Selecting power cable based on duty cycle

[syssafety]

Could anyone point me to resources on calculating the equivalent continuous current rating based on a certain duty cycle, when selecting power cables?

We have an application where we have to select a power cable which carries a max. current of 550A when 'on'.

I have seen a calculation for welding cables which is something like Continuous RMS Current = Current during 'On' time * square root (Duty Cycle)

Thanks in advance.

 

[jbartos]

Suggestion: The duty cycle for the current is actually the current profile. The current profile can be used to obtain the current average. If the ac current is represented by rms profile, one obtains an average of the rms current profile. If the dc current has a profile, then the dc current average is obtained. This is frequently used in stationary battery calculations.

 

[davrom]

Hi syssafety,

It does not appear from your message if there is about a 3P (three-phase) or 1P (one-phase) system, and AC or DC system. This has impact on the number of conductors of the supply (feeding) cable, resp. 3 or 4 conductors for 3P systems (3 conductors for the 3 phases and 1 conductor for ground/safety neutral if provided) and 2 or 3 conductors for 1P systems (2 phases + 1 ground if provided) and on selection of cable (for DC systems the voltage drop is pretty big and consequently the cable size must be increased).
Please be more specific regarding the term "duty cycle". I suppose that there is about the operation of the application, e.g. 1 hour on with 30 min. off, repeating again 1 hour on with 30 min. off etc.. In this case the size of feeding cable can be smaller than the cable required for a continous duty/operation.
You must know that there is a correspondance between the carried current (550 amps in your case) and the size of the cable (by size meaning the cross-sectional area of conductors in square mm in European style, resp. in AWG or 1/1000 cmils in US style). Please let me know what style you prefer. As your current of 550 amps is pretty big, it may result you may need to install 2 or 3 cables in parallel. Furthermore, you must know that there are defined several operations (duty cycles) for applications, most utilized being S1-continous operation, S2-30 min and S2-60 min operation.
The selecting of feeding cable is made based on a selection table wich indicates the maximum current that a cable of a certain size can support for the respective duty cycle. You can find these tables by searching on internet on national regulatory bodies database. I can indicate you the cable size for your application using the tables used in shipbuilding industry (American Beaureau of Shipbuilding and Germanischer Lloyd), but I need more info, resp. duty cycle, voltage, AC or DC system, 3P or 1P system, power factor (if AC system), rated electrical power of application. You can read these data on the name tag of application/equipment.
Regarding the rms value: this means "root-mean-square" value and, following my experience, is only used in short-circuit currents theory - indicates the DC equivalent current of a AC current wich develop in 1 second into a resistor of 1 ohm the same energy developed by the AC current in an electrical network in the entire period. I have never considered this value when I have selected a cable.
As a last note: about rms value and more you can read on

http://users.telenet.be/educypedia/electricity.htm

and clicking on the respective link.

Hope I have been usefuly to you.

 

[jghrist]

Accurate calculation of cable ratings under cyclical loading would require calculation of the transient heating at various times, considering the thermal time constant of the system. The Neher-McGrath method uses a simplified method of modifying the cable external thermal resistance to account for a daily load cycle. See The calculation of the temperature rise and load capability of cable systems, Neher, J.H., and McGrath, M.H., AIEE Transactions, vol. 76, part 3, pp. 752-772, October 1957.

 

[busbar]

A very informal, shot-from-the-hip approach may be an estimate based on the material in NEC Article 630.

 

[alehman]

One thought - in order take advantage of the intermittent duty cycle, the cycles would have to be short because the thermal capacity of the cable is small. If the cycles are sufficiently short, I would think you could use a simple RMS method to calculate the "average" current. The NEC doesn't recognize duty cycle except in a few special circumstances.

If cycle time is long, the thermal capacity of the conductor becomes less significant, and for underground applications the Neher/McGrath method comes into play. The NEC does recognize N-M for underground installations.

 

[jbartos]

Suggestion: Cables are relatively inexpensive items in comparison with other hardware, e.g. transformers, generators, etc. Therefore, it is not unusual to see cables and transmission lines to be rated for the worst case loading pattern.