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Joined: Oct 2014
Posts: 5
PN1 Offline OP
New Member
I have an installation where i have a 125Amp MCCB to a 50Kva transformer. My cable size is 35MM. The distance of the conductor is approximately 300 feet. My question is. Is the 35MM(2/0 conductor of the right size for the distance of the cable run. Insight will be most appreciated.

Joined: Dec 2001
Posts: 2,495
Gut reaction: no.

Reasoning: while tables do vary, we can safely assume that the rating of copper wires is roughly the same around the world. According to the Austrian table, 35 mm2 is only good for 119 amps if there are three current carrying conductors (i.e. a 3-phase system, the neutral is not considered a current-carrying conductor in this case, only in single phase applications) and at 30 C ambient temperature. At 25 C we can go up to 128 amps but considering you're in Kuwait I seriously doubt you would get by without exceeding that temperature even if the cable is run underground. Sooo, even without doing any voltage drop calcs it's at the very best borderline.

I quickly did an online voltage drop calculation and that read 1.9%. Way too much for a main feeder, you'd want more like 0.5% for anything close to European standards (most European countries specify no more than 4% in the entire fixed installation).

Unless this is a "make do" situation definitely upgrade to 50 mm2 or downgrade to 100 A or smaller MCCB (and even that results in a rather bad 1.5% voltage drop).

Joined: Jun 2004
Posts: 1,273
Temperature de-rating is a major concern.

In Las Vegas, Nevada, USA, everything has to be up-sized -- and the authorities want every conductor above ground whenever this is at all practical.

In Kuwait it has to be a hot as Las Vegas. You have to design for the heat -- worst case.

So it's also important to use insulation that has the highest temperature rating that's financiall viable. (It being understood that MI is totally impractical.)

The last time I looked cross-linked polyethylene was the favored insulation. ( XHHW-2 for Americans )

I would expect that the local trade has already resolved the insulation type for your climate.

BTW, 125A for a 50kVA transformer doesn't add up.

I'm suspicious that your climate forces everyone to use oversized thermal circuit breakers. You may be running far fewer amps than us temperate climate electricians are used to.

Without actually knowing your loading (amps & volts) I hesitate to run the numbers.

Joined: Oct 2014
Posts: 5
PN1 Offline OP
New Member
Thanks for the insight. The cable is rated to 50cecius. Originally I wanted to use a 30Kva transformer but out here it is near impossible to locally find step dowen transformers from british to american voltage. Again thank you.



Joined: Dec 2001
Posts: 2,495
50 C conductor temperature?!? That'll force you to derate even further!

Is your MCCB supposed to protect the transformer? Then I'd say start by sizing that appropriately and then calculate cable size etc.

Joined: Jul 2002
Posts: 8,412
Likes: 3
What voltage is this system?

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