0100 Db 0902

Data Bulletin 0100DB0902R04/1104/2011

Replaces 0100DB0902, 07/2009

Guide to Low Voltage Transformer Protection and Selective CoordinationRetain for future use.

© 2011 Schneider Electric All Rights Reserved ™

Introduction Use this guide for selecting transformer primary circuit breaker, secondary circuit breaker and the largest branch circuit breakers for total selective coordination. The circuit breaker selections conform to the NEC Article 450 requirements for transformer protection while providing total selectivity for the circuits.

See How to Use this Guide, on page 2, for additional information.

Use the Guidelines below for tips on working through systems for selective coordination.

NOTE: It is still necessary to perform an engineering study to determine the specific requirements of each system.

Guidelines

Know your state and municipal code Understand your state and local municipality’s adoption and enforcement regarding the National Electrical Code (NEC). Part of understanding local code adoption and enforcement is understanding the threshold of selective coordination required—0.1s (TCCs) or total, for the full operating range of the protective devices (which typically uses look-up tables). This guide assumes total selective coordination is required.

Lighting Panelboard Recommendations • Do not feed lighting panelboards from other lighting panelboards unless there is a transformer in between them.

• Better levels of selective coordination are available with 225 A and larger panelboards.

• Consider using main lugs panelboards.

Bottom-Up Coordination When selectively coordinating a system, work from the loads towards the source(s), coordinate the branch panels first, then the power distribution panels, then the switchboard or switchgear.

Sizing Upstream Circuit Breakers • The upstream circuit breaker should be at least one frame size larger than the downstream circuit breaker. This may necessitate increasing the size of panelboards and feeder conductors.

• Very high levels of short circuit selective coordination may be achieved by using high ampere frame electronic trip circuit breakers with low ampere sensors and/or lower ampere rating adjustments.

Overlapping Time Current Curves (TCCs) • When there is no overlap (at any point) of the TCCs below the available short circuit current at the downstream panel, the circuit breakers are fully coordinated.

• When there is overlap in the short circuit zone, use the short circuit selective coordination tables in document number 0100DB0501, “Short Circuit Selective Coordination for Low Voltage Circuit Breakers” or

Guide to Low Voltage Transformer Protection and Selective Coordination 0100DB0902R04/11Data Bulletin 04/2011

© 2011 Schneider Electric All Rights Reserved2

document number 0100DB0604, “Selectivity Guidelines for Square D Panelboards”.

Automatic Transfer Switch (ATS) Withstand Ratings

Make sure automatic transfer switches have adequate withstand ratings.

• It may be necessary to relocate the switch, or

It may be necessary to increase the size of the switch.

Mixing of Overcurrent Protective Devices (OCPD)

• Time-Current Curves (TCC) may be used so long as the fault current does not exceed the instantaneous trip point of the upstream breaker

• OCPD from different manufacturers or different types cannot be mixed if the fault current is outside of the parameters describe above.

Arc Flash Incident Energy • Consider how selective coordination impacts the arc flash Protective Personnel Equipment (PPE) levels.

• It may be possible to reduce arc flash incident energy with Zone Selective Interlocking (ZSI).

Nesting Time-Current Curves The time-current curve of a thermal magnetic circuit breaker can sometimes be nested underneath the time-current curve of an upstream electronic trip circuit breaker.

Circuit Breaker Withstand Capacity Molded case circuit breakers (UL 489) will perform to the following levels:

• LA/LH MC (Mission Critical) circuit breaker—Selectively coordinate up to 22 kA with 120 V Circuit Breakers (most economical)

• D-Frame MC (Mission Critical) circuit breaker - Selectively coordinate up to 30kA with 120V circuit breakers

• PowerPact PG/PK and RG/RK circuit breakers—Selectively coordinate up to 65 kA with 277 V Circuit Breakers

• Masterpact NW circuit breakers—Selectively coordinate up to 100 kA with larger molded case circuit breakers

• Refer to 0100DB0501 for complete, total selectivity tables.

Field Adjustment Do not neglect to properly adjust circuit breakers in the field as all Square D low voltage circuit breakers are shipped from the factory with all but the ampere-rating switch in the lowest position.

Rarely needed, but as a last resort . . . • Change the Upstream Circuit Breaker Type to an Insulated Case Circuit Breaker or a Low Voltage Power Circuit Breaker

• Reduce the voltage if the desired level of selective coordination cannot be achieved using a 480Y/277 Vac panelboard. Consider feeding a 208Y/120 Vac panelboard through a transformer.

• Split up some of the loads and use multiple smaller transformers—avoid feeding a 100 A/208 V panel from another 208 V panel.

Insert Impedance through longer run of wire or higher impedance transformers or reactors.

How To Use This Guide 1. Determine the available fault current on primary side of transformer and choose the appropriate reference table.

a. 35 kA / 18 kA

b. 65 kA

c. 100 kA

2. Locate the transformer size on far left of chart. Options are available up to 225 kVA.

3. Based on the specification requirement for aluminum or copper transformer windings, read the available circuit breaker combinations for

0100DB0902R04/11 Guide to Low Voltage Transformer Protection and Selective Coordination04/2011 Data Bulletin

© 2011 Schneider Electric All Rights Reserved 3

primary, secondary (main), and branch circuit breakers for a selectively coordinated system.

4. Choose the circuit breaker combination that best suits your system requirements.

a. The primary circuit breaker should be selected as the device feeding the transformer, whether in a panelboard or circuit breaker enclosure.

b. The secondary circuit breaker should be selected as the main breaker in the panelboard downstream of the transformer; or optionally, could be mounted in a circuit breaker enclosure to feed a main lugs only panelboard.

c. The branch circuit breakers are listed to show the maximum size that can be applied to a full selectively coordinated system. Additionally, notation is made to present which style of QO(B) circuit breaker is required based on current rating and number of poles.

5. Reference data bulletin 0100DB0501 if additional circuit breaker selections are required upstream from the primary circuit breaker to complete the system. This data bulletin is a comprehensive resource for selectively coordinated circuit breaker combinations within the same voltage system.



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Qu

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Ref

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Gu

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for

Se

lect

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Co

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n B

ase

d o

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5 k

A A

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ault

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on

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t Circ

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.

Tab

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Sel

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ve C

oo

rdin

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Dev

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Co

mb

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s (6

5 kA

)

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150

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)(0.

95)

410

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QO

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70

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125–

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per

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412

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250

100

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No

tes:

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CC

sho

ws

over

lap

bet

wee

n th

e se

cond

ary

D-M

C a

nd t

he b

ranc

h ci

rcui

t br

eake

r ab

ove

the

inst

anta

neou

s tr

ip.

b.

Use

d S

KM

"F

unct

ion"

feat

ure

to s

how

tw

o di

ffere

nt p

rimar

y ci

rcui

t bre

aker

cho

ices

on

sam

e T

CC

.1 V

alue

s re

pres

enta

tive

of im

peda

nce

valu

es fo

r S

quar

e D

tran

sfor

mer

s.2 A

llow

prim

ary

trip

ra

ting

to in

crea

se to

250

% o

f rat

ed F

LC (

NE

C 4

50.4

); s

econ

dary

ma

in tr

ip r

atin

g ca

nnot

exc

eed

125

% o

f rat

ed F

LC.

3 S

elec

tive

with

2 p

ole

QO

(B)

thro

ugh

80A

rat

ing

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elec

tive

with

3 p

ole

QO

(B)

thro

ugh

60A

rat

ing

.5 B

ranc

h ci

rcui

t bre

aker

s w

ith a

mpe

re r

atin

gs o

f 50%

, or

less

, of t

he D

-MC

mai

n ci

rcui

t bre

ake

r ar

e se

lect

ive.

V

alue

s sh

own

in t

his

colu

mn

corr

esp

ond

to th

e m

inim

um D

-MC

trip

in c

olum

n to

the

left.

AB

CD



Qu

ick

Re

fere

nc

e G

uid

e f

or

Sel

ecti

ve C

oo

rdin

atio

n B

as

ed o

n 1

00 k

A A

vail

able

Fa

ult

Cu

rre

nt

on

48

0 V

ac P

rim

ary

Inpu

t Circ

uit B

reak

er

Mai

n C

ircui

t Bre

aker

Bra

nchC

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aker

s

Tran

sfor

mer

AB C

D

No

tes

ab

ou

t u

sin

g t

his

Qu

ick

Re

fere

nce

Gu

ide

:

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ble

is n

ot m

ea

nt

to r

epl

ace

a s

ele

ctiv

e c

oo

rdin

atio

n sy

ste

m s

tudy

. Th

e p

urp

ose

of

this

gui

de

is f

or

act

iviti

es

tha

t occ

ur b

efo

re t

he f

inal

sy

ste

m s

tud

y, s

uch

as

quo

tatio

n a

nd

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tive

de

sign

ch

ang

es,

to

incr

eas

e th

e c

ha

nce

of d

evi

ce c

omb

ina

tions

bei

ng

se

lect

ivel

y co

ord

ina

ted.

The

de

vice

com

bin

atio

ns p

rese

nte

d in

this

gu

ide

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pre

sen

t a

leve

l of

"to

tal"

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ect

ive

co

ord

ina

tion,

wh

ere

th

e d

evi

ces

we

re c

ho

sen

ba

sed

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axim

um le

t-th

rou

gh

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rre

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NE

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r tr

an

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llow

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om

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atio

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480

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20

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con

dar

y.

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pere

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600/

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QO

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per

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225T

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320

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CC

Not

es:

a. T

CC

sho

ws

ove

rlap

betw

een

the

seco

ndar

y D

-MC

and

the

bran

ch c

ircui

t bre

aker

abo

ve th

e in

stan

tane

ous

trip

.b.

Use

d S

KM

"F

unct

ion"

feat

ure

to s

how

two

diffe

ren

t prim

ary

circ

uit b

rea

ker

choi

ces

on s

am

e T

CC

.1 V

alu

es r

epre

sent

ativ

e of

impe

danc

e va

lues

for

Squ

are

D tr

ansf

orm

ers.

2 A

llow

prim

ary

trip

rat

ing

to in

crea

se to

250

% o

f rat

ed F

LC

(N

EC

450

.4);

se

cond

ary

mai

n tr

ip r

atin

g ca

nnot

exc

eed

125%

of

rate

d F

LC.

3 S

ele

ctiv

e w

ith 2

po

le Q

O(B

) th

roug

h 80

A r

atin

g.4 S

ele

ctiv

e w

ith 3

po

le Q

O(B

) th

roug

h 60

A r

atin

g.5 B

ranc

h ci

rcui

t bre

ake

rs w

ith a

mpe

re r

atin

gs o

f 50%

, or

less

, of t

he D

-MC

ma

in c

ircui

t bre

aker

are

sel

ectiv

e. V

alu

es s

how

n in

thi

s co

lum

n co

rres

pond

to th

e m

inim

um D

-MC

trip

in c

olum

n to

the

left.

AB

CD












































































































Electrical equipment should be installed, operated, serviced, and maintained only by qualified personnel. No responsibility is assumed by Schneider Electric for any consequences arising out of the use of this material.

Square D™ and Schneider Electric™ are trademarks or registered trademarks of Schneider Electric. Other trademarks used herein are the property of their respective owners.

Schneider Electric USA, Inc.3700 Sixth Street SWCedar Rapids, IA 52404 USA1-888-SquareD (1-888-778-2733)www.schneider-electric.us


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