Armoured Cable - What Size for 80' run??

[Lionheart]

Hi guys

I need to run an armoured cable from our electric supply to our new garage build. It sits around 80 feet from the house, and I will be using power tools etc in the garage.

What size of cable would I need to cover an 80 foot run?

 

[RF Lighting]

How long is a piece of string?

 

[plugwash]

I'd be looking at a 32A circuit, running some quick calcs on TLC's volt drop calculator with a distance of 30M (a litte more than you specified but people often underestimate cable length) and using the "lighting circuit" volt drop rules (since this will almost certainly be supplying lighting) you'd be looking at 10mm.

 

[Belboz]

I think what you are getting at is the idea that a longer run of cable will require a bigger size due to a drop because of the distance. The professionals on here will tell you the answer but I don't think it makes any significant difference.

The key issue is what you will be running the other end i.e. the size of what you will be expecting to supply.

You need to work out exactly what you will be having first e.g. just one socket and a light will require less thickness than 4 sockets and 3 lights.

If you work this out then ask on here they may tell you the answer you seek.

 

[ericmark]

Not enough information I use I use this calculator it has more the really required but has de-fault values hope it helps.

 

[Lionheart]

Hi guys

The use at the other end will be relatively heavy. I'd run a ring of sockets (6 sockets, plus quite a few lights (it's a four car garage).

I'll be using power tools for sure, plus heater, air con unit etc. Doing a lot fo work on cars, so all related tools really.

Does that help?

 

[plugwash]

You need to work out exactly what you will be having first e.g. just one socket and a light will require less thickness than 4 sockets and 3 lights.

Number of sockets and lights isn't massively relavent. Lights are a relatively trivial load and sockets are just connection points.

What really matters is the equipment that is likely to be in use at the same time. You need to think about what equipment is likely to be in use at once, look at it's current requirements, round up to the next standard breaker size and then calculate a cable size to match.

My initial post was based on the assumption that this was a fairly normal domestic garage and so 32A would be ample but lionheart's latest post is now implying it's a place used for fairly serious work on cars.

I suspect 32A will still be sufficient if there is only one person working in the garage at a time and the heating/aircon is a small portable unit (e.g. one that can be run off a 13A plug) but if multiple people are likely to be using power tools in there at once i'd want to go higher than that.

 

[Belboz]

Thanks plugwash.

That's what I was trying to get at when I said

'The key issue is what you will be running the other end i.e. the size of what you will be expecting to supply.'

I just didn't explain it as well as you did.

 

[Lionheart]

Thanks guys!

No, it would be me working solo. I am going to run a 32 amp ring main and also lights.

General power tool use, plus heater or air con unit. Nothing amazingly drastic, but it certainly will be worked in daily (by one person).

What do you think would be suitable?

 

[plugwash]

As I said earlier based on a 32A circuit used to supply lighting and a slight rounding up of your distance I came up with 10mm. That assumes that the limiting design critera is volt drop (it USUALLY is but there may be exceptions) and that the circuit is fed from the origin of the installation.

If you are going to be getting an electrician to connect up and certify it is important that you get them involved from the start, since they have to be satisfied that the type, size and installation method of the cable are satisfactory. They will likely also be able to source the cable cheaper than you will.

 

[Lionheart]

Hi guys

Problem has arisen. We need to run the cable a different route, which means it's nearer the 45m - 50m mark

Would 10mm still be suitable?????

 

[JohnW2]

Problem has arisen. We need to run the cable a different route, which means it's nearer the 45m - 50m mark ... Would 10mm still be suitable?????

Strictly speaking, not quite (in terms of lighting voltage drop with 32A and 50m). The next size up is 16mm².

Kind Regards, John

 

[EFLImpudence]

Strictly speaking, not quite (in terms of lighting voltage drop with 32A and 50m).

Oh, that's a bit strict. Isn't 49m (less final lighting circuit) the actual limit for 3%.

 

[JohnW2]

Strictly speaking, not quite (in terms of lighting voltage drop with 32A and 50m).

Oh, that's a bit strict. Isn't 49m (less final lighting circuit) the actual limit for 3%.

I did say "strictly" You're right at 70°C (i.e. PVC), but I think the limit is more like 45m (less final lighting circuit and anything else upstream of the SWA) at 90°C, so it depends on what cable is being used.

You and I both know what the pragmatic answer is, in either case, but that's a different matter from 'advising'!

Kind Regards, John

 

[ericmark]

With a correction factor of 0.94 I get volts just within limits at 50 meters 6.6 volts using 10mm cable when using a 32A supply.

I have many times questioned using a 32A supply in a house to feed a 32A plus a 6A MCB in a garage as in theory the house MCB will trip first. Yet feeding with a 32A MCB to a garage consumer unit with 2 x 16A and 1 x 6A MCB I would consider OK even if garage had 4 x 16A I would still consider it to be OK.