Today, we will
explain the NEC Rules related to HVAC System Power Sizing
Calculations.
The National
Electrical Code (NEC) And HVAC Systems – Part One
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First, we need to differentiate between the following load calculations:
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:
The below figure indicate the definition and extent of each
expression; 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?
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First:
The HVAC System Contribution in Service/Feeder Load Calculations
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The NEC calculate service/feeder
load by two different methods:
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First:
NEC Standard Method
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Important!!!
Cooling Fan Motor
= Blower Motor = evaporator Motor = air handler Motor. All have the same meaning.
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Important!!!
Based on
Rule#5, in case of split or packaged units:
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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.
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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.
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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.
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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.
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Important!!!
The heat pump can
work as an air conditioner in summer and as a heater in winter.
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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.
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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
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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.
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Second: NEC
Optional Calculation Method
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Rule#1: Application of NEC
Optional calculation method
NEC Optional
calculation method will be used if the following condition is verified:
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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.
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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:
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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.
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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.
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First: For
Single Dwelling Units
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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:
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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.
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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.
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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.
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Important!!!
Depending on
the design, the heating system and the air conditioning system might be
noncoincident loads.
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Important!!!
Similarly,
220.82(C) requires that only the largest of the six choices needs to be
included in the feeder or service calculation.
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Second:
Multifamily Dwelling
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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Third: Two Family
Dwelling (That are Supplied By a Single Feeder)
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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.
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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.
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Important!!!
In accordance
with 220.85, it is necessary to calculate by both methods and then select the
lesser of the two loads.
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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.
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Forth: Existing
Dwelling Unit
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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.
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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:
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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.
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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 %
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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.
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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.
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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.
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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.
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Load (kVA)
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Percent of Load
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First 8 kVA of load at
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100
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Remainder of load at
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40
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Important!!!
The NEC
optional calculation method for an existing dwelling shall Include all of the
existing loads and the new loads.
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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:
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Important!!!
The percentages
in below image shall be used for existing and additional new loads.
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Important!!!
The NEC
optional calculation method for an existing dwelling shall Include all of the
existing loads and the new loads.
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For more information and solved examples, you need to review the following articles:
- Applied NEC Rules for Heating and air conditioning loads
- Calculation of Heating and air conditioning loads for Single Family Dwelling
- Calculation of Heating and air conditioning loads for MultiFamily Dwelling
- Calculation of Heating and air conditioning loads for Two Family Dwelling
- Calculation of Heating and air conditioning loads for Existing Dwelling
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
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Article
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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.
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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
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HVAC Equipment Power Rating Calculations – Part Two
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