Technical information

The size/calibration of the protection of the branch circuit must not be more

than the low est v alue between the two following:

•the size of the BCP is established by calculating the size required for the

largest motor in the assembly (by doing this the product of the sizing

current of the largest motor by a coefficient in the table, has already been

seen for the individual motor start) plus the sum of the full load currents of

the remaining motors and the nominal currents of all the other loads in the

group; or

•the size of the BCP is chosen in such a way that all the power components

do not exceed the value of the current specified in the marking as regards

the group installation and the type of protection is the type specified in

the same marking (for this purposes,the term fuse refers to a branch

circuit protection connected with fuses and the term circuit breaker refers

o an automatic cutout at reverse type).

Finally it should be noted that the group installation can only be carried out

with automatic fuses and circuit breakers of the “inverse time” type: the

o v erload r elay of T ype E cannot be used as BCP f or a g r oup installation.

EXAMPLE

A calculation is required of the size/calibration of a branch circuit protection for

the following motor group installation: a 10 hp (7.5 kW) motor and two 5 hp

(3.5 kW) 480 V motors.The rating plate current is unknown

The FLA tables are: 14 A for the large motor,7.6 for the other motors.

The branch circuit protection consists of a delayed fuse: its coefficient for the

calculation of the BCP is 175%.

The protection size must be lower than:

14 x 1.75 Avalue calculated as for individual motor start

7.6 AFLA

for a total of 29.7 A.The size of the fuse immediately below is 25 A.

In the previous calculation the restrictions imposed by the downstream

components have been ignored because generally larger sizes/calibrations are

acceptable.

If,on the other hand, the maximum protection allowed delayed fuses of 20 A, this

value would prevail over that calculated (25 A) and the fuses would have to be of

sizes no greater than the 20 A allowed.

The CEC allows a group installation in accordance with points a) and b), while it

limits the application of point c) to just machine tools and for wood working.The

CSA does not lay down specific restrictions for the group installation so the BCP

size limit is always obtained through calculation (the largest motor plus the other

loads); maximum values are imposed for the size of the branch circuit branch

circuit protection: 200 A up to 250 V, 100 A from 251 V up to 750 V.

BRANCH CIRCUIT