Limited to 7.5kw shower?

coffeehound

Are you running on single-rate electric or E7 ?  I guess the former with your ASHP

danrv

Still on the old E10 tariff with the favourable cheap rate hours (switches to peak at 5.00pm).
I’ll see how the next bill looks but the Eon cheap rate isn’t far off a competitive normal rate.

GDB2222

coffeehound said:

GDB2222 said:

Section62 said:

grumbler said:

jamesd said:

Plumbworld has a list of requirements which includes showers above 7kW up to 9kW on 6mm cable with runs up to 18m

IMO, up to 18m (and 35m)  restriction on the distance from the fuseboard is absolutely nonsensical. The bigger the distance, the better as the current gets only smaller.

This undermines credibility of all their other requirements.

The longer the cable the greater the voltage drop.

The length of cable of a given CSA passing a given current has to be limited to ensure the total voltage drop isn't excessive.  If it is, then a larger CSA cable has to be used.

I guess Grumbler is saying that, if the voltage drop is too big, the expected output of the shower would not be achieved. That would not be dangerous, as long as the cable is not getting too hot?

Wonder whether the solenoid valve might start to play up if fed too low a voltage?  Also there are those instant showers that contain a small pump to boost pressure from a loft cistern, and they could overheat if under-volted, I suppose.

You really need to do the voltage drop calculation, rather than speculating rather wildly about what might go wrong. 

With a 30 m cable run, the voltage drop is 3.3% for a 6mm2 cable, or 2% for a 10mm2 cable. That’s clearly not going to affect the operation of the shower, except it will be a tad cooler with the thinner cable as a bit more of the energy will be lost in the cable.

The only real issue is whether the cable will overheat, and that depends on how well thermally insulated it is. The key here is that none of the cable must overheat, so it and the shower need to be sized based on the most insulated bit of the cable. 

There’s a calculator here 
https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

coffeehound

^ your voltage-drop calcs look wrong, but I agree with the general conclusion.  

From a previous thread, here's a guide to the de-rating:



BTW, it's fine to speculate about things -- so long as it's made clear that's all it is

GDB2222

Using your table, the voltage drops I quoted look okay. I can take no credit for that, as I just took the figures from the online calculator. 

For example 7.3 x 30 x 32 = 7 v, which is 3%.

I am puzzled what calculations  you can have made?  

coffeehound

I did a calc for 8.5 kW last night (after a few beers so might be worng but) R load = 6.7765 ohm.  R of 60m of 6mm sq = 0.19 ohm.  Total circuit R = 6.9665.  Cable R as % of total = 2.73% so same ballpark, but a bit different from 3.3%

GDB2222

I was doing a 30m calculation, so that's a difference. I can't remember what the OP said his cable length was, and can't be bothered to look.

From your table, the voltage drop is 7.3mV/A/m. The current for 8 kW is around 32 A - anyway, that's what I took it as. Length = 30 m. Multiply that together, and you get a 7 V drop. (Not that I did that, as I just used the online calculator.) 

But, it does look like your table and the online calculator are giving comparable results. 

coffeehound

Ah okay the fact that we were using different loads will have an effect.  I think the tables take into account the fact that effective length is x2 the length of the run.  

From danrv's question, it does look marginal to run even 7.5 kW shower from 6 sq mm when under insulation.  Think in that situation I'd rather do the plumbing to run the mixer shower from the E10-heated tank.

Section62

coffeehound said:

From danrv's question, it does look marginal to run even 7.5 kW shower from 6 sq mm when under insulation.  Think in that situation I'd rather do the plumbing to run the mixer shower from the E10-heated tank.

Same here - if you've got hot water heated at cheap rate then it makes sense to use that, rather than pay to heat water at the rate when you use it.

The current E10 rates might have marginal benefit compared to a really good single tariff, but I wouldn't expect that to last as we all get 'encouraged' to move to TOU charging via our smart meters.

Section62

GDB2222 said:

I was doing a 30m calculation, so that's a difference. I can't remember what the OP said his cable length was, and can't be bothered to look.

I'm not sure the OP will be affected.

But the reason why I mentioned volt drop was because it is something which becomes more of an issue as the length of cable increases, therefore (in response to grumbler's point) manufacturers/retailers do need to specify a maximum length of cable for which they believe a certain CSA is adequate for their product.  And because the actual supply voltage for any individual customer is an unknown quantity (to the manufacturer/retailer), the quoted figures need to err low to account for the worst case scenario.

It's one of the disadvantages of manufacturers/retailers giving generic advice vs employing an electrical engineer to do a site-specific assessment.