Sump Pump Diagram
What fuses would I need for a 7.5kW motor with a 3 phase 415V supply?
I’ve been given a project to install a motor to drive a sump pump to pump water out of a cable tunnel. My employers have specified that I need a 7.5kW motor and have a 3 phase 415V supply which will be fed from a switchboard 300m away. What size fuses would I need for this and what type of cable would I need for this distance and this supply? Also I am required to draw a circuit diagram for the motor control circuit, the motor is required to start automatically as well as manually. What other protection would I need for a motor control circuit of this nature.
Thanks, if you have any information that I require it would be much appreciated.
step-1 find the full load current
7.5kw is about 10hp
up to 5hp — 0.84pf
5-20hp ——- 0.86pf
I = 1000 * kw / (1.732 * pf * E)
I = 1000 * 7.5 / (1.732 * 0.86 * 415) = 12.13 amp
if you have the full load current shown in the nameplate of motor, skip step-1 calculation
because the name plate rating is a tested value
step-2 nec table 40
full load — overload protection
current—– fixed — adjustable — fuse — breaker
12a ——— 15a —- 15a ————- 40a — 30a
13a ——— 20a —- 16.25a ——– 40a — 40a
normally before the 40a branch circuit fuse protection you need a magnetic starter. SQ.D magnetic starter pre-wired with thermal overload protection for the motor, the magnetic starter located near the motor. the magnetic starter start by a push button switch and latched in, it can also be automatically started by water level sensing switch. you may wants to connect some indicator lights. if the calculated full load current is 12.13a, use 13a circuit
step-3 voltage drop
v = (21.4)(L) / CM
L = distance one way in ft = 300m * 3.28 = 984ft
12awg —- 6 530CM
10awg — 10 380 CM
for 12 awg
v = (21.4)(984) / 6 530 = 3.22%
for 10awg
v = (21.4)(984) / 10 380 = 2.03%
the nec allowable voltage drop is 5%
so you can use 3c-12 awg 1000v or 600v cable, if the motor is subjected to water splash, you have to use insulation suitable for such application, in the termination of motor, don’t used tape, use heat shrinkable tubing, 3M or raychem has it
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