DLO cable ampacity and impedance modeling
I am working on a study where a plant has used a bunch of DLO type cabling for the distribution cable throughout the plant. I had two questions I was looking into regarding how to view/model the DLO cable.
1) What is the best reference to use for the ampacity of DLO cables? More modeling purposes can it be cross-referenced to a standard size cable? For instance can a 535kCMIL be viewed as a 500MCM cable when looking at ampacity?
2) What is the best way to model these DLO cables from an impedance standpoint? In other words if these cables are being represented as impedances in a SS study what is the best way to model them. Are they close enough to the standard cable types for which most software packages have impedance models for? Again can a 535kCMIL be cross referencd to a 500MCM cable?
All DLO manufacturers have properties and ampacity tables available on the web. a 535KCMIL will have a greater ampacity than a 500KCMIL.
Originally Posted by mull982
What software package are you using? If you're using something like CYMCAP, I'm pretty sure that the cable impedance for standard cable sizes is contained in their library, and you should be able to arrange your cables as desired, set the maximum permissible temperature (for your specific installation), and determine the allowable ampacities. If not, they do have an add-on for calculating cable impedance....I'm sure other packages have similar tools (never used it, can't say how it works...).
I'd be cautious using the ampacity tables from the manufacturer, unless your conductor is clearly oversized and your actual environmental conditions are conservative compared to the 30deg C environmental temp that is usually specified. You may also want to ensure that the temperature of your conducts stays below the high withstand temp of DLO cable, too - in this case you're best to proceed building a steady-state thermal model to evaluate your cables. Good luck!
When looking at cable sizing and protection per the NEC can you use the manufactures publised ampacities or must you use some value in the NEC tables for the cable size?
Originally Posted by jjkind
The software package i'm using is SKM
There is really only one table to use for most purposes under 600V and that is the 75 degree chart in the code.
Originally Posted by mull982
many times manufacturers claim their cable can take huge amounts of current by comparison to this chart becasue the cable and insulation can take it but the terminations at the devices almost never can take that kind of heat.
IEEE Std 141 - Recommended Practice for Electric Power Distribution for Industrial Plants - gives some good guidance on conductor selection (see pg. 571, 12.4.2 - conductor selection).
Conductor size is based on the following criteria:
a) Load current criteria as related to loadings, the NEC requirements, thermal effects of the load current, mutual heating, losses produced by magnetic induction, and dielectric losses
b) Emergency overload criteria
c) Voltage drop limitations (3-5%, as specified by the NEC)
d) Fault current criteria (the Std has a table for allowable ampacities in a table of clearing times vs. conductor sizes)
e) Frequency criteria
f) Hot-spot temperature criteria
g) Length of cable in elevated ambient temperature
h) Equipment termination requirements
Goes on to say that "large groups of cables should be carefully considered as deratings due to mutual heating may be limiting.."
I'd highly suggest to the original poster, if he hasn't already, to contact the cable distributor and ask your impedance question (please post their reply here, too, as I'm curious to know if this depends on manufacturer or if the defaults in your software can be used...I've never had to model DLO cable, just URD cable, but I've seen the question raised for DLO cable in a wind turbine application). Then, once you've modeled the steady-state temperatures around the cables, go back to the NEC and look at what kind of de-rating it gives you. I'd use the smaller ampacity between your software and the NEC, just to be conservative (as you noted, you'd have to interpolate an ampacity somehow, since 535kcmil is not a listed size...or perhaps check in the IEEE S-135 source document, perhaps they have a method there for calculating the ampacity of 535kcmil)
Finally, as Bob mentions, check the temp. limitations at your terminations and compare that to the conductor heating.
You may need to check your fault current criteria, but that should be it to the process.
I looked at the SKM software and unfortunately there is no trial to download, though it would be interesting to try it out.
Last edited by jjkind; 10-30-11 at 09:24 PM.
I was able to find a cable model to use in SKM for modeling the DLO cable for impedance in a fault study.
My question now is what ampacity to use for these cables when analyzing these cables for adquate protection in a system. If I am looking to see if these cables are adequately protected by their OCPD should I use the standard manufacturer published tables or use some table in the NEC?