Cable Ampacity Calculations
Cable Ampacity Calculations
(OP)
Gents
I am working on Cable Ampacity Calcs on EDSA software. Three single phase cables are going to a transformer and three out going feeders (9 cables-single phase cables) are placed in a concrete duct bank - all cables have concentric neutral wires for return current. The cables are designed to carry neutral current equal to 1/3rd of phase current.
The software can model the cable with concentric neutral wires - but there is no means to add this return current.
My problem is that how to take this neutral current in to account - do I have to add this neutral current into phase current and calculate total temperature rise?
- or does the software takes into account the neutral current once concentric neutrals are meodelled?
- or anything else?
- Can anybody give an idea?
Thanks
I am working on Cable Ampacity Calcs on EDSA software. Three single phase cables are going to a transformer and three out going feeders (9 cables-single phase cables) are placed in a concrete duct bank - all cables have concentric neutral wires for return current. The cables are designed to carry neutral current equal to 1/3rd of phase current.
The software can model the cable with concentric neutral wires - but there is no means to add this return current.
My problem is that how to take this neutral current in to account - do I have to add this neutral current into phase current and calculate total temperature rise?
- or does the software takes into account the neutral current once concentric neutrals are meodelled?
- or anything else?
- Can anybody give an idea?
Thanks






RE: Cable Ampacity Calculations
Thanks
RE: Cable Ampacity Calculations
The EDSA model can only show concentric neutral wires - there is no means to show whether the neutrals are grounded or not.
RE: Cable Ampacity Calculations
I understand that you use "single end grounding" method - only at one end? Is this correct?? If it is, you do not have to worry about the cable concentric neutral (assuming that the concentric neutral wires have a jacket isolating them from the ground) as it will not carry any current.
If, like you said, there is no place to inform software what kind of grounding is I would call EDSA support and ask them how they modelled the algorithm for calculation. It is obvious that the grounding methods affect the cable ampacity if it is different than described above.
RE: Cable Ampacity Calculations
RE: Cable Ampacity Calculations
The power loss in sheath or screen [lambda1] consists of losses caused by circulating currents [lambda'1] and eddy currents [lambda"1]. The eddy current in this case is negligible so only circulating current could be taken into consideration as: Ineutral ^2*Racneutral/ (3*Imainconductor^2*Racmain).
Racneutral could be Rdcneutral for 90oC if the insulation is XLPE and Racmain could be calculated from Rdcmain recalculated for 90oC and adding skin effect and proximity effect as per IEC60287.
RE: Cable Ampacity Calculations
RE: Cable Ampacity Calculations
RE: Cable Ampacity Calculations
RE: Cable Ampacity Calculations
I believe It is possible to model the effect of the circulating sheath currents without actually having a knowledge of the current magnitude.
If the temperature of the sheath is assumed to be maximum rated sheath temp (say 90Deg) then you could calculate its effect as part of a thermal circuit.
This would be effective for giving max ampacity of the cable. but may fall down in dynamic ratings and FEA studies at say half rated current.
Just a thought.
Andrew
RE: Cable Ampacity Calculations
RE: Cable Ampacity Calculations
Good point. I dont know my modelling well enough to comment.
Anyone know where to get humble pie?
Andrew