have a large cable or conductors to pull.
If this unit only has one feed then you have a large cable or conductors to pull.
kwired
Electron manager
- Location
- NE Nebraska
many nameplates will have a list of possible heat strips to be installed and the installer is supposed to mark which heat strip has been installed.
The nameplate will usually tell you requirements for supply circuit based on which heat strip is marked as the one installed.
The nameplate will usually tell you requirements for supply circuit based on which heat strip is marked as the one installed.
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
You not only have 20 kw x 125% you also must add in the blower.
I sure hope this is not a house. Ouch what a bill
I sure hope this is not a house. Ouch what a bill
kwired
Electron manager
- Location
- NE Nebraska
the bill depends on where you live. 20kw furnaces are very common around here(Nebraska). Electric rates are favorable for electric heat here, utility demand in winter is low. Winter rates are lower than summer rates, and rate usually is bracketed so first XXXX kw/hr are at $.XXXXX next XXXX are at $.XXXXX, and then anything over XXXX is at $.XXXXX.
cant tell you what the X's are right now.
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
Well here it is about 14 cents a kwh and we are far far frome getting as cold as you
first 1,000 kwh 6.886 energy charge plus 4.611 fuel charge
above 1,000 kwh 7.977 energy plus 5.611 fuel
this months bill $345
I do have about 3600 sf but 6 1/2 days out of week only cool and heat 1500 sf
have heat pumps for most of it and propane for pool table room
Does 424.22 (B) apply in my situation? If so it makes it alot easier.
Also, this 125% thing is confusing me. It contradicts this:
220.51 Fixed Electric Space Heating. Fixed electric
space-heating loads shall be calculated at 100 percent of the
total connected load. However, in no case shall a feeder or
service load current rating be less than the rating of the
largest branch circuit supplied
So is it 100% or 125%?
Also, this 125% thing is confusing me. It contradicts this:
220.51 Fixed Electric Space Heating. Fixed electric
space-heating loads shall be calculated at 100 percent of the
total connected load. However, in no case shall a feeder or
service load current rating be less than the rating of the
largest branch circuit supplied
So is it 100% or 125%?
This is a heat Pump. The 20kW is supplemental heat. It does not run all the time, only when it need it.
424.22(B) does apply.
Art 220 is for calculating the load. You have to refer to the applicable Codes for the actual wiring. 424.5 specifies 125%
All heat pumps, by Code, should have a nameplate giving you specifics on the branch circuit wiring including the min ckt amps and max overcurrent and how many circuits are needed.
On some units the heat and compressor can run simultaneously.
The manufacturer data (424.28, 440.4) is a requirement and is there to give you all that info.
In this jurisdiction, the job woud not be approved without the required data plate.
Art 220 is for calculating the load. You have to refer to the applicable Codes for the actual wiring. 424.5 specifies 125%
All heat pumps, by Code, should have a nameplate giving you specifics on the branch circuit wiring including the min ckt amps and max overcurrent and how many circuits are needed.
On some units the heat and compressor can run simultaneously.
The manufacturer data (424.28, 440.4) is a requirement and is there to give you all that info.
In this jurisdiction, the job woud not be approved without the required data plate.
The data plate does give MCA, but its does not include the electric heat because it is optional. In Florida you might only install a 10KW heater in it, but up north the same heat pump might require a 20kw heater.
By the way, for some reason my copy of NEC does not have 424.5 After 424.3 it jumps to 424.6
Last edited:
In most heat pumps, it will run the full 20KW when doing a defrost. It will also run some or all of the heat strips when the outside temperature is cold enough that the heat pump can't keep up (typically around 30 to 35F, and the colder it gets the less heat the heat pump can generate).
The article 220 thing is for Service and feeder calculations. The branch circuit size is dictated by 424.22.
typo 424.3...
The auxiliary heat strips are normally supplied with a "stick on" label that gives you the info. The heat pump install manuals I have seen also have charts with this info.
Installs like you describe here would have 1 circuit to the heat pump, and for the 20 kW auxiliary wither two circuits or 1 circuit with factory internal fusing breaking it down to 2 circuits.
Most often I see a properly sized load center (normally 125 amp) with 1 breaker for the condenser (based on nameplate data) and (2) 60s for the heat
Here, we reject it and that info always becomes available.
kwired
Electron manager
- Location
- NE Nebraska
A heat pump installer that I frequently work with usually installs a lockout thermostat to shut down the heat pump when the outdoor temperature falls below the point where the efficiency is compromised. This is essentially a SPDT thermostat switch that switches from normal heat thermostat lead to backup heat thermostat lead, and the backup heat comes on immediately on call for heat.
Many of the newer digital thermostats with an outdoor temperature sensor option have this feature in the programming.
As far as I know most heat pumps simply operate the reversing valve during defrost, which causes the unit to go into cooling mode - Heat is transfered from inside to outside just like during air conditiong season. this will remove the frost on the outdoor coil fairly quickly and then the unit resumes normal operation.
Last edited:
kwired
Electron manager
- Location
- NE Nebraska
After thinking about it im pretty sure heat pump defrost are pretty much all the way I described in previous post.
The heat pump indoor coil on almost all air handlers that use electric heat strips is ahead of the heat strip in the air stream. So turning on the heat strip would be pointless as far as defrosting is concerned. I guess that it would still be putting heat into the heated space if the heat strip were on but the defrost cycle is short enough that it is still pointless.
The heat pump indoor coil on almost all air handlers that use electric heat strips is ahead of the heat strip in the air stream. So turning on the heat strip would be pointless as far as defrosting is concerned. I guess that it would still be putting heat into the heated space if the heat strip were on but the defrost cycle is short enough that it is still pointless.
- Status