- Location
- Connecticut
- Occupation
- Engineer
See 430.24
loads of RTUs with ELec Heat
- Thread starter raberding
- Start date
- Status
See 430.24
But that's for motors, and we are talking RTU's that include electric heat.
- Location
- Connecticut
- Occupation
- Engineer
430.24 is for Several Motors or Motor(s) and Other Load(s). An RTU with electric heat fits that definition.
Electric-Light
Senior Member
Well it depends on the chassis design as far as the sequence of airflow. If it passes through heater before the AC, dehumidification is practically impossible and it pretty much guarantees the two won't operate together.
If AC comes first, it's possible to dehumidify, then reheat but I don't know if that is actually done given how incredibly inefficient it is.
clg then reheat is a common method to dehumid
the other won't do anything
say you want 70 deg air
return air is 75
when you cool below dewpoint (say to 45 with dp of 55) you condense moisture out
then reheat back to 70 for temp demand
when you heat first (say to 95) the dewpoint is not changed
then cool to 70 for temp demand
no dehumid since you never cooled below 55
it is relatively eff compaired to other methods
especially capital cost since you already bought the coills and only need the arrange them properly
desiccant is another method
much lower humid but much more $$$
a psychrometric chart illustrates it better than I can
- Location
- Massachusetts
Pretty common in my area although it will be AC into a gas fired furnace.
It is a low up front cost way to take care of high humidity on cool days.
I am sure you know this but as a point of general info for others this is also why most RTUs (in my area anyway) have two to four stages of cooling so that the amount of cooling produced is just enough that the AC runs long enough to pull the humidity out of the building.
Electric-Light
Senior Member
So you'd have to check the design of the RTU. I've seen both types.
If it's return -> heat -> evaporator -> supply, it can not happen due to practical design limitations.
If it's return -> evaporator-> heat -> supply, it could function, if both the thermostat/humidistat and the control module in the RTU would allow both to activate at the same time.
If it's return -> heat -> evaporator -> supply, it can not happen due to practical design limitations.
If it's return -> evaporator-> heat -> supply, it could function, if both the thermostat/humidistat and the control module in the RTU would allow both to activate at the same time.
But when I look at it again, it says 125% of the largest motor, plus 125% of the continuous loads, and heat is a continuous load per 424.3.
Electric-Light
Senior Member
Get the installation or service manual for the RTU. There will be a page with a table telling you the ampacity and allowable OCPD range to use for the tonnage variations of the same product and the installed electric heater kW. The ampacity from equipment manual do not need further compensation. The RTU is listed as a unit, so you don't break down things down manually.
AC units are allowed to be fed from a larger breaker than the conductor size would suggest if the conductors meet the circuit ampacity given on the unit and the OCPD size is in the range of what is given on the equipment plate without making further adjustments. The motor overload is delegated the task of overload protection for the conductor while the magnetic trip provides short circuit protection.
kwired
Electron manager
- Location
- NE Nebraska
Each is describing different situations, 424.3(B) is referring to to branch circuits for fixed electric space heating.
430.24 is a motor or several motors and other loads and is talking about a common conductor supplying such loads.
If you are calculating a service or feeder - you need to know the largest motor plus all other loads that can run simultaneously to get the minimum allowable conductor ampacity.
I think you need the largest motor, all the other loads that can run simultaneously, and 125% of the other simultaneous continuous loads including electric heat. That's what 430.24 is saying. 424 says electric heat is a continuous load.
- Location
- Connecticut
- Occupation
- Engineer
Heat is NOT a continuous load per 424.3. 424.3 says fixed electric heating shall be CONSIDERED a continuous loaf for the purposes of BRANCH CIRCUIT SIZING.
424.3 has no impact on feeder sizing.
Still don't agree. I see nothing that says that applies only to branch circuits.
Again, the handbook commentary says it impacts the sizing of feeders and services.
And 215.2 says feeders for continuous loads shall be sized at the noncontiguous load plus 125% of the continuous load.
kwired
Electron manager
- Location
- NE Nebraska
take a look at exception 2 to 430.24.
That refers back to 424.3B. More evidence for my side of the argument.
- Location
- Connecticut
- Occupation
- Engineer
The Title of section 424.3 is BRANCH CIRCUITS
The Title of subsection 424.3(B) is BRANCH CIRCUIT SIZING
A feeder is NOT a branch circuit. 424.3(B) does NOT apply to feeders.
Sure, but both the handbook and 215.3 seem to indicate that this still has a direct effect on the feeder or service sizing.
- Location
- Connecticut
- Occupation
- Engineer
The handbook is not the Code. 215.3 doesn't say anything about electric heat.
Compare 424.3 to other sections that say load shall be considered continuous load. Some sections say that load shall be considered continuous specifically for sizing branch circuits. Some sections say the load shall be considered continuous, period. The former does not apply to feeders, it is specifically for branch circuit sizing. The latter applies to branch, feeder and service conductors.
kwired
Electron manager
- Location
- NE Nebraska
Isn't deciphering NEC fun sometimes?
- Status