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Max length of control cable from MCC to control station next to motor
2

Max length of control cable from MCC to control station next to motor

Max length of control cable from MCC to control station next to motor

(OP)
Hello,

I'm sizing the control cables that go from an MCC to the respective control stations next to each motor (Start+Stop Pushbuttons on the field)

In some cases the cables reach up to 500 meters long, and I would like to know how I can choose the right cable for each case (IEC-mm²).

In general, we are planning to use 7x2.5mm² cables as a standard in the project

In the link below, I found information about the effect of capacity on long cables, but I hope someone can give me some advice based on their experience, as we are buying a Schneider Electric Okken MCC and the interposing relays were not yet defined

Link: http://www.moeller.net/binary/ver_techpapers/ver94...

Best regards,

Juan

PS: I hope to have posted this topic in the right place, if not, please let me know

RE: Max length of control cable from MCC to control station next to motor

If you use AC control then your choice of control voltage is very important. I did a few calcs a while ago for a typical mini-contactor or heavy pilot relay, and concluded that 110V / 120V AC was around the sweet spot for maximising distance taking into consideration volt-drop, pull-in current, capacitance, etc. I probably still have the spreadsheet somewhere.

If you use DC control then you can go a long way at 120V DC, and a lot further at 240V DC, but DC is hard on your switching devices.

RE: Max length of control cable from MCC to control station next to motor

(OP)
Scotty, thanks for your replay

Luckily, we are using 110 Vac as our control voltage

If you find that spreadsheet, I would appreciate it if you can share it

I was researching with our vendor and he gave me some tables of their own, but I would like to have a more generic calculation, in case the same situation happens in the future with other equipment

Sincerely,
Juan

RE: Max length of control cable from MCC to control station next to motor

This sheet is designed for the the 50Hz world. The underlying calculation is based on work done by ICI many years ago.

The results seem to be fairly conservative - we had no problems in the installation we designed using this spreadsheet.

RE: Max length of control cable from MCC to control station next to motor

(OP)
Thank you Scotty! Such a good spreadsheet

I wasn't considering the disconnection time as a limitation in cable length

You have downloaded it from the internet or created it yourself? If possible I would like to unlock the spreadsheet to follow the calculation procedure

PS: What is ICI? Those acronyms are not known in my country (Argentina)

Regards,
Juan

RE: Max length of control cable from MCC to control station next to motor

Pay the guy with a star Juan! :)

Keith Cress
kcress - http://www.flaminsystems.com

RE: Max length of control cable from MCC to control station next to motor

I created it a while ago, I will have to see if I can remember the password. I'm sorry - I didn't realise it was locked.

ICI was Imperial Chemical Industries, a huge chemical conglomerate in the UK. The company was broken up during the late 1980's and 1990's, with the loss of many jobs and to the detriment of the UK's chemical industry. https://en.wikipedia.org/wiki/Imperial_Chemical_In...

Keith - thanks. winky smile

RE: Max length of control cable from MCC to control station next to motor

(OP)
Keith, thanks for the advice, I'm new to the forum and I do not know the culture haha

Scotty, thanks again!

RE: Max length of control cable from MCC to control station next to motor

Howdy Juan,
With AC control and long-lines; the other issue that is good to avoid is the sealing in (ie holding-in) of a coil. This is a result of leakage current caused by the line-to-line capacitance between parallel conductors within the same cable assembly.
As Scotty pointed out the best way to avoid this is to use DC voltage and interposing control relays. I would think that you could make it work with 24Vdc (even at 500m).
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Max length of control cable from MCC to control station next to motor

(OP)
Hi GroovyGuy,

Thanks for your contribution

Actually i can not imagine a control circuit in which I do not keep a contactor coil o auxiliary relay coil sealed during the motor operation

I understand that the use of an auxiliary relay still has the problem of the cable capacitance, am i right?

PS: I have not made any calculations yet but I think that with 24vdc the command cables must be quite oversized so as not to have a high voltage drop

RE: Max length of control cable from MCC to control station next to motor

Hi Juan,
Yes the issue with leakage-current, caused by the system capacitance, is actually worse with small AC relays, than (say) as compared to
trying to dropout the contactor without a relay.

I think you should consider doing the calc with 24Vdc and 500m; it might work with standard conductor sizes. I have seen this kind of distance done before, at 24Vdc, with #14awg conductor size. Obviously you want to select a relatively sensitive relay.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Max length of control cable from MCC to control station next to motor

Solid-state relays are useful because of the wide tolerance on input voltage. It's worth including a burden of about 1kΩ across the input to reduce the susceptibility to transients and impulses. SSR's can work at distances in excess of a mile on a 24V DC loop. I'm not advocating it as good design, but it's a way out of someone else's messed up design when you inherit the problems.

RE: Max length of control cable from MCC to control station next to motor

(OP)
Hi,

Our control voltage is already chosen (120 VCA) but i'll keep in mind your advices for a future

Thank you!

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