Cable type and length having an effect on fault calculations

[mull982]

I have recently been playing with SKM to see what effects differenct cable arrangments had on fault currents. I noticed the following and was interested in hearing comments, or explanations to some of the questions that resulted from my modeling.

I first noticed that a 3/c tray cable had a lower overall impedance when compared to 3 single conductor cables in a conduit, the reason being that the reactance of the 3/c cable is less with the resistances being the same. Does anyone know why the reactances are lower for a 3/c cable? From what I can tell this can impact fault currents from a modeling perspective however mostly in the higher cable sizes? Do others agree?

I also noticed that cable reactances are lower when installed in PVC conduit vs rigid steel conduit. What causes these reactances to be lower?

Also when doing data collection in the field I was wondering how close one needed to be when measuring cable distances and how this distance accuracy would effect calcuated fault currents. From playing around with a few models I noticed that for larger cable sizes around 500MCM a 10% difference in cable lengths resulted in about a 5% differnece in fault currents and this seemed to be linear. For smaller cable sizes a 10% difference seemed to have a more dramatic effect on fault currents that didn't really change much between different 10% differences. Does anyone know from experience or a rule of thumb what % accuracy one should have when determining cable lenghts for fault studies, or what the relationship is for differences in lenghts?

 

[mayanees]

my practice

my practice

mull,

Here's my two cents on cable types and lengths.

Fault current is in fact reduced when the cable is installed in magnetic raceway, based on the magnetic effect of the conduit, which is consistent with your reactance statement.

As far as accuracy of cable lengths, I always used to say +/- 10% as a general rule, and then any resultant calculation that approached the limit of a device would be scrutinized for accuracy.
But Arc Flash has made it that much more important to be accurate with feeder lengths. I still apply a similar line of thinking though, that if a calculated HRC level is marginal, further scrutiny is warranted.
I try to get the electrical foreman's pull sheets where the lengths are very accurate.

John M

 

[don_resqcapt19]

The reactances are lower for a cable than for conductors in a raceway because the conductors are closer together and held in a specific formation within the cable.

 

[ron]

I have recently been playing with SKM to see what effects different cable arrangements had on fault currents. I noticed the following and was interested in hearing comments, or explanations to some of the questions that resulted from my modeling.

I first noticed that a 3/c tray cable had a lower overall impedance when compared to 3 single conductor cables in a conduit, the reason being that the reactance of the 3/c cable is less with the resistances being the same. Does anyone know why the reactances are lower for a 3/c cable? From what I can tell this can impact fault currents from a modeling perspective however mostly in the higher cable sizes? Do others agree?

I also noticed that cable reactances are lower when installed in PVC conduit vs rigid steel conduit. What causes these reactances to be lower?

Also when doing data collection in the field I was wondering how close one needed to be when measuring cable distances and how this distance accuracy would effect calculated fault currents. From playing around with a few models I noticed that for larger cable sizes around 500MCM a 10% difference in cable lengths resulted in about a 5% difference in fault currents and this seemed to be linear. For smaller cable sizes a 10% difference seemed to have a more dramatic effect on fault currents that didn't really change much between different 10% differences. Does anyone know from experience or a rule of thumb what % accuracy one should have when determining cable lengths for fault studies, or what the relationship is for differences in lengths?

I agree with the previous statements.
Conductors held closer in a cable together offer lower reactance.
Steel will act as an choke (inductor), which is why PVC offers higher calculated fault currents.
Regarding accuracy; be as accurate as you can. Lengths in an existing building is practically impossible. In a new installation, often the installer has conductor lengths because they need it for their estimating and the inevitable conductor pull.