Sensible cable gauge for 20A spur load?

[Eric the Viking]

I would think 4mm (CSA is sensible), but I may be over specifying it - can anyone put me straight please?

It's for a water heater pulling 15 to 20A.

TIA, E.

 

[novocaine]

you need to do the calcs for that Eric. this will need the length of cable so get you ruler out.
this could be of help, but use it at your own risk.
http://www.paigewire.com/pumpWireCalc.a ... eSupport=1

 

[Lurker]

Depends how long a run it is.
Most likely a bit over specified, but only downside is extra cost of the cable which is probably fairly negligible.

Edit: wot Dave said.
Further edit: what fuse are you planning to fit?

 

[Eric the Viking]

It's got a 30A breaker fitted. The data plate says it's around 4.5kW, so that's about 19-20A at 240V.*

It's the whole-house water heater in my son's new place in the USA, and it doesn't work.

In UK terms it looks like 1.5mm twin+earth to the heater, and I think that is far too small (not least because the earth is weedy!). I suspect the heater isn't new, and was a quick replacement to sell the house, and it's possibly (probably) not even wired in properly.

I've been trying to help him diagnose it remotely, and it dawned on me overnight that irrespective of what's actually wrong, the wire gauge is obviously too small for the task (never mind that there are several breakers in series too). I'm not yet sure how far the run is to the incomer and main disty board, but it's off a sub board in the basement, and through another isolating box, both of which have 30A breakers in them.

I think 4mm cable (say 12 gauge wire, which is a bit smaller) should be good. I calculate the element resistance should be around 0.08 ohms (yes I know it will change with temperature).

I hope we're going over there in February, so I'll probably have to spend a bit of time trying to work out what's routed through where for him. Meanwhile he'll have to get the mess sorted out by a professional or two - plumber and sparks. I wanted to be sure I'm being sensible about the wire gauge.

Thanks all, E.

*Not 120V - it has a twin phase connection.

 

[9fingers]

Unless it is a huge length 2.5 mm is adequate

To back this up the approved easy fix for a broken loop ring main is to split the end at the cu and fit each half with a 20 a breaker.
Bob

 

[Eric the Viking]

Thanks Bob, it makes sense.

I didn't know it was a 'thing' in the USA, but apparently my son has insurance, covering appliances in the new house that don't work as they should, so he has invoked this and is expecting a plumber and/or electrician to investigate it. If the unit or the wiring proves faulty (I'm certain the wiring is wrong, for starters) the insurance covers it.

My expectation is that the water heater only looks like it's wired-in (there's some T+E going from it into a breaker box, but operating the breaker does nothing), and it's probably been disconnected (or not connected in the first place) because it's faulty. It looks like it has been recently fitted (new wire nuts and flexible pipework) but the heater itself is old and rusty - incongruous. The property had been unoccupied for a year whilst it was "renovated", and the previous owner has a plumbing firm. I think the original one, which would have been 20 years old or so, failed and this one (secondhand) was put in so it all looked normal.

But then I have a suspicious mind...

E.