Dwelling Units - NEC Optional Method Calculation
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8/17/2019 Dwelling Units - NEC Optional Method Calculation
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NEC Optional calculation method is applicable o
FIRST: SINGLE-FAMILY DWELLING
!" per NEC ##$%#&!'NEC Optional calculation meth
+- Dwelling unit must be ser(ed by a single +#$#$-
#- 0he ser(ice-entrance or feeder conductors ha(e a
Note* 1f the ser(ice-entrance ampacity calculate
Step-1 : Calculating general lighting an gener
)r"ceure
Step-*: Calculating S&all-appliance +ranch circ
)r"ceure
!s per ##$%#&2', for dwelling units, minimumgeneral lighting and general-use receptaclesload is / 3!ft#
Calculate the 4oor area for each 4oor of DwellingUnit in ft#
Calculate the re5uired number of small-appliancebranch ciruits in the dwelling unit
!s per NEC section ##$%#&2'', each #-wiresmall-appliance branch circuit load is calculatedat no less than +,6$$ (olt-amperes
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Step-: Calculating Launr' +ranch ciruit l"a
)r"ceure
Step-.: Fa!tene in )lace Appliance! L"a
7ilo(olt-amperes &83!' shall be considered e5ui(
0he :astened-in-place !ppliances load, regardles
0he nameplate of the :astened-in-place !pplian
/ater heater
Re%rigerat"r
Free0er
Calculate the re5uired number of laundry branchciruits in the dwelling unit
!s per NEC ##$%#&2'', each #-wire laundry
branch circuit is calculated at no less than+,6$$ (olt-amperes
1f 0he fastened-in-place appliances ;oad includinfastened-in-place appliances, add the nameplat
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i!h/a!her
i!p"!al
Range h""
&icr"/ae
&ini Re%rigerat"r
in!t h"t
ir"ning center
/ine Clr
A &"re Appliance!
A &"re Appliance!
A &"re Appliance!
T"ta
Step-4: Cl"the! Dr'er! L"a
! clothes dryer is not a re5uirement for a load ca
0he dryer load is not sub
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7ilo(olt-amperes &83!' shall be considered e5ui(
0he >ousehold coo8ing appliances load is not su
0he nameplate load of the >ousehold coo8ing a
=ange
9all-mounted o(en
Counter-mounted Coo8ing units
Step-9: M"t"r!
9hen calculating the feeder or ser(ice load of a
Do not include heating and air conditioning e5ui
1nclude only permanently connected motors that
Motor?+
Motor?#
Motor?/
Motor?
Step-: De&an "% General L"a
!s per NEC section ##$%#&2'&', add the namep
Bermanently connected motors must be added tof the motors may ha(e already been added to t
:or example, a 8itchen waste disposer could bealready been added to the load calculation as fa
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Calculate the demand o
Calculate the de
Step-:6eating an air c"niti"ning l"a! #F;
!s per NEC section ##$%# &C', for >eating and
a- +$$ percent of the nameplate rating&s' of the
b- +$$ percent of the nameplate rating&s' of the
d- 6 percent of the nameplate rating&s' of elect
e- $ percent of the nameplate rating&s' of elect
f- +$$ percent of the nameplate ratings of electri
c- +$$ percent of the nameplate rating&s' of thepre(ented from operating at the same time as t
Ca!e
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)ut the Large!t L"
T;TAL DEMAND L;AD F
SEC;ND: SINGLE-FAMILY DWELLING
1f the total ser(ice load for a multifamily dwellin
+- 1f No dwelling unit is supplied by more than o
#- Each dwelling unit is e5uipped with electric c
/- Each dwelling unit is e5uipped with either ele
:or multifamily dwellings without electric coo8in+- Calculate the load of multifamily dwellings as
#- Calculate the load of multifamily dwellings as
6 percent of the nameplate rating&s' of electricspace heating
Ca!e
$ percent of the nameplate rating&s' of electricspace heating
Ca!e
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/- "elect the lowest load
Step-:6eating an air c"niti"ning l"a! #F"r
!s per NEC section ##$%, for Multifamily Dwelli
9ith the optional method multifamily dwelling lo
9ith the optional method multifamily dwelling lo
!ir Conditioni
"pace >eati
)ut the Large!t L
Step-@: 6"u!e L"a! #%"r Multi-Fa&il' D/elling
7ilo(olt-amperes &83!' shall be considered e5ui(
!s per ##$%&2', >ouse loads shall be calculate
!pplying the 0able ##$% demand factor to hou
T;TAL L;AD F;E
Step-1B: Appl'ing the Ta+le **B>. e&an %a
write the number of dwellin
from table ##$% , de
9ith a heat pump, the compressor &and accompsystem load K the maximum amount of heat tha
>eat pump ;oad A he air conditioning system loconditioner
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T;TAL DEMAND L;AD
T6IRD: TW;-FAMILY DWELLING
the total load of two-family dwelling as per NEC "tan
the total load of 0hree-family dwelling as per NEC Op
T;TAL DEMAND L;AD
F;RT6: EISTING DWELLING
0he purpose of doing ser(ice and feeder calcula
!s per NEC section ##$%, !dditional loads may
+- 0he maximum demand 83! data for a minimu
#- 0he maximum demand at +#6 percent plus th
/- 0he feeder has o(ercurrent protection in accoNO0E"*
:or condition?# in =ule?+ abo(e, apply the NEC
9here* Existing ;oad 3alue A Max demand 3alu
1f condition?# in =ule?+ abo(e is not (eri)ed, y
1n accordance with ##$%6, where two dwelling uwith NEC optional method, the lesser of the two
Berforming the optional method load calculationload calculation
:or condition?+ in =ule?+ abo(e, 1f the maximupower demand o(er a +6-minute period' continuser(ice, based on the initial loading at the startshall include by measurement or calculation the
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Ca!e#A( : Where Aiti"nal Air-C"niti"nin
step?+* Existing ;oad 3alue in 3! as per NEC "tandr
if data are not a(ailable to ma8e calculation of this s
step?/* 0otal ;oad A Existing ;oad 3alue K New ;oa
step?* !pply ##$%/&!' demand factor for total ;oa
Calculate the demand
Calculate the de
Ca!e #( : Where Aiti"nal Air-C"niti"ning E
0he larger connected load of air-conditioning or spac
0he following percentages shall be used for exis
a' !ir-conditioning e5uipment at +$$
b' Central electric space heating at +$$
c' ;ess than four separately controlled space he
step?+* Existing ;oad 3alue in 3! as per NEC "tandr
if data are not a(ailable to ma8e calculation of this s
the calculation of electrical load as per NEC Opti
only dierence is the amount of load that is rate
step?+-optional* Existing ;oad 3alue in 3! as per NE##$%&+' x +#6 A
step?#* Calculate New ;oad as per NEC "tandard Me
A
additional loads for of air-conditioning or space-h
step?+-optional* Existing ;oad 3alue in 3! as per NE##$%&+' x +#6 A
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step?#* select the larger load from air-conditioning l
a' !ir-conditio
b' Central elect
c' ;ess than four separately
the larg
step?* 0otal ;oad A Existing ;oad 3alue K New ;oa
step?6* !pply ##$%/&!' demand factor for total ;oa
Calculate the demand
Calculate the de
step?/* Calculate New ;oad as per NEC "tandard Me A
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welling Units - NEC Optiona
ly for a single dwelling unit, an existing dwelling unit, a multif
d will be used if the following condition is (eri)ed*
(olt or #$%.+#$-(olt set of /-wire ser(ice or feeder conductors
ampacity of at least +$$ amperes
by the optional method is less than +$$!, re-calculate with u
l receptacle! l"a! #e$cept %"r !&all-appliance an Launr
N"te
uit!, l"a
N"te
Under any conditions, for dwelling units, donPt use (alue lessthan / 3!ft#, there are no exceptions 9hile the designercan choose a higher (alue based on the existing designconditions
0he 4oor area for each 4oor shall be calculated from theoutside dimensions of the dwelling unit
0he calculated 4oor area shall not include open porches,
garages, or unused or un)nished spaces not adaptable forfuture use &li8e some attics, cellars, and crawl spaces'
!s per NEC #+$+&C'&+', 1n each dwelling unit, two or more#$-ampere small-appliance branch circuits must be pro(ided
the designer assign the number of small-appliance branchciruits based on the exisiting condition &space dimension ,number of re5uired small appliances, etc'
DonQt exceed the permissible loading of a +#$ 3, #$- amperebranch circuit which is #$$ 3!
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N"te
alent to 8ilowatts &89'
s of the number, will not be derated as re5uired per NEC secti
es that are fastened in place, permanently connected or locat
Nu&+er "% appliance!
!s per NEC #+$++&C'', 1n each dwelling unit, !t least one#$-ampere branch circuit shall be pro(ided
g water heater is gi(en, do not include water heat load to therating of the water heater to the general loads co(ered in ##
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l"a in #2A( 3
lculation, "8ip this step if there is no clothes dryer
and factor as re5uired by table ##$6 for NEC "tandard calc
e(en if it is less than 6$$$ watts
alent to 8ilowatts &89'
N"te
T"tal l"a "% cl"the! r'er! 3
! combination of clothes washer and clothes dryer will behandled in calculations as it is a clothes dryer
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alent to 8ilowatts &89'
b
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f 0he :irst +$,$$$ 3! or ;ess at +$$
and of =eminder, if any, at $
De&an "% General L"a in #2A( 3
Single -Fa&il' /elling "nl'(
ir-Conditioning ;oad, 0he largest of the following six selection
air conditioning and cooling
heat pump when the heat pump is used without any supplem
ic space heating if less than four separately controlled units
ic space heating if four or more separately controlled units
c thermal storage &E0"' and other heating systems where the
Nameplate=ating
heat pump compressor and 6 percent of the supplemental ele supplementary heat, it does not need to be added to the su
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%r"& that "% !i$ ca!e! a+"e
;E SINGLE- FAMILY DWELLING 3
is 8nown, and there is a re5uirement to supply the building b
e feeder
o8ing e5uipment
tric space heating or air conditioning, or both
range &using gas coo8ing ranges or other and may use electper NEC standard method
per NEC optional method with adding electric coo8ing &based
s, multiple ser(ices and feeders may be installed to supply p
not the same as the number on the ser(ice, it will be necessa
method, for a multifamily dwelling, 0able ##$% GOptional Ca
ded all of the conditions for using table ##$% listed abo(e a
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L"a %r"& Step-1 t" Step-9 in #2A( 3
Multi-Fa&il' /elling "nl'(
ng, the demand factors of 0able ##$% shall be applied to th
ad calculation, the air conditioning load is calculated at +$$
ad calculation, the load for space heating units must be adde
g ;oad Nameplate =ating A
g Unit Nameplate =ating A
a %r"& the #( L"a! a+"e
"nl'(
alent to 8ilowatts &89'
in accordance with NEC "tandard method
e loads of multifamily dwellings is not permitted >ouse loads
6"u!e L"a in #2A( 3
LTI- FAMILY DWELLING 3
t"r %"r Multi-%a&il' /elling
units in a multifamily dwelling building A
termine the Demand :actor (alue A
nying motors' and some or all of the electric heat can be ent can be on while the air conditioner compressor is energiJed
d K the maximum amount of heat that can be on while the aircompressor is energiJed
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;E MLTI- FAMILY DWELLING 3
ard Method A
tional Method for Multifamily dwelling A
F;E TW;- FAMILY DWELLING 3
ions for an existing dwelling unit is to determine if the existin
be connected to existing ser(ices and feeders under the follo
m +-year period &or the /$-day alternati(e method from the e
e new load does not exceed the ampacity of the feeder or rati
dance with #$, and the ser(ice has o(erload protection in
standard Calculation method to get the total load as follows*
for a +-year period from ##$%&+' x +#6 , New ;oad A Con
u need to increase ampacity of the feeder andor rating of th
nits are supplied by a single feeder and the calculated load aoads shall be permitted
for two dwelling units without performing the standard metho
demand data for a +-year period is not a(ailable, the calculously recorded o(er a minimum /$-day period using a recordif the recording 0he recording shall re4ect the maximum demlarger of the heating or cooling e5uipment load, and other loa
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E=uip&ent "r Electric Space-6eating E=uip&ent I! N"
method A
ep, then apply step?+-optional
A
f 0he :irst %,$$$ 3! or ;ess at +$$
and of =eminder, if any, at $
De&an T"tal L"a in %"r E$i!ting D/elling nit - Ca!e #A(
uip&ent "r Electric Space-6eating E=uip&ent I! t" e In!ta
-heating, but not both, shall be used
ing and additional new loads*
ting units at +$$
method A
ep, then apply step?+-optional
nal Calculation Method :or !n Existing Dwelling Unit Case &
at +$$ percent, in ##$%#&2' for single family dwelling it was
"tandrd method A Max demand 3alue for a +-year period from
thod A Continuous loads x +#6 K Non-continuous loads x +$$
eating, so the following will be applied for these loads*
"tandrd method A Max demand 3alue for a +-year period from
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ad , central electric space heating and less than four separately co
ing e5uipment at +$$ A
ic space heating at +$$ A
ontrolled space heating units at +$$ A
r load from a, b, c A
A
f 0he :irst %,$$$ 3! or ;ess at +$$
and of =eminder, if any, at $
De&an T"tal L"a in %"r E$i!ting D/elling nit - Ca!e #(
thod A Continuous loads x +#6 K Non-continuous loads x +$$
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l Method Calculation
amily dwelling, two dwelling units, a school, an existing installation and
sing the standard method
Receptacle!(
Calculati"n
:loor area in ft# A $
General lighting an general receptacle! l"a! 3 $
Calculati"n
Number of "mall-appliance branch circuits in the dwelling unit A $
0he general lighting load is calculated by multiplying the 4oor area &inunit by / 3!ft#
0he "mall-appliance branch circuitsP load, for dwelling units, is calculatenumber of "mall-appliance branch circuits by +,6$$ 3!
!s per #+$6#&2'&+'Exception#, !n ind(idual branch ciruit is permitted for =eat +,6$$ 3! 1f you will apply this rule,choose Number of =efergiator Circuits fr
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S&all-appliance +ranch circuit!, l"a 3 $
Calculati"n
Number of ;aundry branch circuits in the dwelling unit A $
Launr' +ranch circuit!, l"a 3 $
on ##$6/
ed to be on a speci)c circuit will be used &E(en 1f less than 6$$$ 9att3
Na&eplate Rating "% appliance!
0he ;aundry branch circuitsP load, for dwelling units, is calculated by mu;aundry branch circuits by +,6$$ 3!
! combination of clothes washer and clothes dryer will be handled in caclothes dryer, is there a combinationR "elect answer from :##
general load as per ##$%#&2'&/'d 2ut if the water heater was not incl%#&2'
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ulation method
Calculati"n
9rite the Nameplate =ating of Cothes dryer $
Number of Clothes Dryers A $
$
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66 for NEC "tandard calculation method
used
Nameplate =ating
oad current of permanently connected motors by +#6 percent
connected motors that are supplied by house power
Nameplate =ating
ntly connected motors to the general loads co(ered in ##$%#&2' 1nclu
calculated by the NEC optional method !lthough motors are listed sep
pliance or as a permanently connected motor 1t is not necessary to ad
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demand of 0he :irst /,$$$ 3! or ;ess A
demand of & +#$,$$$ 3! - /,$$$ 3!', if any A
&load in 83!' shall be included*
ntal electric heating
usual load is expected to be continuous at the full nameplate (alue
supplemental electric heat nameplate rating &case?/'
ectric heating for central electric space-heating systems 1f the heat pupplementary heat for the total central space heating load
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$
$
y multiple feeders, do not
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larger of the air-conditioning load or the )xed electric space-heating lo
of the nameplate rating
to the calculation at the nameplate rating
Nameplate =ating
$
$
$
must be calculated as per NEC standard method
$
rgiJed at the same time "o, 0he load contribution of a heat pump A th
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$
3!
3!
$
ser(ice or feeder is of suLcient capacity to ser(e a re5uired additional
wing conditions*
ception' is a(ailable
ng of the ser(ice
ccordance with #/$$
otal ;oad A Existing ;oad 3alue K New ;oad
tinuous loads x +#6 K Non-continuous loads x +$$
ser(ice to be able to add new loads to an existing dwelling unit
per NEC standard method exceeds that for three identical units calcula
load calculation is permissible, but the result could be larger than the
ted load shall be permitted to be based on the maximum demand &meg ammeter or power meter connected to the highest loaded phase of t
and of the feeder or ser(ice by being ta8en when the building or spaceds that may be periodic in nature due to seasonal or similar conditions
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t t" e In!talle>
3!
3!
3!
3!
demand of 0he :irst %,$$$ 3! or ;ess A
demand of & +#$,$$$ 3! - /,$$$ 3!', if any A
in #2A( 3
lle>
3!
3!
' is almost identical to the calculation method in ##$%#&2' for single f
+$ 73! but in ##$%/ for existing dwelling it will be % 73!
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trolled space heating units
3!
3!
3!
3!
3!
$3!
demand of 0he :irst %,$$$ 3! or ;ess A
demand of & +#$,$$$ 3! - /,$$$ 3!', if any A
in #2A( 3
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a new restaurant
ft#
3!
$ $
+
Ciruit #
t#' of a dwelling
by multiplying
rigeration e5uipmentom :+/
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3! /
N .
Ciruit $ N
3!
'
$
$
$
ltiplying number of
lculations as it is a
ded with the
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$
$
$
$
$
$
$
$
$
$
$
$
3!
3!
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$
$
$
$
$
$
$
$
$
$
$
$
e all motors that
arately, some or all
motors that ha(e
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$
$
$
$
$
$
p compressor is
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$
$
$
3!
2A
number of feeders
sary to perform
itsI will be used if
ed by using
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B
ad
3!
$
2A
air conditioning
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2A
2A
loads or not
ted in accordance
standard method
sure of a(eragehe feeder oris occupied and
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$
$
$
mily dwelling 0he
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$
$
$