How can i accurately calculate the amp rating of an installed armoured cable?

Bit of back ground:

The garage is detached from the house and is powered by an armoured cable which is dug into the ground and runs the length of the garden.

The armoured cable then runs into a second Consumer Unit in the garage. The cable is a 3 core of old colour wiring (Red, Black and Yellow). The cable has been installed for the last 30 years.

The armoured cable is protected by a 16amp MCB in the house Consumer unit.

My Plan:

I can measure (using digital calipers) the diameter of the armoured cable at a junction box in the garage. Using the diameter i can then work out the cross sectional area: A=pi times radius squared. Looking at the cable i would expect the figure to be around 2.5mm2. This is just a guess at the moment.

Once i have the cross sectional area of the armoured cable how do i accurately calculate amps it can carry?

I don't want to over load armoured cable and play on the side of caution.

Thanks

Ash

If you can get to see the oversheath it will have the cable size embossed upon it.

Failing that you can get an approximate size by taking the overall diameter of the cable over the sheath and looking it up in cable tables.

Once the size is established you can look up the current in the cable tables too.

A 3c 2.5 mm armoured cable is 14.1mm overall diameter and can carry
24 amps.

Apologies its 42 amps must be dixlexicts!

[Edited on 30/12/13 by big-vee-twin]

Once you know the csa of the conductor and the length of the cable run you can use this Cable calculator link to work out if it is up to scratch.

Stu

I'm sure there's a formula for it, but if you know the mm˛ then try this http://www.cable-ratings.co.uk/
According to the table 2.5mm˛ 3 core can carry 42 amps when buried and has a voltage drop of 16mV per metre.

[Edited on 30/12/13 by Smoking Frog]

Thanks for the swift replies. I would love to just look at the numbers embossed on the cable but that not an option unfortunately. Way too easy for my luck...

I have had a quick look at the cable calculator. I think i may have been worrying about nothing, but need to verify.

Please correct me if i have made a stupid mistake.

The main house consumer unit MCB that protects the cable is rated to 16amps so the load will never exceed 16amps or 3.5 KW on the cable.

By my calculations for a 16amp load a cable of 1.5mm is the minimum and could take upto 21amps.

So if the cable does turn out to be 2.5mm i'm well in.

With a load of 14A the maximum cable length for 1.5mm is 30.77m to keep you at 5% volt drop which is the maximum for non lighting ccts but im sure you have lights in the garage therefore you maximum volt drop should be 3% which at 30m means that for 14A the cable should be 2.5mm and at 15A needs to go up to 4mm. and jumps to 6mm at 23A.

RD

[Edited on 30/12/13 by rallyingden]

[Edited on 30/12/13 by rallyingden]