Short Circuit Contribution for Motors less than 50hp

[ish1284]

I am performing short circuit calculations and equipment evaluation on a 480V bus with a large amount of induction motor loads that are rated under 50hp. I am using the assumption that the contribution from each motor is 6.5xLRA. The combination of all of these motors results in a significant amount of fault contribution.

The fault contribution combined with the utility contribution cause the need for the UL1558 switchgear which feeds these loads to be rated 85kA. Customer is fighting that the motor contribution should not be included because all of the motors are under 50hp and they want to buy 65kA gear (most likely to save money).

I understand this to be acceptable practice when it comes to arc flash calculations because the inertia is not enough to sustain an arc, but I'm not so sure about low voltage short circuit calculations. Any insight out there would be appreciated!

 

[Jraef]

I think you may have started off with one big error (or maybe just typed it wrong). Start there. It’s not 6.5x LRC, it is based on FLC. LRC is already 6xFLC, so you are using 39x FLC!

 

[ish1284]

I think you may have started off with one big error (or maybe just typed it wrong). Start there. It’s not 6.5x LRC, it is based on FLC. LRC is already 6xFLC, so you are using 39x FLC!

Oops, that was a typo in the original post, I meant 6.5xFLA in the original post. My calculation is based on 6.5xFLA.

 

[Jraef]

Oops, that was a typo in the original post, I meant 6.5xFLA in the original post. My calculation is based on 6.5xFLA.

Well, I know of nothing that lists any HP value at which you don't have to count it. The term I have seen used is "if significant", which leaves it up to you to decide I guess. But to that point, I have this in a guide put out by Cooper / Bussman a few years ago, saying;

Motor short circuit contribution, if significant, may be
added at all fault locations throughout the system. A
practical estimate of motor short circuit contribution is to
multiply the total motor current in amps by 4. Values of 4
to 6 are commonly accepted.

http://www.cooperindustries.com/con...rary/BUS_Ele_Tech_Lib_Electrical_Formulas.pdf


So if you decide to use 4, does that make a difference?

Also, and I can't find where I have this, but at one time I read a procedure that, when adding motor contribution to determine the fault level AT THE GEAR (in my case MCCs), you can factor in the cable lengths and sizes for the motor feeders. In that case as I recall, I was dealing with 8 x 400HP motors so it wasn't that hard to pull off. A large number of small motors may end up being quite the task, but I bring it up because that may be why the Cooper / Bussman document said to use a factor of 4.

 

[topgone]

[MENTION=89959]Ish[/MENTION],
Use the motor's LRC per unit as the factor when doing the motor contribution estimates. Say you have a locked-rotor code G, 500HP, 480V motor. The locked-rotor current in pu for that motor is 6.4! You just can't choose an arbitrary number there.

Let's do the math:
FLA of 500 HP, 480V motor = 590A
Locked-rotor code G = 6.3 kVA per HP; so = 6.3 X 500 = 3150 kVA
Locked-rotor current = (3150 x 1000)/(1.732 X 480) = 3788.9 A
Locked-rotor current per unit = 3778.9/590 = 6.4!

 

[xptpcrewx]

Short Circuit Contribution for Motors less than 50hp

I am performing short circuit calculations and equipment evaluation on a 480V bus with a large amount of induction motor loads that are rated under 50hp. I am using the assumption that the contribution from each motor is 6.5xLRA. The combination of all of these motors results in a significant amount of fault contribution.

The fault contribution combined with the utility contribution cause the need for the UL1558 switchgear which feeds these loads to be rated 85kA. Customer is fighting that the motor contribution should not be included because all of the motors are under 50hp and they want to buy 65kA gear (most likely to save money).

I understand this to be acceptable practice when it comes to arc flash calculations because the inertia is not enough to sustain an arc, but I'm not so sure about low voltage short circuit calculations. Any insight out there would be appreciated!

As Jraef and topgone have pointed out, use locked-rotor code letters and factor in the cable impedance. Also, if some motors do not have the possibility of operating simultaneously, you won’t have to consider those in the contribution.

 

[ron]

I've seen that assumption in some texts such as on page 389 of this text (middle-ish of the page)
http://web.ecs.baylor.edu/faculty/lee/ELC4340/Lecture note/Chapter7_GSO5.pdf

Midway through this article -
https://www.ecmweb.com/design/short-circuit-calculation-methods
"The ANSI/IEEE method, which is described in IEEE Std. C37.010-1979 and its revision in 1999, is used for high-voltage (above 100V) equipment. It calls for determining the momentary network fault impedance, which makes it possible to calculate the close and latch rating of the breaker. It also calls for identifying the interrupting network fault impedance, which makes it possible to calculate the interrupting duty of the breaker. The interrupting network fault impedance value differs from the momentary network fault impedance value in that the impedance increases from the subtransient to transient level.The IEEE standard permits the exclusion of all 3-phase induction motors below 50 hp and all single-phase motors. Hence, no reactance adjustment is needed for these motors. The Chart at left clarifies the ANSI/IEEE procedure."

 

[MrJLH]

never mind my post

 

[MRKN]

I believe the old IEEE 1584 stated you could exclude motors smaller than 37 kW / 50 HP; not sure if that's changed in 2018. I can tell you that exclusion was broadly implemented at the facility I work at.

 

[ish1284]

I believe the old IEEE 1584 stated you could exclude motors smaller than 37 kW / 50 HP; not sure if that's changed in 2018. I can tell you that exclusion was broadly implemented at the facility I work at.

Understood that it can be excluded in terms of arc flash calculations. Hope you're not interpreting that as it can be excluded when you're evaluating equipment ratings?

And to others who have quoted ANSI C37.010, this standard applies to medium voltage motors. C37.13 (for low voltage equipment) does not exclude motors under 50HP. My research has concluded that the small motors must be included in the effort of evaluating equipment ratings but can be excluded for arc flash calculations in low voltage systems.

 

[MRKN]

Hope you're not interpreting that as it can be excluded when you're evaluating equipment ratings?

Your hope is not in vain.

 

[Jraef]

Understood that it can be excluded in terms of arc flash calculations. Hope you're not interpreting that as it can be excluded when you're evaluating equipment ratings?

And to others who have quoted ANSI C37.010, this standard applies to medium voltage motors. C37.13 (for low voltage equipment) does not exclude motors under 50HP. My research has concluded that the small motors must be included in the effort of evaluating equipment ratings but can be excluded for arc flash calculations in low voltage systems.

Yes, the exclusion of <50HP is relating to MV motors under 50HP, which basically don't really exist anyway. That ECM article had a typo in stating that C37.010 applied to "high voltage systems of >100V" (sic); it should have said 1000V.

But IMHO you are still using too high of a value.

From the IEEE Violet Book: 551 IEEE Recommended Practice for Calculating Short - Circuit Currents in Industrial and Commercial Power Systems

For application of ac low-voltage power circuit breakers and both medium and low-voltage fuses, only first cycle calculations are necessary, and IEEE Std C37.13-1990 [B5],IEEE Std C37.41-2000 [B6], and ANSI/IEEE C97.1-1972 [B1] recommend representing all rotating machines in the equivalent circuit based on subtransient reactances regardless of motor rated horsepower. IEEE Std C37.13-1990 [B5] qualifies this by suggesting that motor short-circuit current contributions, for “typical” groups of low-voltage motors lacking detailed information, may be estimated at four times the summation of motor rated currents. A contribution of four times rated current corresponds to a first cycle motor Z =(V/4) = 0.25 per-unit based on motor rated apparent power (kVA) and voltage of 1 p.u.Usually exact motor short-circuit reactances are not readily available and is approximated by using 1/(locked rotor current).

It goes on after that to describe why 4x is used as being a compromise between a system being 75% induction motors at 3.6% and 25% synchronous motors at 4.8x, so if you have no synchronous motors, 4x is even a bit of overkill.

This also is apparently where I saw that the value is lower than LRC due in part to the expected impedance of the cables and equipment.

This reduction might be partly explained by the motor cables and/or overload heater impedances in series with low-voltage motors that are often omitted from the calculation, but a very important factor is the decay during the first cycle of motor current contribution due to collapsing motor flux.

So yes you include them, but not at 6.5x the FLC, use 4x the FLC.

Also, just in case you didn't know this: you can exclude any motors that are on standard (meaning non-regenerative) VFDs.