Size of suppliers fuse vs RCD

[DuncanS]

Here's a scenario, I'm adding two new circuits to a mini consumer unit for my first ever NAPIT inspection... Just noticed that the main switch/RCD is rated to 100amps but the suppliers main fuse is only 80amp, should I change the RCD to one that's 80amp or lower, I don't want the NAPIT spark to think I've over looked anything...
just to add detail the circuits on the board are as follows, oven 32amp breaker! shower 45amp breaker, jacuzzi 20amp breaker.

 

[trev]

It's flow you need as well as pressure. 100 psi at 1 litre a minute is useless but 26 psi at 26 litres a minute will give you a great shower if it has a high enough wattage shower.

See post #42

 

[Meggerman]

Are you seriously telling me that you can't get a hot enough shower on a cold day from a relatively low powered one compared to that Mira you mentioned?

You can have it just as hot, but with a flow rate not a whole lot above a trickle of ****.

 

[jaypp]

from what i remember, the old electric showers temp control just slowed the flow. lovely and warm 'trickle/drip drip' in winter! think they've improved now

 

[NickD]

See above, if the incoming flow rate is X on a 7.6 Kw it's still going to be the same on a 10Kw job

What do you mean by incoming flow rate? There is some maximum flow-through rate of water that a 7kW shower is capable of getting up to X degrees centigrade, right? Well, the 10kW shower can get a lot more water through and get it up to X degrees, so you get a wetter shower.

 

[Meggerman]

from what i remember, the old electric showers temp control just slowed the flow. lovely and warm 'trickle/drip drip' in winter! think they've improved now

They are the same principle now mate just with larger heating elements

 

[mechelec]

But you're still only going to have it as hot as you can stand. Are you seriously telling me that you can't get a hot enough shower on a cold day from a relatively low powered one compared to that Mira you mentioned?
When I was playing rugby the showers were never switched on anyway so I kind of got used to cold showers. Nowt to do with spanking the monkey mind you

Perhaps you live in an area with low water pressure/flow. Round here you could fit a 7.5 KW and have a cold shower at full flow. I have a 10.5 KW and I can turn it to full flow and the water is Luke warm. Just thought you are not using microbore pipe when you plumb yours in are you ??????

 

[trev]

What do you mean by incoming flow rate? There is some maximum flow-through rate of water that a 7kW shower is capable of getting up to X degrees centigrade, right? Well, the 10kW shower can get a lot more water through and get it up to X degrees, so you get a wetter shower.

If the flow rate is X litres per minute at the water isolator upping the wattage of the shower is not going to change that.

 

[NickD]

Thanks, as you can tell I didn't install this unit just adding another two circuits... I'm probably over thinking things and just being paranoid but.... If this unit is protected by an rcd, does that rcd effectively become it's main switch?

Unless it has a main switch within the CU upstream of the RCD, then the RCD is also the CU's main switch. If you look on the certifying forms I believe it lets you specify overcurrent/RCD characteristics of a main switch which has them. I think what your thinking is hinting at is a potentially valid concern about whether your additions have compromised the diversity of the installation such that it it doesn't meet diversity guidelines and so is unacceptably at risk of blowing the DNO fuse. Have a look at the diversity stuff in the 17th edition to see how to work it out. At least I think it's in there.

- - - Updated - - -

If the flow rate is X litres per minute at the water isolator upping the wattage of the shower is not going to change that.

But usually it's the shower that constrains the flow rate at the shower head, not what the upstream supply is capable of, isn't it?

 

[mechelec]

If the flow rate is X litres per minute at the water isolator upping the wattage of the shower is not going to change that.

See #51

 

[Meggerman]

If the flow rate is X litres per minute at the water isolator upping the wattage of the shower is not going to change that.

Ok. another way of looking at it. The more powerful the element the more heat it can transfer to a given amount of water as it passes through.

 

[Brightspark2]

We'll I'm stunned at this...

(not the shower talk)

I'm no troll but this an early apprentice question...

 

[Knobhead]

I appreciate diversity but not so much with a shower, they draw full load from the start, at least that's what my osg tells me...

OSG?

Time you came out if you want to appreciate diversity properly.

 

[trev]

Perhaps you live in an area with low water pressure/flow. Round here you could fit a 7.5 KW and have a cold shower at full flow. I have a 10.5 KW and I can turn it to full flow and the water is Luke warm. Just thought you are not using microbore pipe when you plumb yours in are you ??????

Don't have a shower mate, we're not filthy buggers though.

 

[mechelec]

We'll I'm stunned at this...

(not the shower talk)

I'm no troll but this an early apprentice question...

Are you sure you are not a troll. That sounds like troll talk to me ; )

 

[Meggerman]

Don't have a shower mate, we're not filthy buggers though.

No way!! . Baths eh!! All that water!!