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Ty
pe
s o
f O
ve
rcu
rre
nt
&
Ca
lcu
lati
on
s
NE
C®
Art
icle
10
0 –
De
fin
itio
ns
Overcurrent
An
ycu
rre
nt
ine
xce
sso
fth
era
ted
curr
en
to
f
eq
uip
me
nt
or
the
am
pa
city
of
aco
nd
uct
or.
It
ma
yre
sult
fro
mo
ve
rlo
ad
,sh
ort
circ
uit
,o
r
gro
un
dfa
ult
.
NE
C®
Art
icle
10
0 –
De
fin
itio
ns
Overload
Op
era
tio
no
fe
qu
ipm
en
tin
exc
ess
of
no
rma
l,
full
-lo
ad
rati
ng
,o
ro
fa
con
du
cto
rin
exc
ess
of
rate
da
mp
aci
tyth
at,
wh
en
itp
ers
ists
for
a
suff
icie
nt
len
gth
of
tim
e,
wo
uld
cau
sed
am
ag
e
or
da
ng
ero
us
ove
rhe
ati
ng
.A
fau
lt,
such
as
a
sho
rtci
rcu
ito
rg
rou
nd
fau
lt,
isn
ot
an
ove
rlo
ad
.
No
rma
l Lo
ad
Source
RR
RR
R
R
Load
R
I L=
VS
/R
T
VS
I L I L
Ove
rlo
ad
Source
Wit
hin
th
e n
orm
al
path
RR
RR
R
R
Load
RV
S
I OL=
VS
/R
T
Sh
ort
Cir
cuit
Source
Ou
tsid
e th
e n
orm
al
path
RR
RR
R
R
I SC=
VS
/R
T
VS
Load
R
Ou
tof
the
circ
uit
Ty
pe
s o
f Fa
ult
s
•B
olt
ed
Fa
ult
s
•A
rcin
g F
au
lts
Bo
lte
dS
ho
rt
Cir
cu
it
AB
Arc
ing
Fa
ult
AB
Curr
ent
Thru
Air
Bo
lte
d F
au
lts
–V
ari
ou
s Ty
pe
s A B C Gro
und
N
480Y
/277 V
3φ
/ 4W
Soli
d G
rd
Bo
lte
d F
au
lts
–T
hre
e P
ha
se
A B C Gro
und
N
480Y
/277 V
3φ
/ 4W
Soli
d G
rd
Typic
ally
consi
der
ed t
he
“wors
t
case
” or
hig
hes
t m
agnit
ude
Bo
lte
d F
au
lts
–Li
ne
to
Lin
e A B C Gro
und
N
480Y
/277 V
3φ
/ 4W
Soli
d G
rd
87 %
of
the
thre
e phas
e
bolt
ed f
ault
Bo
lte
d F
au
lts
–Li
ne
to
Gro
un
d
A B C Gro
und
N
480Y
/277 V
3φ
/ 4W
Soli
d G
rd
Typic
ally
much
low
er t
han
3φ
fault
, but
can b
e >
3φ
fault
nea
r X
fmr
term
inal
s
AB
Arc
ing
Fau
lts
–M
any
Var
iab
les
Eff
ect
Cu
rren
t &
Wh
eth
er S
ust
ain
able
Sy
stem
Vo
ltag
e
Gap
sp
acin
g
Av
aila
ble
3φ
Sh
ort
Cir
cuit
Am
ps
Am
ou
nt
of
Co
pp
er V
apo
rize
d
Deg
ree
of
Co
nta
inm
ent
Co
nfi
gu
rati
on
of
Eq
uip
men
t
Ty
pic
ally
do
es n
ot
sust
ain
on
20
8Y
/12
0V
Arc
ing
Fa
ult
s –
Pro
gre
ssio
n
A B C Gro
und
N
480Y
/277 V
3�
/ 4W
Soli
d G
rd
Arc
ing
Fa
ult
s –
Th
ree
Ph
ase
A B C Gro
und
N
480Y
/277 V
3φ
/ 4W
Soli
d G
rd
Can
var
y w
idel
y p
oss
ibly
up t
o
89%
of
3φ
bolt
ed f
ault
Arc
ing
Fa
ult
s –
Lin
e t
o L
ine
A B C Gro
und
N
480Y
/277 V
3φ
/ 4W
Soli
d G
rd
Can
var
y w
idel
y p
oss
ibly
up t
o
74%
of
3φ
bolt
ed f
ault
Arc
ing
Fa
ult
s –
Lin
e t
o G
rou
nd
A B C Gro
und
N
480Y
/277 V
3φ
/ 4W
Soli
d G
rd
Can
var
y w
idel
y
Arc
ing
Fa
ult
s –
Su
sta
ina
bil
ity
A B C Gro
und
N
480Y
/277 V
3φ
/ 4W
Soli
d G
rd
Rule
of
thum
b:
Arc
ing f
ault
s w
ill
typic
ally
not
sust
ain
at l
ess
than
38%
of
3φ
bolt
ed f
ault
Ho
w d
o y
ou
kn
ow
wh
at
the
sh
ort
-cir
cuit
cu
rre
nt
is t
hro
ug
ho
ut
a s
yst
em
?
an
d
Wh
at
are
so
me
ty
pic
al
va
lue
s
MM
MS
B
Short
Cir
cuit
Curr
ents
Var
y D
epen
din
g o
n M
any
Fac
tors
Tra
nsf
orm
er S
ize
& %
Z
Volt
age
Conduct
or
Siz
e &
Len
gth
MM
MS
B
Short
Cir
cuit
Curr
ents
Var
y D
epen
din
g o
n M
any
Fac
tors
60,0
00 A
27,0
00 A
40,0
00 A
60,0
00 A
18,0
00 A
9,0
00 A
500 K
VA
480/2
77V
1
5 %
Z
500 K
VA
480/2
77V
1A
2 %
Z
1500 K
VA
480/2
77V
2
5%
Z
1500 K
VA
480/2
77V
2A
2 %
Z
Sh
ort
Cir
cu
it C
urr
en
t E
xam
ple
s #
1
1500 K
VA
208/1
20V
3
5%
Z
1500 K
VA
208/1
20V
3A
2 %
Z
Ho
w t
o C
alc
ula
te T
ran
sfo
rme
r
Se
con
da
ry (a
ssu
min
g i
nfi
nit
e p
rim
ary
)
I sca
= (
Xfm
r F
LA)
x 1
00
/%
Z
(in
cre
ase
re
sult
by
10
% d
ue
to
UL
tole
ran
ce f
or
tra
nsf
orm
er
imp
ed
an
ces)
500 K
VA
480/2
77V
1
5 %
Z
500 K
VA
480/2
77V
1A
2 %
Z
13,2
22 A
33,0
55 A
1500 K
VA
480/2
77V
2
5%
Z
1500 K
VA
480/2
77V
2A
2 %
Z
39,6
66 A
99,1
65 A
Sh
ort
Cir
cu
it C
urr
en
t E
xam
ple
#1 A
nsw
ers
1500 K
VA
208/1
20V
3
5%
Z
1500 K
VA
208/1
20V
3A
2 %
Z
91,6
08 A
229,0
20 A
208/1
20V
4
4A
6
6A
480/2
77V
5
5A
480/2
77V
40,0
00 A
.40,0
00 A
.40,0
00 A
.
50 f
t # 1
50 f
t 250 k
cm
Sh
ort
Cir
cu
it C
urr
en
t E
xam
ple
#2
Ca
lcu
lati
ng
Sh
ort
Cir
cuit
Cu
rre
nts
Uti
lize
Po
int-
to-P
oin
t M
eth
od
Ste
ps
4,
5 &
6 f
or
3φ
Fau
lts
Sh
ort
Cir
cu
it C
urr
en
t E
xam
ple
#2 A
nsw
ers
208/1
20V
4
4A
40,0
00 A
.
50 f
t # 1
12,3
67 A
.
480/2
77V
5
5A
40,0
00 A
.
20,3
22 A
.
6
6A
480/2
77V
40,0
00 A
.
50 f
t 250 k
cm
28,8
18 A
.
Su
mm
ary
•O
ve
rcu
rre
nt
–o
ve
rlo
ad
s, b
olt
ed
fa
ult
s, &
arc
ing
fau
lts
•Fa
ult
cu
rre
nts
ca
n b
e d
ete
rmin
ed
th
rou
gh
ou
t
dis
trib
uti
on
syst
em
•S
ho
rt c
ircu
it c
urr
en
ts n
ee
de
d t
o a
sse
ss I
.R.,
Co
mp
on
en
t P
ro,
Co
ord
ina
tio
n,
Arc
Fla
sh
Ha
zard
s &
OC
PD
Se
lect
ion