Hello,
What size of circuit breaker can you suggest for 18.5kW motor(3 phase connection)?
Do you think that 40Amps will be enought?

 

Andy1984...
1) Voltage rating?
2) Duty Classification?
3) Special Operating Conditions?
Regards, Phil Corso

 

Hi Phil,

Sorry for late answer,
Its a vacuum pump,
400VAC between live wires, IE3 enginer, nothing special condition,
i apply 84Amps breaker and working fine,
but im wonder if is any tableor formula for select right circuit breaker?
Regards,
Andy

 

Andy1984...
You have answered only the 1st question, and part of the second ! To broaden your knowledge I suggest you try to find out what the Nameplate reveals by asking a mfg of motors in your location.
Regards, Phil Corso.

 

Andy1984...
Both the USA standard, NEMA MG-1-1.40.2 and EU Standard, IEC 60034-1, stipulate "Duty"" be noted on induction-motor nameplates. I am quite sure you will see it on your motor's nameplate !

Furthermore, an 84-Ampere Circuit Breaker is more than double the motor's rated Full-Load current ! In my opinion it is too large and therefore dangerous!
Phil Corso

 

What's the motor FLA? Should come from the nameplate. My quick calc is saying it is circa 32A.

Based on just that, a 40A MCB with D-curve should be sufficient, in my view.

 

Gadha707...
I found that an IEC rated 3-ph, 18.5 kW, 400 V, with an SF equal to S1, has a nameplate current equal to 35 Amperes. The CB selected by Andy is 84 Amperes !

The breaker size he selected does not comply with USA NEC requirements ! But your suggestion is valid !

Regards, Phil Corso

 

Hello Phil , Hello Gadha,
Good morning and very sorry for late reply,

Im not sure if name plate photo will help but i enclosed
32A or 35A are definitely to small for this application,
nominal current for this vacuum pump is:
18.5kW=400V*I ==> I=18.5kW/400V=46.25Amper
but during starting this motor consume much more energy,
It could be even 1.5~2 times more then nominal,
So i think the circuit breaker can not be smaller then 80A

 

Hello Phil , Hello Gadha,
Good morning and very sorry for late reply,

Im not sure if name plate photo will help but i enclosed
32A or 35A are definitely to small for this application,
nominal current for this vacuum pump is:
18.5kW=400V*I ==> I=18.5kW/400V=46.25Amper
but during starting this motor consume much more energy,
It could be even 1.5~2 times more then nominal,
So i think the circuit breaker can not be smaller then 80A
View attachment 2209

We assumed it was a 3ph motor. Your calculation indicates it is a single phase motor. Can you confirm if it is a 3ph or 1ph motor?

It appears that you shared the pump nameplate. Do you have the motor nameplate?

The start up current is generally 6 times the full load current (Could be up to 12 times for a high efficiency motor). My suggestion of the choice of a D-curve should allow ride through during motor start without tripping.

 

Gentlemen... A small,, but important technicality... Energy cannot be consumed nor produced.
Phil

 

Hello Gentlemans

Im sorry but i dont have this motor in my home and i can not make a name-plate photo,
i confirm it is a 3phase motor(i know i make mistake in my above calculation),
During starting this motor dont have easy life,
its connected via v-belt with roots pump,
so i think the resistance comming from air is very strong,

Previously i tryed with 50Amps circuit breaker but was to weak,
Next size i had was 84A

 

If the properly sized breaker (with the right rating and trip curve) trips....then it's protecting the motor from overload and putting in a bigger breaker will damage the motor. This is one reason why fans and pumps often have soft starters (electronic, wye-delta, etc.). By the way, if the starting torque is that high, it'll be hard on the pump as well.

 

Joseph_2...
We now know it's single-phase and it's driving a Vacuum pump!
Phil Corso

 

Phil: The OP confirmed it is a 3ph motor.

Andy: What do you mean by the 50A breaker was too weak? Was it tripping during start up? If it was, the reason could either be a true motor overload condition or breaker with the wrong curve. I suspect it is the latter. What trip curve does your breaker have?

 

I'm not sure where this discussion is taking place so excuse me for mudding things up but I have gotten confused and would like to interject a question. I am studying the NEC and my understanding is that a continuous duty motor > 1 Hp requires a separate overload device or one integral to the motor [2020 NEC 430.32(A)] . Additionally 430.52 which addresses the motor branch circuit short circuit and ground fault protection seems to indicate an inverse breaker used can be 250% of the motor full load current.
So my question is that if the installation describe here was installed per the NEC, the breaker would not be providing overload protection to the motor. The breaker could be 84 amps as it would be under the 250% (2.5 x 35 = 87.5). Am I interpreting the code correctly?

Thanks in advance.

 

Phil: The OP confirmed it is a 3ph motor.

Andy: What do you mean by the 50A breaker was too weak? Was it tripping during start up? If it was, the reason could either be a true motor overload condition or breaker with the wrong curve. I suspect it is the latter. What trip curve does your breaker have?

Hi
Im sorry for late answer
50A breaker was just tripping during starting,
I dont know the trip curve of this breaker,
Now with 84A working fine