breaker switch

[serendipityfox]

Guy in the shop recommended 32A breaker switch for a 45kW electric shower on (4.5mm cable)

Is this OK? How do i calculate?

[stoneyboy]

You will get decisive answers from the likes of Crossy who posts regularly here on the forum but due to the time at present probably won't be about until the morning Thai time.

[Jdietz]

Well let's assume it's a 4.5kW heater for starters, otherwise you'll need a little larger supply and most likely your own transformer.

Ohm's law: U = I * R; P = U * I. so P/U = I => 4,500 (W) / 240 (V) = 18.75A so with some headroom and inrush current, 32A sounds reasonable.

4.5mm cable is rated for 35Amp so again 32A sounds very reasonable.

Now if your heater is indeed 45KW, you're looking at around 190Amps steady current flow...

[Forkinhades]

A 6mm cable should be fine for a 4.5KW shower unit.

No such thing as a 4.5mm cable, or 45KW shower unit.

To get precise calcs, we need to know length of the cable run, and how it is to be run, ie in ceiling space, clipped direct, in conduit. Many many factors to consider.

More important is your earthing arrangements, and safety cut devices.

[Crossy]

4.5kW @ 220V = 20.5A or thereabouts.

I would be using a 32A breaker and 4mm² cable (rated at 30-32A). If you are likely to wish to upgrade, I would pull a 6mm² cable.

If your distribution board does not already have an RCD (Safe_T_Cut) device and you are add a breaker ensure it's an RCBO (earth leakage breaker) which will provide extra protection and also insist that the heater is grounded.

Your shop may be suggesting a 2.5mm² cable, this would be marginal with no room for enhancement. If it's already there it will be fine so long as you don't take 2 hour showers (it will get warm) but if pulling a new cable go with the above.

[Forkinhades]

Personally I would never use 4mm cable with a 32A breaker, especially here in LOS. A 25A would be better suited. Also to note is that mostly MCBs here are on the 'C' curve, which basically means they can handle the inrush of current x 10 of the value of the MCB. So a 32A can handle 320A for a short period of time.

[Crossy]

Personally I would never use 4mm cable with a 32A breaker, especially here in LOS. A 25A would be better suited. Also to note is that mostly MCBs here are on the 'C' curve, which basically means they can handle the inrush of current x 10 of the value of the MCB. So a 32A can handle 320A for a short period of time.

Agreed if you can get a 25A unit.

Assuming this is a dedicated circuit (may be a bad idea) a 4.5kW heater isn't going to overload the 4mm² cable so the breaker just has to provide short circuit and earth leakage protection.

In reality a 20A breaker would stay in forever at 20.5A.

[serendipityfox]

Thankyou, i will continue to study electrical circuits.

[serendipityfox]

ok, my consumer unit does not have an RCD - so i will buy one and a new consumer unit as the current one has no more space.

[serendipityfox]

do i need an isolator switch? the shop assistant and local sparky indicated not.

[Crossy]

do i need an isolator switch? the shop assistant and local sparky indicated not.

Up to you, local isolation of a water heater is not required by the Thai regulations, if you use an RCBO that can act as the circuit isolator.

Before going shopping for a new CU how much expansion are you intending?

500 Baht will buy you a stand alone RCBO for your shower and a little box to put it in, wire directly into the switched side of your incoming breaker. Makes things safe at minimal cost if you're in a rental.

[serendipityfox]

yeah i thought you might suggest that - well the current box is a bit clunky - steel from the 90's, with no spare spaces, so an upgrade may well be in order.

Thanks for the info.

[serendipityfox]

i have taken the ceiling down and it turns out there is a shower circuit there already, albeit with shabby wiring.

there is an earth leakage breaker unit switch - i guess this is the same function as a RCBO?

The current fuse is 20Amp - is this ok for the shower unit? or should i upgrade to 25A?

[Crossy]

If the "earth leakage breaker unit switch" is one of the small stand-alone units it is likely an RCBO.

The current 20A breaker will likely be just fine, if it trips (you like really long showers), yes you could replace it with a 25A unit, but read below.

What size is the existing shower wiring, if it is 2.5mm² (as I suspect it will be) I would not replace the 20A breaker.

[serendipityfox]

thanks, nope existing wiring is 4mm, i will be replacing it with better (unspliced) 4mm cable anyway.

Sum up.. an earth leakage breaker/RCBO provides safety but not as much as an RCD? (and is minimum requirement per thai regs)

[Crossy]

Sum up.. an earth leakage breaker/RCBO provides safety but not as much as an RCD? (and is minimum requirement per thai regs)

Other way round, an RCD provides earth leakage protection, an RCBO provides earth leakage and over-current protection.

[serendipityfox]

ah, RCDs are so expensive!

[Crossy]

ah, RCDs are so expensive!

I've never seen an RCD in the little stand alone packages, RCBOs are about 500 Baht in this style. Great value.

But you have to compare like with like.

The little units are fine for giving protection to your shower or an outside outlet. But they're not up to the task of living in your breaker box where the potential fault currents which they must interrupt are much higher.

You will note that breaker box RCDs and RCBOs are a lot more expensive and the Safe-T-Cut front end units more expensive still.

[serendipityfox]

yes... and a box RCD covers all the circuits