Short Circuit Study - Motor Branch Circuit Protection

[orlyboy]

Hello Everybody,

I am new in the design analysis and we are using analysis software. In the example, the data are given below.
Rated kW = 110 kW
Rated kV = 0.38 kV
Rated PF = Typical
Rated EF = Typical
Slip = 0.8

What will be the size of the Protective Device (PD) of the motor?
Can someone show me the solution and point me to the standard to refer to.

Thanks

 

[rbulsara]

In ANSI world, it is governed by local Codes such as NEC article 430, not a software. If you "need" software for this, you will have difficulty down this career path.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

point me to the standard to refer to

Right after you tell us where in the world this motor is.

The mind reading skills of this group have dropped off in recent years.



David Castor

http://www.cvoes.com

 

[orlyboy]

Ok guys sorry if i am lazy reading the standards perhaps maybe i don't have the reading material and
because i am new in the field of design and i have to ask my friend to provide me electronic copy of NEC.
the sample motor i have posted is from training example of etap. i am just wondering why etap had selected
400 ampere for the size of the circuit breaker.

Here is my calculation base on NEC Part IV, Article 430.51-52, please correct me if my calculation is wrong.

Given:

Rated kW = 110kW
Rated kV = 0.38kV
P.F. = Typical (assume 0.86)

Solution:

I = (Rated kW)/1.732(Rated kV)(P.F)
= 110/(1.732)(0.38)(0.86)
I = 194.34 A

Therefore:

PD = 194.34(250%)
= 485.85 or
PD = 500 A

I also tried to choose 350A and simulate or run the load flow and it doesn't give errors but when
i try 250A it gives error. So, my question is which is reliable?

 

[rbulsara]

Getting the basics right is essential before you can use a software like ETAP properly. Software are no more than sophisticated calculators, they do not make decisions for you.

Laziness or not having a very common standard is not an excuse that works here. Professionals here do not invest their time and share invaluable experience to make up for that either.

This is a site for professionals and you need to be one to get the most out of others' goodwill.

So read the NEC again. Limits in the NEC tables may for maximum value or minimum. See what you find.


Rafiq Bulsara

http://www.srengineersct.com

 

[dheXjhe]

may i know, what kind of PD that you use to the motors?such as Overloas heater, Fuses, LVCB, etc.

as i know, if you want to determine the size of protection you can look what full load ampere on the motor side. so the size of protection must be higher than FLA.

and if you dont mind, please show the alert on ETAP that state errors on the motors.

 

[orlyboy]

I suspect sophisticated softwares comes and derive from fundamental mathematical equation and the bases of decision making.

Here is the etap alert. Kindly share your knowledge on the software.

I choose CB size 150A and it gives alert.
I choose CB size 250A and no alert given.
I choose CB size 400A which is the data given on the training and doesn't give alert.

Thanks to everyone

 

[orlyboy]

Hi dheXjhe,

etap info of 400A CB.

see attached.

Thanks

 

[dheXjhe]

ya, thanks orlyboy for attachment.

but on the pic, i didnt find have alert. as i know, load flow analysis not to calculate the rate of protection. but, just load flow in the system and data will be used on the protection-coordination analysis.

so, if you want to know what size of protection for motor 110 kw, i think you have to consider FLA on motor for normal protection.

 

[sspeare]

When using NEC Table 430.52 to select the Maximum Overcurrent Protection (MOCP) - you do NOT calculate the running Amps from the kW, kV, etc. The NEC requires that you look up the Full Load Current (FLC) in Tables 430.247-250. Then you multiply that FLC by the "Percentage Full-Load Current" you find in Table 430.52.

Example:
Given: 100 HP squirrel cage motor, 3-phase, 460 Volts.

Find: Max Overcurrent Protection, if using Inverse Time (I-T) Breaker.

Solution: Table 430.250 gives FLC = 124 Amps. Table 430.52 allows a max multiplier of 250% for an I -T breaker. So, 124 Amps x 2.5 = 310 Amps. You are allowed to round up to the "nearest standard breaker rating".

My question to you: can you tell this Forum what the "nearest standard breaker rating" is?

 

[burnt2x]

orlyboy,
From what I understand with softwares, programmers require the user to input required data, e.g. power rating, voltage, frequency, etc. before the program executes routines to spit out results. Built-in the program are lots of curves for each protection device, the damage curves for inputed equipment and compare those and see to it that the operating range of the equipment will never cross the limits (damage curves) with some margins. Having said those, you will get alerts whenever the program sees that the protection device you chose is insufficient according to its internal logic. Granting you get a successful run, meaning, there were no alerts, you will have to check with your governing codes if that protection device is allowed. The final result is that you are going to look for the "nearest standard breaker/protection device" as what others had been saying. The catch is, you will do an engineering analysis and then decide on the "nearest to standard" device which will get your design code-compliant.
Hope that helps.

 

[orlyboy]

sspeare,

My calculation on the first (top) page is approximately equal to your example. Please correct me if I am wrong.

My calculation is based on Article 430 Part IV, 430.52(C), Exception No. 1. and, I think that answers also your question: (can you tell this Forum: what the "nearest standard breaker rating" is?).

dheXjhe,

You are absolutely correct, Load Flow analysis module of etap doesn't gives you what rating of CB or PD to a particular equipment, it gives/displays only Power, voltage angle, current, pf, voltage drop.

burnt2x,

That's what I did, all required data to run the load flow is entered and feed.

As you have said:
"(The catch is, you will do an engineering analysis and then decide on the "nearest to standard" device which will get your design code-compliant.)"....you mean to say we can go down to the standard settings which violate Article 430.52(C)(1) Exception No.1.

Thanks for all the replies...

 

[burnt2x]

"Exception No. 1: Where the values for branch-circuit short-circuit and ground-fault protective devices determined by Table 430.52 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, thermal protective devices, or possible settings of adjustable circuit breakers, a higher size, rating, or possible setting that does not exceed the next higher standard ampere rating shall be permitted."

Nothing is edited on the above quote! Never said you go down either. Going down makes your decision a bad one. BTW, it's your call.

 

[rbulsara]

Ratings in Table 430.52 are "Maximum" ratings although the next higher standard size is "permitted". Going down is not prohibited. In fact it is often done, based on engineering analysis/judgment of motor starting current requirements.

Remember, the code does not care if your motor (non-life safety related) does not start or runs backward, they are only meant for safety.



Rafiq Bulsara

http://www.srengineersct.com

 

[sspeare]

orlyboy, et al:

Sorry, I did not intend to say that anything was wrong with your calculation. All I was saying is that the NEC requires you to use Full Load Current (FLC) from the Tables at the end of Article 430.

In your July 13 post you have:
PD = 194.34(250%)= 485.85 or PD = 500 A.

The 500 Amp selection looks good for an inverse time MOCP device. Why spend time trying the 350 Amp, and then the 250 Amp? You already solved the problem with 500 Amps.

Hope the Forum helped; I know I learned a few things!