HVAC Equipment Power Rating Calculations – Part Three



Today, we will explain the NEC Rules related to HVAC System Power Sizing Calculations.

The National Electrical Code (NEC) And HVAC Systems – Part One





First, we need to differentiate between the following load calculations:

  1. Service load calculation,
  2. Feeder load calculation,
  3. Branch circuit load calculation.

And to know the difference between the above load calculation types, we need to understand the NEC definitions for above expressions which are as follows:

  • Service load calculations: are the calculations done for sizing the conductors and equipment for delivering electric energy from the serving utility (service point/ service Cutout) to the wiring system of the premises served (service equipment  or Main LV Switchgear).

  • Feeder load calculations:  are the calculations done for sizing all circuit conductors between the service equipment or Main LV Switchgear, the source of a separately derived system, or other power supply source and the final branch-circuit over-current device (panel boards for example).

  • Branch Circuit load calculations: are the calculations done for sizing the circuit conductors between the final over‐ current devices (panel board breakers for example) protecting the circuit and the outlet(s).


The below figure indicate the definition and extent of each expression; service, feeder and branch circuit conductors.

service, feeder and branch circuit conductors


 
Another difference between the above load calculations is that the service and feeder load calculations are designed based on the contribution of each load type (after applying the demand and diversity factors) in the total load, while the branch circuit load calculation is designed based on the nameplate or actual load of each final outlet/load.

So, the HVAC load calculations in NEC differ according to what we are going to calculate; the HVAC system contribution in the service/feeder load or the branch circuit load?



First: The HVAC System Contribution in Service/Feeder Load Calculations


The NEC calculate service/feeder load by two different methods:
  1. Standard method,
  2. Optional method.




First: NEC Standard Method


Important!!!
Most service load calculations will include heating and/or air conditioning equipment, but not all feeder load calculations will include these types of loads. If the feeder will not supply power to heating and air conditioning equipment, calculate just the general loads on this feeder. If a service will not supply heating equipment calculate only the service for air condition only. If a service will not supply power to heating and air conditioning equipment, ignore this load in service load calculation.


Rule#1: Service load for Room air conditioners (Window or Split)

The load for Room air conditioners shall be calculated at 100 % of its ampere rating which may be indicated on its nameplate and will be used in branch, feeder and service load calculations.


Rule#2: Service load for Fixed electric space-heating loads

As per NEC section 220.51, Fixed electric space-heating loads shall be calculated at 100 percent of the total connected load (not the total load on its name plate where not all the stages will be on). However, in no case shall a feeder or service load current rating be less than the rating of the largest branch circuit supplied.

Rule#3: Central air conditioning and heating system Load

Central air conditioning and heating system Load shall be calculated at 100 % of its nameplate and will be a Noncoincident Load.


Rule#4: Noncoincident Loads

As per NEC section 220.60, where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time for calculating the total load of a feeder or service.


Important!!!
Depending on the design, the heating system and the air conditioning system might be noncoincident loads.
Other examples of noncoincident loads are standby motors and standby pumps.

Rule#5: the air handler (or blower motor) is not a noncoincident load

Although the heating and air conditioning in package units and split systems are noncoincident loads, the air handler (or blower motor) (or evaporator motor) is not. Since the blower motor works with both the heating and air conditioning system, it must be included in both calculations.


Important!!!
Cooling Fan Motor = Blower Motor = evaporator Motor = air handler Motor. All have the same meaning.


Important!!!
Based on Rule#5, in case of split or packaged units:
  • The air conditioning load will consist of compressor load, blower motor and condenser fan motor.
  • And the heat load will consist of electrical heater and blower motor.



Rule#6: Largest Motor in the feeder or service load calculation

As per NEC sections 220.50 and 430.24, when calculating a feeder or service, the largest motor must be multiplied by 125 percent.


Important!!!
Unless it is the largest motor in the feeder or service load calculation, do not multiply the full-load current of the motor by 125 percent.


Important!!!
If we have Many air conditioner compressors with the same rating and this rating is considered to be the largest motors, then apply rule#6 (125%) for one of them only and others will be at 100% of their rating like other loads.


Rule#7: A heat pump with supplementary heat is not a noncoincident load

With a heat pump, the compressor (and accompanying motors) and some or all of the electric heat can be on at the same time (in case of heating mode when the outdoor ambient temperature is very low). The load contribution of a heat pump is the air conditioning system load plus the maximum amount of heat that can be on while the air conditioner compressor is on.


Important!!!
The heat pump can work as an air conditioner in summer and as a heater in winter.


Important!!!
Supplementary heat means that the heat pump is a dual fuel heat pump which consists of electric heat pump and gas furnace all in one unit.


Important!!!
The heat pump with supplementary heat is not noncoincident loads. So, the heat pump with supplementary heat load = air conditioning load + heat load used on the same time



Summary for NEC standard method Based on above rules

First: No demand factor is applied and all the room air conditioner, central air conditioning and central heating are calculated at 100% of its name plate rating while fixed electrical space heating is calculated at 100% of its connected load.
Second: calculate the largest load between the air conditioning load and the hating load.
Third: note that The air conditioning load will consist of compressor load, blower motor and condenser fan motor. While the heat load will consist of electrical heater and blower motor.
Fourth: the heat pump with supplementary heat load = air conditioning load + heat load used on the same time
Fifth: if the largest load (the air conditioning load and the hating load) has motors then you the service/feeder load= 125% of the largest motor load + 100% other loads
Sixth: If we have Many air conditioner compressors with the same rating and this rating is considered to be the largest motors, then apply rule#6 (125%) for one of them only and others will be at 100% of their rating like other loads.



Second: NEC Optional Calculation Method

Rule#1: Application of NEC Optional calculation method

NEC Optional calculation method will be used if the following condition is verified:
  • The service-entrance or feeder conductors have an ampacity of at least 100 amperes.



Important!!!
If the service-entrance ampacity calculated by the optional method is less than 100A, the service/feeder load calculation must be done by the standard method.


Important!!!
In NEC Optional calculation method, for a multifamily dwelling, Table 220.84 “Optional Calculations — Demand Factors for Three or More Multifamily Dwelling Units” will be used if the following conditions are verified:

  1. If No dwelling unit is supplied by more than one feeder,
  2. Each dwelling unit is equipped with electric cooking equipment,
  3. Each dwelling unit is equipped with either electric space heating or air conditioning, or both.




Important!!!
The optional calculation can be used, provided all of the conditions for using table 220.84 listed above are met. Otherwise, the calculation for the multifamily dwelling is performed by using standard calculation method.


Rule#2: Application of NEC Optional calculation method

NEC Optional calculation method is applicable only for a single dwelling unit, an existing dwelling unit, a multifamily dwelling, two dwelling units, a school, an existing installation and a new restaurant.


First: For Single Dwelling Units


Rule#1: Heating and Air-Conditioning Load as per NEC Optional calculation method

As per NEC section 220.82 (C), for Heating and Air-Conditioning Load, The largest of the following six selections (load in kVA) shall be included:

  1. 100 percent of the nameplate rating(s) of the air conditioning and cooling.
  2. 100 percent of the nameplate rating(s) of the heat pump when the heat pump is used without any supplemental electric heating.
  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. If the heat pump compressor is prevented from operating at the same time as the supplementary heat, it does not need to be added to the supplementary heat for the total central space heating load.
  4. 65 percent of the nameplate rating(s) of electric space heating if less than four separately controlled units.
  5. 40 percent of the nameplate rating(s) of electric space heating if four or more separately controlled units.
  6. 100 percent of the nameplate ratings of electric thermal storage (ETS) and other heating systems where the usual load is expected to be continuous at the full nameplate value.



Case#1: If a dwelling has some type of heat other than electric

  • In this case, calculate the air conditioning load only at 100 percent of the nameplate rating.


Important!!!
Where there is more than one air conditioning unit, the calculation method is the same as it is for one air conditioning unit. 


Case #2: Heat pumps equipped with or without electric supplemental heat

  • Supplemental heat is sometimes referred to as auxiliary, backup or even emergency heat. A dual-fuel heat pump is an electric heat pump and a gas furnace all in one. Dual fuel heat pumps can be fueled with natural gas or propane. Because geothermal heat pumps (sometimes referred to as geo-exchange, earth-coupled, ground-source or water-source heat pumps) do not depend on the temperature of the outside air, they may or may not be equipped with supplemental heat.
  • Heat pumps not equipped with supplemental electric heat are calculated exactly the same as the air conditioning equipment specified in case#1.

Case #3: When a heat pump is used with supplemental electric heat

  • In this case, multiply the nameplate rating(s) of the heat pump compressor by 100 percent and multiply the supplemental electric heating for central electric space-heating systems by 65 percent.



Important!!!
Some heat pumps are designed so that only part of the electric heat will operate while the compressor is in operation. If the heat pump compressor is prevented from operating at the same time as the supplementary heat, it does not need to be added to the supplementary heat for the total central space-heating load. 


Case#4: electric space heating by less than four separately controlled units

  • In this case, 65 percent of the nameplate rating(s) of electric space heating if less than four separately controlled units.


Case#5: electric space heating by four or more separately controlled units

  • In this case, 40 percent of the nameplate rating(s) of electric space heating if four or more separately controlled units.


Case#6: using electric thermal storage (ETS) and other heat systems


  • Some electric utilities offer a discounted rate for kilowatt-hours used during certain hours of the day and night. During off-peak hours, customers pay rates that are less than rates during peak hours. During off-peak hours, electric elements are used to heat ceramic bricks. The stored heat is then released throughout the day when rates are higher. Many installations include multiple units placed throughout the house.
  • Since all of the electric thermal storage units could be heating bricks at the same time, no demand factor can be applied to the units.
  • In this case, 100 percent of the nameplate ratings of electric thermal storage (ETS) and other heat systems where the usual load is expected to be continuous at the full nameplate value.

Rule#2: Noncoincident Loads

As per NEC section 220.60, where it is unlikely that two or more noncoincident loads will be in use simultaneously, it shall be permissible to use only the largest load(s) that will be used at one time for calculating the total load of a feeder or service.


Important!!!
Depending on the design, the heating system and the air conditioning system might be noncoincident loads.


Important!!!
Similarly, 220.82(C) requires that only the largest of the six choices needs to be included in the feeder or service calculation.



Second: Multifamily Dwelling


Rule#1: Table 220.84 for Multifamily Dwelling demand factors

As per NEC section 220.84, for Multifamily Dwelling, the demand factors of Table 220.84 shall be applied to the larger of the air-conditioning load or the fixed electric space-heating load.


Rule#2: Calculation of feeder and service loads for air conditioning load in Multifamily Dwelling

With the optional method multifamily dwelling load calculation, the air conditioning load is calculated at 100 % of the nameplate rating.


Important!!!
The air conditioning load is calculated the same way in multifamily dwellings as it is in one-family dwellings. In both types, the air conditioning load is calculated at 100 percent of the nameplate rating.


Rule#3: Calculation of feeder and service loads for electric space heating load in Multifamily Dwelling

With the optional method multifamily dwelling load calculation, the load for space heating units must be added to the calculation at the nameplate rating.


Important!!!
The electric space heating load is not calculated the same way in multifamily dwellings as it is in one-family dwellings. With the optional method one-family dwelling load calculation, it is permissible to apply a demand factor to space heating units. The demand factor depends on the number of units.


Don’t Forget…
When calculating a multifamily dwelling by the optional method, use the larger of the air conditioning loads or the fixed electric space-heating load.

Important!!!
Do not assume the heating load will always be larger than the air conditioning load. The heating system could be gas or oil. The dwelling could also be located in a warm climate where the air conditioning load is larger than the heating load.

Important!!!
Fixed electric space heating is not limited to space heaters and electric strip heat that is part of a package unit or furnace. Fixed electric space heating could also be a heat pump with supplementary heat.


Rule#4: If the Fixed electric space heating is a heat pump with supplementary heat

With a heat pump, the compressor (and accompanying motors) and some or all of the electric heat can be energized at the same time.
So, The load contribution of a heat pump = the air conditioning system load + the maximum amount of heat that can be on while the air conditioner compressor is energized.

Rule#5: house loads of multifamily dwellings

Applying the Table 220.84 demand factor to house loads of multifamily dwellings is not permitted. House loads must be calculated as per NEC standard method.


Rule#6: multifamily dwelling buildings with multiple services and feeders

Sometimes, in large multifamily dwelling buildings, multiple services and feeders may be installed to supply power to different floors or different buildings; in this case, it will be necessary to perform optional load calculation for each feeder.
Generally, if the number of units on the feeder is not the same as the number on the service, it will be necessary to perform more than one load calculation.


Important!!!
If the total service load for a multifamily dwelling is known, and there is a requirement to supply the building by multiple feeders, do not just divide the service load calculation by the number of feeders. Follow rule#6.



Third: Two Family Dwelling (That are Supplied By a Single Feeder)


Rule#1: service and feeder Calculation for Two family dwelling

In accordance with 220.85, where two dwelling units are supplied by a single feeder and the calculated load as per NEC standard method exceeds that for three identical units calculated in accordance with NEC optional method, the lesser of the two loads shall be permitted.


Important!!!
The optional method load calculation for two dwelling units involves using the optional method load calculation procedures for multifamily dwellings, but not for two dwelling units. Perform the load calculation procedures in 220.84, but calculate these two dwelling units as if there were three identical units.


Important!!!
In accordance with 220.85, it is necessary to calculate by both methods and then select the lesser of the two loads.


Important!!!
Performing the optional method load calculation for two dwelling units without performing the standard method load calculation is permissible, but the result could be larger than the standard method load calculation.



Forth: Existing Dwelling Unit


Important!!!
The purpose of doing service and feeder calculations for an existing dwelling unit is to determine if the existing service or feeder is of sufficient capacity to serve a required additional loads or not.


Rule#1: Conditions Of adding new loads to An Existing Dwelling Unit

As per NEC section 220.87, Additional loads may be connected to existing services and feeders under the following conditions:
  1. The maximum demand kVA data for a minimum 1-year period (or the 30-day alternative method from the exception) is available.
  2. The maximum demand at 125 percent plus the new load does not exceed the ampacity of the feeder or rating of the service.
  3. The feeder has overcurrent protection in accordance with 240.4, and the service has overload protection in accordance with 230.90.



Important!!!
For condition#1 in Rule#1 above, If the maximum demand data for a 1-year period is not available, the calculated load shall be permitted to be based on the maximum demand (measure of average power demand over a 15-minute period) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. The recording shall reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include by measurement or calculation the larger of the heating or cooling equipment load, and other loads that may be periodic in nature due to seasonal or similar conditions.


Important!!!
For condition#2 in Rule#1 above, apply the NEC standard Calculation method to get the total load as follows:

Total Load = Existing Load Value + New Load

Where:
Existing Load Value = Max demand Value for a 1-year period from 220.87(1) x 125%
New Load = Continuous loads x 125 % +  Non-continuous loads x 100 %


Important!!!
If condition#2 in Rule#1 above is not verified, you need to increase ampacity of the feeder and/or rating of the service to be able to add new loads to an existing dwelling unit.


Rule#2: Conditions Of Application Of NEC Optional Calculation Method For An Existing Dwelling Unit

As per NEC section 220.83, The NEC optional calculation method for an existing dwelling can only be used if the dwelling is supplied by a single-phase service. The service can be fed from a 120/240-volt (V) or 208Y/120V system but must be a 3-wire system.


Important!!!
For existing dwelling unit, it shall be permissible to calculate the total load by using the NEC optional calculation method in the following two cases:
Case#1: Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is Not to Be Installed. 
Case#2: Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is to Be Installed. 


Case#1: Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is Not to Be Installed

 
Rule#3: Calculation of electrical load as per NEC Optional Calculation Method For An Existing Dwelling Unit – Case (A)

In this case, the calculation is almost identical to the calculation method in 220.82(B) for single family dwelling. The only difference is the amount of load that is rated at 100 percent, in 220.82(B) for single family dwelling it was 10 KVA but in 220.83 for existing dwelling it will be 8 KVA.

The following percentages shall be used for existing and additional new loads:

Load (kVA)
Percent of Load
First 8 kVA of load at
100
Remainder of load at
40


Important!!!
The NEC optional calculation method for an existing dwelling shall Include all of the existing loads and the new loads.


Case#2: Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is to Be Installed


Rule#4: Calculation of electrical load as per NEC Optional Calculation Method For An Existing Dwelling Unit – Case (B)

In this case, the calculation is identical to that of case (A), But In this case, there are additional loads for of air-conditioning or space-heating, so the following will be applied for these loads:

  1. The larger connected load of air-conditioning or space-heating, but not both, shall be used.
  2. The following percentages shall be used for existing and additional new loads:

  • Air-conditioning equipment at  100%
  • Central electric space heating at %100
  • Less than four separately controlled space heating units at 100%



Important!!!
The percentages in below image shall be used for existing and additional new loads.




Important!!!
The NEC optional calculation method for an existing dwelling shall Include all of the existing loads and the new loads.



For more information and solved examples, you need to review the following articles:


In the next article, we will explain in details the NEC Rules for HVAC System branch circuit load Calculations. So, please keep following.

The previous and related articles are listed in below table:
Subject Of Previous Article
Article
Summary of heating and cooling systems,
Parts Consuming Power in HVAC Systems,
Types of motors used in HVAC Systems,
Types of pumps used in HVAC/refrigeration,
Parts consuming power as per used unit/system,
Motor Nameplate for Air Conditioner Motor Applications.


The Common Types Of Motors Used In HVAC Industry
First: the Common Types of Motors Used in HVAC Industry to drive Compressors

1- Single-Phase Hermetic Motors:

1.1 Split Phase (SP),
1.2 Capacitor-Start, Induction-Run (CSIR),
1.3 Capacitor-Start, Capacitor-Run (CSCR),
1.4 Permanent Split Capacitor (PSC),
2- Poly-Phase Hermetic Motors.
Second: the Common Types of Motors Used in HVAC Industry to drive Fans
1- Shaded-Pole Motors
HVAC System Units And Ratings
Energy Conversions In Air Conditioning / Refrigeration Systems

HVAC Equipment Power Rating Calculations – Part Two





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