fuse and conductor sizing 3

[NAZ55]

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?

 

[rbulsara]

Sounds like American voltages and terminology. Look up NEC 430 for feeder sizing for multiple motors.

Rafiq Bulsara

http://www.srengineersct.com

 

[fx007]

You are welcome to use my calculation sheet. Please PM me your email so I can send it. I tried directly from here with 0 success. It's a PDF.
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.

 

[fx007]

OK, it was just a firewall. Here you go. Regards

 

[NAZ55]

fx007 your sheet is very basic. It doesn't account for phase to line current conversion. Also I am not sure why you are multiplying the largest motor with 1.75. NEC 430.24 says to multiply the largest motor by 1.25.

 

[wareagle]

The NEC allow you to size the breaker by 2.5 x FLA of the largest motor plus the sum of the FLA of the other motors.
Conductor size is 1.25 x FLA of largest motor plus the FLA of the other motors.

 

[NAZ55]

wareagle where did you get 2.5 from in NEC?

 

[LionelHutz]

The NEC rules are different if this is a piece of equipment with multiple motors or multiple pieces of equipment each with their own motor. So, which one is this?

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.

 

[NAZ55]

These are single phase fan motors on an Auto

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

 

[NAZ55]

By the way, Lionel, I disagree with your comment on line and phase currents being the same when referring to conductors. It depends on, which circuit you are sizing the conductors for.
As I mentioned in my OP phase current is 45A and line current is 77A.

 

[fx007]

Well, I am not sure that you can have them mixed and mashed by NEC connected to a single circuit. Good luck with AHJ.

 

[wareagle]

NEC TABLE 430-152 allows a breaker to be set at 250% of the motors FLA. In the case of several motors, you set the breaker for the largest motor + the sum of the other motors FLA.

 

[LionelHutz]

Auto? Is this a vehicle of some sort?

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.

 

[NAZ55]

Thanks Lionel for your last response. I agree with your deduction for using 100A. My concern was the motor starting amps and if they would blow 100A NON fuses. After looking at the time-current curve for 100A fuse I realized that 100A NON fuse could withstand 200A for 25 sec, which is plenty of time for the motors to reach FLA.