60hz vs 50hz Cable Impedance Values

[confused-engineer81]

Is using the 60Hz cables in SKM's equipment library close enough to their 50Hz counterparts. If not, how do I find information for Copper, Aluminum, in both magnetic and non magnetic conduit in all typical trade sizes so I can accurately model a 50Hz System in SKM?

I'm working on a project that uses a 50Hz electrical system. In SKM there is an option to run studies in 50Hz instead of 60Hz, which I have set correctly. My confusion is that there are cables listed as 50Hz, and cables listed as 60Hz in the SKM library (see image below). I've spot checked similar trade size, quantity, material, etc. the cables have different reactance and resistance values. I understand that Reactance is a function of Hz, so the same cable with 60Hz vs 50Hz would give different Impedances. The options for 50Hz cables in the SKM library are extremely limited and I need more cable types (magnetic vs non magnetic conduit, etc.)

I've tried googling the problem and searching this forum for other answers, but couldn't find anything. SKM tech support doesn't have these on file and asked me to provide them with the cable information. Which i don't know where to find.

If not an answer, I would even appreciate some guidance on where to look to find the information

 

[xptpcrewx]

Is using the 60Hz cables in SKM's equipment library close enough to their 50Hz counterparts. If not, how do I find information for Copper, Aluminum, in both magnetic and non magnetic conduit in all typical trade sizes so I can accurately model a 50Hz System in SKM?

I'm working on a project that uses a 50Hz electrical system. In SKM there is an option to run studies in 50Hz instead of 60Hz, which I have set correctly. My confusion is that there are cables listed as 50Hz, and cables listed as 60Hz in the SKM library (see image below). I've spot checked similar trade size, quantity, material, etc. the cables have different reactance and resistance values. I understand that Reactance is a function of Hz, so the same cable with 60Hz vs 50Hz would give different Impedances. The options for 50Hz cables in the SKM library are extremely limited and I need more cable types (magnetic vs non magnetic conduit, etc.)

I've tried googling the problem and searching this forum for other answers, but couldn't find anything. SKM tech support doesn't have these on file and asked me to provide them with the cable information. Which i don't know where to find.

If not an answer, I would even appreciate some guidance on where to look to find the information
View attachment 2557016

What type of study are you doing?


Sent from my iPhone using Tapatalk

 

[confused-engineer81]

What type of study are you doing?


Sent from my iPhone using Tapatalk

Short Circuit, but ill be using the same model for an arc flash study eventually

 

[mbrooke]

Short Circuit, but ill be using the same model for an arc flash study eventually

With smaller conductors sizes under 16mm2 you can typically ignore X.

For ground fault loop impedance assume 70-75*C, for short circuit (AIC) assume 30*C AC resistance- this will always yield values that are a bit more conservative and will account for reactance being ignored.

 

[xptpcrewx]

With smaller conductors sizes under 16mm2 you can typically ignore X.

For ground fault loop impedance assume 70-75*C, for short circuit (AIC) assume 30*C AC resistance- this will always yield values that are a bit more conservative and will account for reactance being ignored.

30*C and 75*C are not conservative temperatures for short-circuit analysis.

 

[xptpcrewx]

What type of study are you doing?


Sent from my iPhone using Tapatalk

Although 50Hz isn’t in my realm, I would guess this information should be made available through a manufacturer or IEC standard?

Just as an estimate, you may be able to adjust the NEC reactance values in Chapter 9 Table 9. Multiply reactance values by a factor of 50/60 = 0.833.

 

[mbrooke]

30*C and 75*C are not conservative temperatures for short-circuit analysis.

You don't use 75*C for short circuit analysis.

 

[xptpcrewx]

You don't use 75*C for short circuit analysis.

I’ve noticed you use the term “ground fault loop impedance” pretty often. In the context of short circuit analysis, there isn’t such a parameter or value you could enter into a software model. Based on a formula I’ve seen you post on another thread regarding this term, it is nothing more than the total impedance as seen by a ground fault (and an oversimplified version of it).

The OP is working with SKM and is left to model the system with lumped components. While the OP can certainly enter impedance sequence components, modeling is done on an individual component level and not a total impedance level so references to “ground fault loop impedance” wouldn’t be applicable from a modeling perspective.

 

[mbrooke]

I’ve noticed you use the term “ground fault loop impedance” pretty often. In the context of short circuit analysis, there isn’t such a parameter or value you could enter into a software model. Based on a formula I’ve seen you post on another thread regarding this term, it is nothing more than the total impedance as seen by a ground fault (and an oversimplified version of it).

The OP is working with SKM and is left to model the system with lumped components. While the OP can certainly enter impedance sequence components, modeling is done on an individual component level and not a total impedance level so references to “ground fault loop impedance” wouldn’t be applicable from a modeling perspective.

Total system impedance at the fault point is the sum of all individual lumped components up to the fault point. Loop impedance is simply the worse case (lowest) L-G fault current likely to be encountered in any portion of the system and it is easy to model.


The hand equation is over simplified, but it beats electricians having to do sequence components.


AIC uses the lowest conductor temps, EFLI/GFLI uses the highest operating or short circuit temps.

 

[xptpcrewx]

Total system impedance at the fault point is the sum of all individual lumped components up to the fault point. Loop impedance is simply the worse case (lowest) L-G fault current likely to be encountered in any portion of the system.


The hand equation is over simplified, but it beats electricians having to do sequence competents.

So if it’s worst case (lowest fault current), then it wouldn’t apply to short-circuit analysis device/equipment evaluation - which is most likely what the OP is doing. For Arc-Flash, this becomes more relevant but unbalanced/single-phase is not commonly done.

 

[mbrooke]

So if it’s worst case (lowest fault current), then it wouldn’t apply to short-circuit analysis device/equipment evaluation - which is most likely what the OP is doing. For Arc-Flash, this becomes more relevant but unbalanced/single-phase is not commonly done.

It would certainly apply to equipment evaluation and wire sizing.

Since I see metric wires sizes and 50Hz, Table 41.1 in IEC 60364-4-41 or BS7671 would be mandatory.

If following NFPA-70 the discretion is left to the engineer but in the end it must not violate 250.4 (A) (5).

In order to protect life and property both the min and max fault currents must be known. FWIW in some cases L-G fault current is higher than balanced 3 phase fault current which would apply to the AIC rating.

 

[xptpcrewx]

It would certainly apply to equipment evaluation and wire sizing.

Since I see metric wires sizes and 50Hz, Table 41.1 in IEC 60364-4-41 or BS7671 would be mandatory.

If following NFPA-70 the discretion is left to the engineer but in the end it must not violate 250.4 (A) (5).

In order to protect life and property both the min and max fault currents must be known. FWIW in some cases L-G fault current is higher than balanced 3 phase fault current which would apply to the AIC rating.

Minimum fault current never applies to device/equipment evaluation. Nor does it apply to sizing EGC’s from an NEC perspective - you have 250.122 for that. As far as 250.4(A)(5) goes, it’s irrelevant to device/equipment evaluation. Devices and equipment are rated and tested under 3-phase bolted-fault conditions so anything having to do with the ground fault current path is not relevant 99% of the time.

 

[mbrooke]

Minimum fault current never applies to device/equipment evaluation. Nor does it apply to sizing EGC’s from an NEC perspective - you have 250.122 for that.

Have you read the IEEE green book? It mentions multiple times that 250.122 is only a minimum size and not adequate under all short circuit conditions encountered in the real world.

If equation 240.92 B is applied to wire type EGCs the results show that exceptionally high short circuit currents, delayed clearing, or reliance on short time rather than instantaneous elements can heat the wire beyond 150*C, sometimes above 1,850*C.

An overheated EGC will cause the loss of the fault clearing path while possibly igniting wire insulation and building material.

As far as 250.4(A)(5) goes, it’s irrelevant to device/equipment evaluation. Devices and equipment are rated and tested under 3-phase bolted-fault conditions so anything having to do with the ground fault current path is not relevant 99% of the time.

It is exceptionally relevant, a primary driver above all else. Under high short circuit conditions 3 cycle tripping as provided by MCCBs is not enough to protect small EGCs, and as such sub-cycle (low-peak) current limiting fuses must be employed. Under short circuit limited conditions (such as faults in very long runs) the instantaneous pickup may no be reached, so either a faster breaker must be employed, ground fault protection applied, or the EGC up-sized.

 

[xptpcrewx]

Have you read the IEEE green book?

Yes. I don't want to be rude but do you have the technical background to understand and actually apply it?

It mentions multiple times that 250.122 is only a minimum size and not adequate under all short circuit conditions encountered in the real world.

You are twisting what the Green Book says just to fit it into your view and insisting on making an unnecessary connection to the OP. Simply put, the Green Book tells you to follow the NEC as a minimum safety standard. At the same time, the NEC doesn't claim to prescribe EGC's that will be adequate under all short-circuit conditions and neither have I - you are making up your own problem to debate about here.

One thing the Green Book suggests is that the EGC minimum sizes in the NEC may not provide efficient or practical use of certain equipment or when using unreasonably long conductor/raceway systems and configurations. There is nothing surprising or disputable about this (it is well known and the basis for Article 90 of the NEC). Moreover, the NEC is what governs. The Green Book is an excellent resource but it is also a bit dated, at times written too generally, and has no authority as an installation standard.

If equation 240.92 B is applied to wire type EGCs the results show that exceptionally high short circuit currents, delayed clearing, or reliance on short time rather than instantaneous elements can heat the wire beyond 150*C, sometimes above 1,850*C.

Maybe you don't know this, but that equation is simply the cable damage curve and is something properly evaluated in protective device coordination analysis on a TCC. Furthermore, if you have any experience with cable damage curves, you will know this is practically almost never an issue since the damage curve is so far to the right of the OCPD TCC - even with short-time and instantaneous max'd out (most power system engineers don't even bother showing the cable damage curve for this reason).

BTW, not sure why you are citing Part VIII of Article 240 because these are essentially a set of exceptions to the general rules of Article 240. They only apply to supervised industrial installations and require that qualified/competent personnel design, install, operate and maintain the system - so talking about melting copper in a situation where you are already applying exceptions which offer less protection is counter productive to this discussion. If you aren't a licensed engineer or under the supervision of one, you shouldn't be applying this equation - this is only allowing you to push equipment protection out further and is contrary to your point. Similarly, if you have no experience using this equation (which it sounds like you don't) you probably shouldn't bring it up.

An overheated EGC will cause the loss of the fault clearing path while possibly igniting wire insulation and building material.

So what. Unrelated to this post and my comments about short-circuit analysis device/equipment evaluation.

It is exceptionally relevant, a primary driver above all else.

As stated, 250.4(A)(5) it irrelevant to device/equipment evaluation. If you don't know what that is, please look it up. I don't know how to make it more clear that equipment is rated and tested on a three-phase bolted-fault basis. Anything you insist about ground-faults and EGC's goes out the window when it comes to three-phase balanced faults.

Under high short circuit conditions 3 cycle tripping as provided by MCCBs is not enough to protect small EGCs, and as such sub-cycle (low-peak) current limiting fuses must be employed. Under short circuit limited conditions (such as faults in very long runs) the instantaneous pickup may no be reached, so either a faster breaker must be employed, ground fault protection applied, or the EGC up-sized.

Again, basically everything you have mentioned is irrelevant to what I am talking about - namely short-circuit analysis device/equipment evaluation (which is what I believe the OP is doing). I think you are missing the forrest for the trees by being too focused on your "ground fault loop impedance" knowledge and perhaps looking for ways to talk about it? (you seem to keep bringing it up on this message board)

If you want to talk about "ground fault loop impedance" let's start another thread specifically for that topic and you can tell us everything you think we need to know about it. No hard feelings.

 

[xptpcrewx]

Total system impedance at the fault point is the sum of all individual lumped components up to the fault point.

It may not be the sum. It can be the series, parallel and series-parallel equivalent combination looking back from the fault point.

Loop impedance is simply the worse case (lowest) L-G fault current likely to be encountered in any portion of the system and it is easy to model.

Loop impedance is not current. Loop impedance is loop impedance. Moreover, I don't think it has anything to do (intrinsically) with being the best-case, worst-case, highest or lowest. You may choose to evaluate the loop impedance under whatever condition or wire parameters that satisfies your type of analysis.

The hand equation is over simplified, but it beats electricians having to do sequence components.

I don't know a single electrician that does a hand equation to determine EGC other than just reference 250.122 and maybe occasionally bump it up a size or two based on practical judgement/experience. Similarly, I don't know a single engineer who calculates sequence components by hand for EGC's. Not because it's hard, but because it's not necessary and not typical. This is what software is for... presuming you want to do it in the first place. Based on my experience it's not something designers, engineers or contractors are really concerned about anyway.

AIC uses the lowest conductor temps, EFLI/GFLI uses the highest operating or short circuit temps.

I think what you mean is fault-duty calculations and not AIC (since AIC is a rating). You could still use a better value than 30*C.

 

[mbrooke]

It may not be the sum. It can be the series, parallel and series-parallel equivalent combination looking back from the fault point.

If you have parallel conductor sets those count. Parallel paths like building steel, duct work, ect are ignored.

Loop impedance is not current. Loop impedance is loop impedance. Moreover, I don't think it has anything to do (intrinsically) with being the best-case, worst-case, highest or lowest. You may choose to evaluate the loop impedance under whatever condition or wire parameters that satisfies your type of analysis.

Impedance dictates current flow during a fault for a given voltage.

E/R=I

Because current flow dictates how fast a breaker opens, the lowest reasonable value must be must be used.

I don't know a single electrician that does a hand equation to determine EGC other than just reference 250.122 and maybe occasionally bump it up a size or two based on practical judgement/experience. Similarly, I don't know a single engineer who calculates sequence components by hand for EGC's. Not because it's hard, but because it's not necessary and not typical. This is what software is for... presuming you want to do it in the first place. Based on my experience it's not something designers, engineers or contractors are really concerned about anyway.

Not typical ≠ not necessary.












Real world:

https://www.reddit.com/r/electricians/comments/lnleu9

40 years ago the average electrician, engineer, contractor, ect believed that a ground rod could open a breaker. Many thought 3 wire subpanels were legal. That 10-2 could be used for dryers.

I think what you mean is fault-duty calculations and not AIC (since AIC is a rating). You could still use a better value than 30*C.

AIC must assume the highest current that could be involved, as well as fault duty and conductor withstand.

If wire runs through an unheated area, underground, ect than values below 30*C would actually be necessary.

Conductor withstand:

 

[mbrooke]

You are twisting what the Green Book says just to fit it into your view and insisting on making an unnecessary connection to the OP. Simply put, the Green Book tells you to follow the NEC as a minimum safety standard. At the same time, the NEC doesn't claim to prescribe EGC's that will be adequate under all short-circuit conditions and neither have I - you are making up your own problem to debate about here.

Over ruling, the NEC does prescribe (mandate) that EGCs be adequate under all short circuit conditions through the wording of 250.4 (A) 5.

One thing the Green Book suggests is that the EGC minimum sizes in the NEC may not provide efficient or practical use of certain equipment or when using unreasonably long conductor/raceway systems and configurations.

The words the IEEE use are "adequacy", "safe", "insulation damage", "not capable of protecting", "thermal damage", "short circuit rating"- words which are applicable to the direct safety of life and property vs "efficient" or "practical" which describe mere inconvenience.

Not just long long raceways, but also high short circuits currents, selective coordination and and device time current curves directly relate to the thermal withstand of the EGC.

Thus calculating and knowing the short circuit current is very much relevant to the OP.

There is nothing surprising or disputable about this (it is well known and the basis for Article 90 of the NEC).

Of course! 90.1 C takes us to IEC-60364 which is the core of NFPA-70.

IEC-60364 sets forth the requirements of every electrical code on earth- including earth fault loop impedance, maximum disconnection times, adiabatic limits, ect as explicitly detailed, elucidated and mandated in the standard.

Moreover, the NEC is what governs. The Green Book is an excellent resource but it is also a bit dated, at times written too generally, and has no authority as an installation standard.

Of course, the NEC simply requires that an over current device opens and that take place without creating a danger in the process. That is all is needed from a legal stand point.

90.1 A tells us the code is not an instruction or design manual.

How to go accomplishing it is where other resources come in like education, technical reports, books, apprenticeship, ect, ect.

Maybe you don't know this, but that equation is simply the cable damage curve and is something properly evaluated in protective device coordination analysis on a TCC.

Capable damage is a function of current and time.

 

[mbrooke]

Maybe you don't know this, but that equation is simply the cable damage curve and is something properly evaluated in protective device coordination analysis on a TCC. Furthermore, if you have any experience with cable damage curves, you will know this is practically almost never an issue since the damage curve is so far to the right of the OCPD TCC - even with short-time and instantaneous max'd out (most power system engineers don't even bother showing the cable damage curve for this reason).

Under ideal conditions and assumptions, yes.

BTW, not sure why you are citing Part VIII of Article 240 because these are essentially a set of exceptions to the general rules of Article 240. They only apply to supervised industrial installations and require that qualified/competent personnel design, install, operate and maintain the system - so talking about melting copper in a situation where you are already applying exceptions which offer less protection is counter productive to this discussion. If you aren't a licensed engineer or under the supervision of one, you shouldn't be applying this equation - this is only allowing you to push equipment protection out further and is contrary to your point. Similarly, if you have no experience using this equation (which it sounds like you don't) you probably shouldn't bring it up.

240.92 B validates this discussion as it shows what time a conductor will overheat for a given current.

Adiabatic limits and physics behind it remain the same regardless if an ungrounded tap conductor or an EGC.

I've run the equation more than enough times to know that delayed clearing (or high short circuit currents faster than a device can respond) will melt any given conductor.

I can even show you some examples.

So what. Unrelated to this post and my comments about short-circuit analysis device/equipment evaluation.

If you seeing nothing wrong with putting life and property in danger, one ought not to be to be practicing engineering.

As stated, 250.4(A)(5) it irrelevant to device/equipment evaluation. If you don't know what that is, please look it up. I don't know how to make it more clear that equipment is rated and tested on a three-phase bolted-fault basis. Anything you insist about ground-faults and EGC's goes out the window when it comes to three-phase balanced faults.

Rated and tested, I get that.

However time current curves and trip time still hold applicable to L-G faults.

Again, basically everything you have mentioned is irrelevant to what I am talking about - namely short-circuit analysis device/equipment evaluation (which is what I believe the OP is doing). I think you are missing the forrest for the trees by being too focused on your "ground fault loop impedance" knowledge and perhaps looking for ways to talk about it? (you seem to keep bringing it up on this message board)

If you want to talk about "ground fault loop impedance" let's start another thread specifically for that topic and you can tell us everything you think we need to know about it. No hard feelings.

Impedance and system voltage determines short circuit current. Short circuit current dictates device selection.

I bring do bring up AIC ratings, grounding and bonding, and conductor de-rating because I see it frequently ignored, misunderstood or misapplied.

 

[synchro]

... I'm working on a project that uses a 50Hz electrical system. In SKM there is an option to run studies in 50Hz instead of 60Hz, which I have set correctly. My confusion is that there are cables listed as 50Hz, and cables listed as 60Hz in the SKM library (see image below). I've spot checked similar trade size, quantity, material, etc. the cables have different reactance and resistance values. I understand that Reactance is a function of Hz, so the same cable with 60Hz vs 50Hz would give different Impedances.

Just as an estimate, you may be able to adjust the NEC reactance values in Chapter 9 Table 9. Multiply reactance values by a factor of 50/60 = 0.833.

I agree with Xptpcrewx about how to adjust the reactance values because they'll be inversely proportional to frequency with a given inductance.

With regard to resistance values, I would compare the AC resistance at 60 Hz with the DC resistance for the conductors that you're considering. For example, the difference between the AC and DC values is negligible unless the cable is larger than 1/0 for copper. And the difference will be even less for 50 Hz than with 60 Hz. So if the difference between the 60 Hz and DC resistances is less than a few percent for your cable of interest, I'd go with the 60 Hz resistance values for a 50 Hz analysis and be done with it.

If you really want to dig into some details, the following has some equations showing the frequency dependence of the skin effect and proximity effect in conductors:

myCableEngineering.com > Conductor Resistance

Electrical cable sizing software. Current capacity to BS 7671, ERA 69-30 and IEC 60502. Impedance and voltage drop to IEC 60909 and CENELEC CLC/TR 50480. Cloud based - any device, anywhere.

mycableengineering.com

 

[synchro]

I agree with Xptpcrewx about how to adjust the reactance values because they'll be inversely proportional to frequency with a given inductance.

myCableEngineering.com > Conductor Resistance

Electrical cable sizing software. Current capacity to BS 7671, ERA 69-30 and IEC 60502. Impedance and voltage drop to IEC 60909 and CENELEC CLC/TR 50480. Cloud based - any device, anywhere.

mycableengineering.com

Looking back at my own comment, the reactance is of course directly proportional to frequency in an inductance and so you'll need to scale down the 60 Hz reactance by 50/60 as mentioned by Xptpcrewx to get the reactance at 50 Hz.