SWA current-carrying capability question

A 1.5mm2 3 core SWA cable is typically about 12mm dia and will have a maximum current carrying capacity of about 15A, but I wouldn't normally want to exceed 10A max.

Or could be 2.5mm^2 two core if the armour.

Higher if XLPE ... depending on installation method.

I read it as 18A from the tables. But, with a 10A load, you'll be dropping

(you could of course insulate the earth at the destination and add a RCD there, with a local earth spike making it a TT install)

The long term current limit is set by the insulation temperature.

There are two common types of SWA - PVC insulated (so called "thermoplastic insulation"), and XLPE ("thermosetting insulation").

The former has a maximum conductor temperature of 70 deg C, the latter

So XLPE can carry a higher current - but only in cases where the termination equipment is also capable of operating with 90 deg conductor temps.

The "buried in ground" (directly, on in conduit) is a reference method D install. For a PVC SWA that actually *increases* the current carrying capacity a little bit, and for XLPE it reduces it a bit.

So from table 4D4A - 70 deg C multicore armoured cables A reference method C install would allow 21A, and a reference method D would allow 22A

For the XLPE cable, that would be (from table 4E3B) reference method C:

So with PVC cable and a 20A MCB, you could run at the full 20A, and with XLPE you could use a 25A MCB if you can find one.

However the voltage drop might be borderline, since when operating at max temp the voltage drop per meter per amp is 29mV at 70, or 31mV at

1. That would translate to 10.4V or 11.2V assuming 18m of cable. (you can have 11.5V for normal circuits, but only 6.9V for lighting circuits)

So either is fine for "normal" use. For charging an EV, then a "granny" charger is ok, you might also be able to go for 5+ kW charger, but not the full 7.3kW of a 32A circuit.

Many thanks for the full and interesting reply, and thanks too to all those that responded.

I would have answered your post sooner, but was waiting for the afternoon shade to come round so I could take the cover off the wall-mounted junction box after isolating the circuit.

The diameter of the SWA is 16.8mm and not as previously stated. It contains three cores of red, blue, and yellow. These individual cores are very close to 4mm diameter. Red is connected to house red, blue to house blue, and yellow is connected to both the house green/yellow and the tab of a washer that holds the fixing gland in place. All of these conductors look to be the same diameter.

From the link you gave, this would strongly indicate conductors of 6mm^2 with a cable current rating of 42A (PVC) or 53A (XPLE) presumably in free air.

This is far more current than might be required, and presumably this cable could be run at 10 to 16A continuously (although the latter isn’t a likely modus), the circuit breaker being rated at 16A.

Yup, today was not a good one to be sat in the sun! It was 29 for a bit here.

Yup SWA cores are normally all the same diameter (and three core would typically carry three phase, so they would need to be!)

Yup 6mm^2 gives far more options. Also by combining the armour with a core, that also gives you enough area of CPC to function as a main bonding conductor as well since you have 6mm^2 of core and 23mm^2 [1] of armour, which you can divide by 2.255 to get the "copper equivalent" size of 10.2mm^2, or over 20mm^2 in total for your CPC. Which is plenty for a main bond [2] to pass the adiabatic heating under fault conditions check [3]

Many thanks once again for a full and interesting reply. It would seem that this cable has been well spec’d and installed. With the right breakers in the house and garage CUs, it could carry 7kW if necessary.

One slight fly in the ointment is that the thin steel nut that holds the gland to the house box on the outside wall appears to have corroded and delaminated, although the brass earthing washer underneath it and everything else there seems fine. I can see no evidence of water ingress.

Replacing this with a proper earthing nut seems to be a straightforward exercise, so I’ll get one ordered toot sweet.

Thanks for the links, they made interesting reading!

Indeed... it makes sense when installing feeds to outbuildings to overspec the cable since it is a one off cost, and often small compared to the cost of the whole job.

Yup those skinny nuts are a bit crap really. Hence the number of after market nuts that do a better job and make it easer to terminate the earth connection without needing to rely on the "banjo" tag. Some examples here:

Only thing is you can't do it without disconnecting all the wires :-)

Glad they helped... those pages seem to get quite a few visits.