32A to 16A 3 phase question

[john87]

3 minutes ago, GarethM said:

@john87 pretty much was going to be my next question as extension leads say 4mm+ get very pricey and liable to get damaged.

 

Any chance you could bury SWA closer to the job ?. The cost of a short extension would be peanuts 😀

That is a good idea!!

[GarethM]

Obviously that is 11.5A per phase, depending on the motor type it can be 3 or even has high as 7 times at start up and then voltage drop.

 

I've supplied 4kw stuff with 20m x 2.5mm, but it was always a risk as one damaged core and it's in the skip.

 

In the end just advised the customer to move the supply to within 10m as a 4mm+ extension is like getting a remortgage.

Edited February 29 by GarethM

[john87]

1 hour ago, GarethM said:

Obviously that is 11.5A per phase, depending on the motor type it can be 3 or even has high as 7 times at start up and then voltage drop.

 

I've supplied 4kw stuff with 20m x 2.5mm, but it was always a risk as one damaged core and it's in the skip.

 

In the end just advised the customer to move the supply to within 10m as a 4mm+ extension is like getting a remortgage.

 

Yes, it is 11.5A per phase, but then with the PF and Eff, it will be just over 16A per phase.

 

Calculations are difficult as we do not know anything about the supply characteristics.. You are supposed to have a maximum volt drop of 5% in general and 3% for lighting, so lets say we will go for 3% in our extension cable as the rest will be used up in the installation cabling..

 

I could not find an online calculator that actually worked, so i will do the sums manually later and post back..

 

john..

[john87]

Hi there,

 

Done some number crunching for you..

 

Just to carry 16A with the required max volt drop over that distance, [35m] works out at 2.5mm2 cable.

 

BUT, as i said the governing factor will be EFLI [so that the circuit breaker operates] and, do not forget that to START the motor will take about 6 to 10 times this current, so you are in practice, to start the motor reliably you are going to want a much larger cable or the motor simply will not start [and contactors will drop out due to undervoltage] and you will end up in a right mess..

 

So, taking an educated guess at the SUPPLY characteristics, you want the following, assuming the breaker protecting the circuit is a C32.

 

I am also assuming that you require a neutral too

 

IF, you want an armoured cable to bury in the ground, you want a 10mm2 four core, and you will use the armour as CPC [earth]

Note, you CANNOT use an armoured cable as "flex", it is not designed to be repeatedly bent, and besides, it would be terrible to handle.. This cable would be about 21mm in diameter and weigh about 36kg.

 

IF, you want flex, you want something called "HO7" This is the most beautifully supple, lovely, rubber covered stuff, Lovely to handle even in the freezing cold. With this, you want 10mm2 FIVE core..This cable would be about 24mm in diameter and weigh about 35kg.

 

The armoured cable will cost you about £264, plus the glands and adaptable boxes to terminate it into, and someone that knows what they are doing to fit the glands and terminate it for you.. [this is very tricky to do properly unless you know what you are doing]

 

If you want the HO7 flex, it will cost you about £390, plus a "trailing socket" for one end and a plug for the other..

 

Any questions, ask..

 

john..

 

[GarethM]

The extension is HO7RN-F, just ensure it doesn't get twisted as the cores will break over repeated coiling and uncoiling, it's thin copper wires after all.

 

You can get CY cable extension, it's braided for a little extra protection from damage but is like wrestling a bear in gloves whilst greased up like a mackerel.

 

You should be okay on 6mm extension, usual rules of don't leave it coiled up in use etc.

Edited March 1 by GarethM

[john87]

2 hours ago, GarethM said:

The extension is HO7RN-F, just ensure it doesn't get twisted as the cores will break over repeated coiling and uncoiling, it's thin copper wires after all.

 

You can get CY cable extension, it's braided for a little extra protection from damage but is like wrestling a bear in gloves whilst greased up like a mackerel.

 

You should be okay on 6mm extension, usual rules of don't leave it coiled up in use etc.

 

As you say you can get CY and SY, [the CY has a copper braid and the SY a steel one] but they are not intended as power cables, [they do not meet any standards for this] but for control circuits.

 

The braid is to stop EMF interference affecting whatever they control They are also HORRIBLE to handle as you say, especially in the cold. You can also have problems caused by the braid if the cable is damaged as the braid is not sufficient to act as a CPC, but IS sufficient to "liven up" whatever it is connected to.

 

The HO7 [HO7RN-F] is specifically made and INTENDED as "flex", that it why it has the "F" bit standing for "flexible conductors" which is why they ARE fine, that is the idea of it, they will flex without cracking in the same way as you could bend a sheet of paper to and fro as much as you liked, but if you did the same with thick card, it would fail before long.

 

Have a read of this..

https://www.elandcables.com/media/13rlt2rk/ec-statement-on-the-use-of-sy-cy-yy-cables-rina-18th-ed-with-summary.pdf

 

john..

[sandspider]

Sorry to hijack the thread, but I occasionally use a 7T forest master log splitter with a 25m extension cable. Think the splitter is 2200kw. I know using it with an extension lead longer than 10m isn't ideal, but it seems to work fine, and has done for years now. Am I damaging it?

[GarethM]

Most extension leads bought pre made will have a power rating on them.

 

Think a 25m is literally on the limit, so nothing else running off the extension or uprate to something that's 2.5mm instead of the basic 1.5mm

[Dan Maynard]

3 hours ago, sandspider said:

Sorry to hijack the thread, but I occasionally use a 7T forest master log splitter with a 25m extension cable. Think the splitter is 2200kw. I know using it with an extension lead longer than 10m isn't ideal, but it seems to work fine, and has done for years now. Am I damaging it?

Other thing about your log splitter is the motor starts under no load, so you don't have the worst case situation John is talking about above when the compressor melted.

[sandspider]

Makes sense, thanks.

 

I seem to remember FM don't recommend using one with an extension lead longer than 10m, but it seems to work...