what is SCCR if it doesn't prevent arc flash?

[MTW]

Here's you one that wasn't a bolted fault or short circuit, but it burned long enough to take out a 480V 750KVA bank on a pole.

The cause was accumulated grinding dust with condensing humidity in the middle of a wet winter.

 

[malachi constant]

Tangentially related to this subject... In article 440 (Air-Conditioning and Refrigerating Equipment), article 440.10 requires the SCCR to exceed the available fault current on motor controllers or industrial control panels of multimotor and combination-load equipment. My question is...what motors does this generally include and exclude? My niche is office / K-12 / government / healthcare...not so much industrial/manufacturing/etc.
* chillers, condensing units, cooling towers - obviously yes
* pumps related to such equipment - ??
* packaged air handlers / RTUs that include integral cooling - obviously yes
* packaged air handlers that push heated/cooled air, but do not include integral cooling - ??
* exhaust fans - assume no
* commercial kitchen freezer and coolers
* what else?

(I am aware of the similar requirements in 620.51 for elevator control panels. And of course switchboards, switchgear and panels. Mainly interested in what motors need to be remembered.)

Thank you!

 

[kwired]

Tangentially related to this subject... In article 440 (Air-Conditioning and Refrigerating Equipment), article 440.10 requires the SCCR to exceed the available fault current on motor controllers or industrial control panels of multimotor and combination-load equipment. My question is...what motors does this generally include and exclude? My niche is office / K-12 / government / healthcare...not so much industrial/manufacturing/etc.
* chillers, condensing units, cooling towers - obviously yes
* pumps related to such equipment - ??
* packaged air handlers / RTUs that include integral cooling - obviously yes
* packaged air handlers that push heated/cooled air, but do not include integral cooling - ??
* exhaust fans - assume no
* commercial kitchen freezer and coolers
* what else?

(I am aware of the similar requirements in 620.51 for elevator control panels. And of course switchboards, switchgear and panels. Mainly interested in what motors need to be remembered.)

Thank you!

I think you are misunderstanding what it is saying.

All it means is the equipment SCCR needs to be higher than the available fault current, which is the general rule for pretty much all applications when it comes to SCCR.

One reason they maybe put this wording in art 440 is basically a reminder to verify. Many times manufacturers of this equipment do not actually test or at least do not mark them with a rating and the default rating in such cases is 5 kA. Usually not a problem as branch circuit wiring often limits available current at the unit. But really short circuit length could get you over 5kA available at the equipment in some cases.

 

[malachi constant]

I think you are misunderstanding what it is saying.

All it means is the equipment SCCR needs to be higher than the available fault current, which is the general rule for pretty much all applications when it comes to SCCR.

One reason they maybe put this wording in art 440 is basically a reminder to verify. Many times manufacturers of this equipment do not actually test or at least do not mark them with a rating and the default rating in such cases is 5 kA. Usually not a problem as branch circuit wiring often limits available current at the unit. But really short circuit length could get you over 5kA available at the equipment in some cases.

Hi kwired, I understand what it is asking there. My question is "what is air conditioning and refrigeration equipment" as I can't find those terms defined in the NEC.

For context - our engineering team does a pretty good job of getting this requirement addressed, or at least it is very rare that we get asked by an inspector to document and get caught in a bad place. We have our chiller and AHU/RTU specs written for equipment to be supplied as 65kAIC unless otherwise noted. We often get asked (understandably) by manufacturers to provide the calcs prior to them ordering the mechanical equipment, because they can save a decent chunk of change with a 5kAIC or 22kIC rating, etc.

Our team would like to add a column to our mechanical schedules for SCCR, which will encourage us engineers to actually include the motors in the calcs and get info onto the mech schedules prior to bids being due. The question that freezes us is "which motors do and don't need SCCR coordinated". Like, right now we aren't doing short circuit calcs for exhaust fans and I think that is correct. We aren't doing calcs for pumps and I think, maybe, that is OK. We are doing them for chillers/CUs and AHUs/RTUs. Just trying to get clarity on which mechanical equipment we should and shouldn't be performing short circuit calcs on.

 

[jim dungar]

The question that freezes us is "which motors do and don't need SCCR coordinated".

It is not about the motors. It is about the motor controllers.

This section was added to article 440 because people were ignoring 110.10 which has been in the code for decades.

 

[kwired]

My question is "what is air conditioning and refrigeration equipment" as I can't find those terms defined in the NEC.

See art 440.1. Not necessarily a definition there but it does tell us what art 440 does apply to. Key words in there is "hermetic refrigerant motor-compressors" If it doesn't have one of those it is likely either a 422 motor driven appliance or if not that plain art 430 motor application

 

[malachi constant]

OK...in NEC 2023 I found 430.83 (F) A motor controller shall not be installed where the available fault current exceeds the motor controller’s short-circuit current rating.

So all motors [technically motor controllers, but I'm going to be loose with my language and say "motors"], all motors everywhere, always, should have their available fault current calculated and compared against the controller's SCCR?

I mean, realistically a 1/6 HP exhaust fan is probably under 5kAIC every time (though I haven't really played around with our studies to test that out).

But in absence of established "we know with certainty this equipment will be fine except in the most extreme circumstances", we should be paying attention to available fault current and SCCR - is that right?

This is different from my lived experience of expecting the inspector has eyes on the chiller and elevator SCCRs, and rarely but ~sometimes~ we get asked to verify appropriate ratings on RTUs...and that's it. Are inspectors just assuming the smaller motors are good so they are only looking at the largest ones? For instance I'm surprised we haven't been hit on pumps yet - they can be large, and they are usually located near the service, so I doubt an SCCR of 5kAIC is sufficient.

 

[kwired]

OK...in NEC 2023 I found 430.83 (F) A motor controller shall not be installed where the available fault current exceeds the motor controller’s short-circuit current rating.

So all motors [technically motor controllers, but I'm going to be loose with my language and say "motors"], all motors everywhere, always, should have their available fault current calculated and compared against the controller's SCCR?

I mean, realistically a 1/6 HP exhaust fan is probably under 5kAIC every time (though I haven't really played around with our studies to test that out).

But in absence of established "we know with certainty this equipment will be fine except in the most extreme circumstances", we should be paying attention to available fault current and SCCR - is that right?

This is different from my lived experience of expecting the inspector has eyes on the chiller and elevator SCCRs, and rarely but ~sometimes~ we get asked to verify appropriate ratings on RTUs...and that's it. Are inspectors just assuming the smaller motors are good so they are only looking at the largest ones? For instance I'm surprised we haven't been hit on pumps yet - they can be large, and they are usually located near the service, so I doubt an SCCR of 5kAIC is sufficient.

Motors don't really need a SCCR. They don't have to carry short circuit currents and be expected to still function after doing so as when they end up carrying a high level fault current they typically are done for and need replaced or rebuilt. A breaker, bus bar, motor controller, etc. however needs to be able to withstand the possible fault current it could carry during a fault event at something it is simply passing current for, and is expected to operate again after the fault condition is cleared.

Smaller loads don't necessarily get overlooked, but anyone that has some experience at doing fault current calculations will see a pattern with small conductors and especially when the length of those small conductors starts to increase as the resistance of the conductors is very current limiting once you get past even just 10 to 25 feet of conductor length. Remember the SCCR the component in question are subjected to is what matters. If they are 100 feet away from the service equipment with 30 kA available at the service equipment that 30kA only applies at the service equipment. If you are 100 feet away and on a branch circuit of only 6 AWG conductors (I ran quick calculation with such figures with a 480 volt three phase circuit) there was only 5500 amps available fault current at the end of that circuit. still a concern if the equipment there is only 5kA rated but certainly not a problem for 10 kA rated equipment.

 

[jim dungar]

mean, realistically a 1/6 HP exhaust fan is probably under 5kAIC every time (though I haven't really played around with our studies to test that out).

Do a short circuit analysis comparison using various available currents and 20' and 50' lengths of #14, #12, and #10 conductors. You will find you can rarely see more than 5kA until you start to have tremendously large fault currents on your 15A and 20A circuits. 30A circuits probably need to be a lot more than 50' to get below 10kA. Larger conductor sizes aren't as good at reducing fault currents.

UL allows most equipment to have a 5kA SCCR without testing so this is a good value to aim for.

 

[malachi constant]

Jim & kwired - sounds like I need to run some test studies to see which loads can be considered safe and which loads deserve scrutiny. I agree with the "generally speaking smaller loads are fine" but in our area the utility has some very large short circuit values at commonly-sized 208V pad mounts (52-77kA at 300-500kVA xfmrs), so 120V 20A/1P motors fed out of those main electrical room could be an issue.

Do they make fractional transfer and exhaust fans with larger SCCRs? Cabinet unit heaters with their 1/30 HP fans? We for sure will have CUHs fed with a short run from a panel the main electrical room - we will almost certainly run into some that are over 5kAIC. I'm going to get some exasperated calls from our mechanical equip reps, ha ha.

My understanding is motor controllers are not on the list of equipment that requires the available fault current to be marked or documented. (Such as the requirement in 440.10(B) for AC/refrigeration equipment, or 408.6 for switchboards and panelboards). Meaning a study (or at least an appropriate level of "design thought") is tacitly required for all motor controllers by 430.83 (F) - but absent a requirement for the available fault current to be explicitly marked/documented it is unlikely that inspectors are looking at it there.

 

[jim dungar]

We for sure will have CUHs fed with a short run from a panel the main electrical room - we will almost certainly run into some that are over 5kAIC.

Yes you will.
We often find these problems when doing complete short circuit studies.
It is not unheard of to require a circuit have 20 to 30' of conductor coiled someplace in order to lower the fault current.

I'm going to get some exasperated calls from our mechanical equip reps, ha ha.

Article 110.10 has been in the NEC for some 50 years. Equipment vendors should not be surprised when told the need to supply higher SCCR.

 

[malachi constant]

It is not unheard of to require a circuit have 20 to 30' of conductor coiled someplace in order to lower the fault current.

I've considered that solution to deal with an 800A chiller - but needed a lifeline much longer than 30' so abandoned it. It is nice to know that we can put that in our back pocket for sticky situations with smaller motors. I assume one should provide signage/labeling of some sort to advise future contractors on the purpose of the coil?

 

[don_resqcapt19]

Jim & kwired - sounds like I need to run some test studies to see which loads can be considered safe and which loads deserve scrutiny. I agree with the "generally speaking smaller loads are fine" but in our area the utility has some very large short circuit values at commonly-sized 208V pad mounts (52-77kA at 300-500kVA xfmrs), so 120V 20A/1P motors fed out of those main electrical room could be an issue.

Do they make fractional transfer and exhaust fans with larger SCCRs? Cabinet unit heaters with their 1/30 HP fans? We for sure will have CUHs fed with a short run from a panel the main electrical room - we will almost certainly run into some that are over 5kAIC. I'm going to get some exasperated calls from our mechanical equip reps, ha ha.

My understanding is motor controllers are not on the list of equipment that requires the available fault current to be marked or documented. (Such as the requirement in 440.10(B) for AC/refrigeration equipment, or 408.6 for switchboards and panelboards). Meaning a study (or at least an appropriate level of "design thought") is tacitly required for all motor controllers by 430.83 (F) - but absent a requirement for the available fault current to be explicitly marked/documented it is unlikely that inspectors are looking at it there.

Very unlikely...using Bussmann's free app, FC², starting with 100 kA, I get less the 5kA and the end of a 30' run of 12 AWG.

Also, as far as I know motors do not have an SCCR. The motor controllers and other control devices that carry the actual load current do.

 

[D. Castor]

NFPA 70E considers testing part of 'normal' activities

I think this depends on the type of test being conducted. If you are doing voltage testing on exposed live parts, it's not considered normal operation, and an arc flash hazard evaluation and maybe PPE is needed. NFPA 70E does allow testing to be done without an Energized Work Permit - one of three exceptions to the requirement.

 

[jim dungar]

I think this depends on the type of test being conducted. If you are doing voltage testing on exposed live parts, it's not considered normal operation, and an arc flash hazard evaluation and maybe PPE is needed. NFPA 70E does allow testing to be done without an Energized Work Permittant - one of three exceptions to the requirement.

I did not say an incident energy analysis and resultant PPE was not required.
My point is that NFPA 70E says to perform a risk analysis. It considers an operating system to be less risky than one that has just been wired or one that has experienced a fault.

The fact a fault did not exist before being deenergized is one factor that can be considered while evaluating the risk of an arc flash. Look at the table for deciding what/when PPE is required.

 

[MyCleveland]

Very unlikely...using Bussmann's free app, FC², starting with 100 kA, I get less the 5kA and the end of a 30' run of 12 AWG.

Also, as far as I know motors do not have an SCCR. The motor controllers and other control devices that carry the actual load current do.

Don
I am no expert on conductor damage curves, but the idea of serving anything in smaller wire sizes exposed to 100k seems like a poor design choice.
Would it not be better to serve smaller panels first to get the AFC down considerably before serving any of the smaller loads ?

 

[kwired]

Don
I am no expert on conductor damage curves, but the idea of serving anything in smaller wire sizes exposed to 100k seems like a poor design choice.
Would it not be better to serve smaller panels first to get the AFC down considerably before serving any of the smaller loads ?

Proper overcurrent protection should interrupt the circuit before conductor is damaged. Conductors can handle well over their NEC ampacity for short time periods, but those short time periods are still much longer compared to the milliseconds that it likely takes for overcurrent devices to operate in such conditions. Look at how small NEC allows an EGC to be compared to what size the circuit conductors need to be as a general rule. If you see an EGC melted down it usually isn't because of fault current but maybe because of a welder not placing his work ground lead close to the work.

 

[don_resqcapt19]

Don
I am no expert on conductor damage curves, but the idea of serving anything in smaller wire sizes exposed to 100k seems like a poor design choice.
Would it not be better to serve smaller panels first to get the AFC down considerably before serving any of the smaller loads ?

The impedance of the conductor acts as a current limiting resistor, so even with 100kA available at the source, the maximum current with a bolted fault at the end of a 30' run of 12AWG will be less than 2.5kA (yes, I put the wrong number in my previous post). The one cycle withstand current for 12 AWG is 2.7kA. So this one is a bit close. You would have to find the time trip table for your breaker as the actual time trip curves do not give you that detail.

However, you will have to find some way to limit the fault current on the line side of the 20 amp breaker that feeds these loads. Unlikely you will find one suitable for use with that high AIC.

 

[MyCleveland]

The impedance of the conductor acts as a current limiting resistor, so even with 100kA available at the source, the maximum current with a bolted fault at the end of a 30' run of 12AWG will be less than 2.5kA (yes, I put the wrong number in my previous post). The one cycle withstand current for 12 AWG is 2.7kA. So this one is a bit close. You would have to find the time trip table for your breaker as the actual time trip curves do not give you that detail.

However, you will have to find some way to limit the fault current on the line side of the 20 amp breaker that feeds these loads. Unlikely you will find one suitable for use with that high AIC.

I wasn’t picky nits on your numbers…understood your point.
I looked over prior threads and MOD - JD , I believe, was making the point that if the OCPD was UL 489 then the clearing would inherently protect the wire per the damage curves.

 

[jim dungar]

I wasn’t picky nits on your numbers…understood your point.
I looked over prior threads and MOD - JD , I believe, was making the point that if the OCPD was UL 489 then the clearing would inherently protect the wire per the damage curves.

Yes. Even fused switches are tested with conductors so a 20A 100kAIC current limiting fuse would also protect #12 conductors.