fuse and conductor sizing
fuse and conductor sizing
(OP)
I have a fuse and conductor sizing problem I need help with:
Problem:
MOTOR
Quantity: 32 motors
motor size: 1/3 hp
FLA : 3.6 A @230V
LRC : 14 A
connected phase to phase
SERVICE
240V THREE PHASE SERVICE.
I am trying to determine what size conductor and fuses to use for circuit protection.
My worst case scenario per phase-phase is 6 motors running while six are starting.
SOLUTION:
From FLA
11 x 3.6 + 1.25 x 3.6 = 44.1A = phase current
line current = 44.1A x SQRT(3) = 77A
I therefore can use # 6 wire and 100A NON fuse
but with 6 motors starting at the same time and six running I have
6 x 14 + 6 x 3.6 = 105.6 A = phase current
line current = 105.6A x sqrt(3) = 183A
which is more than what #6 wire and 100A NON fuse could withstand.
Any suggestions?
Problem:
MOTOR
Quantity: 32 motors
motor size: 1/3 hp
FLA : 3.6 A @230V
LRC : 14 A
connected phase to phase
SERVICE
240V THREE PHASE SERVICE.
I am trying to determine what size conductor and fuses to use for circuit protection.
My worst case scenario per phase-phase is 6 motors running while six are starting.
SOLUTION:
From FLA
11 x 3.6 + 1.25 x 3.6 = 44.1A = phase current
line current = 44.1A x SQRT(3) = 77A
I therefore can use # 6 wire and 100A NON fuse
but with 6 motors starting at the same time and six running I have
6 x 14 + 6 x 3.6 = 105.6 A = phase current
line current = 105.6A x sqrt(3) = 183A
which is more than what #6 wire and 100A NON fuse could withstand.
Any suggestions?






RE: fuse and conductor sizing
Rafiq Bulsara
http://www.srengineersct.com
RE: fuse and conductor sizing
1. Amps for conductors - use NEC to find appropriate conductor for it.
2. C.B. should be sized by Total Amps (or a fuse). do not forget to multiply it by around 2,5 for fuse; use HACR CB for motors, size approx 1,25x.
Good luck with your project.
RE: fuse and conductor sizing
RE: fuse and conductor sizing
RE: fuse and conductor sizing
Conductor size is 1.25 x FLA of largest motor plus the FLA of the other motors.
RE: fuse and conductor sizing
RE: fuse and conductor sizing
Can you confirm if these are 3-phase or single phase motors?
FYI, when referring to conductors, the phase and line currents are the same.
RE: fuse and conductor sizing
connected to the AC supply as follows:
12 connected bw A-B phase
10 connected bw A-C
10 connected bw B-C
AC Source is a delta delta transformer bank
RE: fuse and conductor sizing
As I mentioned in my OP phase current is 45A and line current is 77A.
RE: fuse and conductor sizing
RE: fuse and conductor sizing
RE: fuse and conductor sizing
With regards to wire, the phase current is typically referred to as the current that flows in each phase of a 3-phase wire or cable. The current flowing in the cable is the current in the cable, there is no internal wye or delta connection in a cable.
You have a somewhat unique case. You have a 3-phase feeder cable which is split to feed a bunch of single-phase loads and those loads conbine to create a delta connected load. Your conversion from motor load per "leg" to line current makes sense now that you have specified this.
Your first calculation is a little conservative but basically correct. If you wanted to be closer the rated currents are phase A = 70A, phase B = 70A and phase C = 62A. You can then add 25% of a single motor FLA to each current to get the minimum NEC required. Installing a 100A feeder circuit seems to be perfectly reasonable for this system.
RE: fuse and conductor sizing