Motor Starting Current

How often do you find a motor that draws 10I_n for more than one second? What is the LRA/FLA code letter for this motor?

That is why in practice, there needs to be co-ordination between the motor overload device, which is there to protect the motor, and the circuit protective device(s) which is/are there to protect the cable. Once again, British Standard 7671 gives full protocols for carrying this out and there is no need to look any further.

It's a strange motor that draws 10x its full load current on starting; most draw something in the region of 5.

If in doubt, consult a qualified Electrical Engineer locally.

Most of the motors would draw only 6 to 8 times the rated full load current during starting. A 'D' Type with instantaneous response current 10 to 20 times would not respond to this.

According to NEC Table 430.7(B) Locked-Rotor Indicating Code Letters

for all motor from code S to V starting current will be more than 10*Irated

According to IEC 60034-12 K=Slock/kw maximum could be 10-13 that means Istart= 8.5-11*Irated.

However, usually maximum Ilock=8*Irat- in case of DOL start at 100% rated voltage.

At start, the voltage drops- it is permissible up to 20% drop. In this case Istart=8*0.8=6.4*Irat

NEC Table 430.52 Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices limits for the Inverse Time Breaker Iset=2.5*Irat.

A usually time-current trip curve breaker will trip in 10 sec for 3 times breaker current rated.

That means 3*2.5=7.5 Imotor rated.

Usually the starting time is less than 2 sec and may be-for high inertia load-up to 6 sec.

You need a special breaker with a delayed time-current curve for Istart= 10*Irat and 10 sec starting time.

In any case, one has to protect the motor against overload by an Overload Device.

See NEC 430.32 for Continuous-Duty Motors. [or elsewhere for shorter duty].

Sorry Jraef, but I cannot agree that the numbers showing in NEC codes table are referring to

inrush current. I have not so large field experience, indeed-since I was only a designer all my life.

As is already said, inrush current duration is up to 1 cycle. So, for a usually breaker is not important, at all, as minimum reaction time may be 60-80 msec [3-4 cycles].

In the following article:

Avoid Nuisance Tripping with Premium Efficiency Motors

http://www1.eere.energy.gov/manufacturing/tech_deployment/pdfs/avoid_nuisance_motorsys_ts6.pdf

it is noted:

"While NEMA Design B standards limit LRC, no standard limits the peak-inrush current.

Fortunately, peak-inrush current usually is not a problem because it lasts only a few milliseconds. However, it can be an issue when the motor controller uses instantaneous magnetic-only circuit protectors that react in less than a single AC cycle"

See also CR4 HHung o.p. from 9/17/2010

"Induction Motor, Inrush Current vs. Locked Rotor Current"

7anoter4, Apples and oranges. You are correct with regards to the US NEC tables, but that is not the question that was asked. They were asking about IEC Mini Circuit Breaker trip curves, something that has no direct correlation to the NEC. There is no NEC table that addresses this issue, although it is mentioned in the exceptions to table 430.52 (going by memory here, I'm on my iPad).

I agree, of course.Thanks.