Is this inline electric heater too many amps?

[Roz]

I've contacted various electricians to get quotes and only one of them has said that our studios' indicated inline water heater at 11kw would not work for our supply. The water heaters at this level e.g. STROM need a 48amp supply, and apparently our whole supply from western power is only 60amp. As we're only on electric and no gas, the electrician has said that it's asking for trouble if we leave the water heater in the studio when the house is finished and people in the house are using the oven, other electical items, and then someone in the studio uses the shower. 

 

As we've only had one person say this, I thought I would get thoughts here. If we downscale the size of the water heater we will get a lower flow rate which would be annoying. I also really dont like the look of electric showers, hence going down this route to begin with. 

[Jeremy Harris]

He's right, it exceeds the maximum allowable load under the diversity rule.  Best bet is to ask your DNO if you can have a 100 A supply instead of the 60 A one.  Whether or not this is possible depends very much on your local grid load, as often the supply cables used for a 60 A supply are the same size as those for a 100 A supply.  The DNO may ask you for a contribution towards local grid reinforcement, though, which might be costly.  Still worth asking, as sometimes 60 A supplies were put in before local grid reinforcement work was undertaken, so there may be spare capacity.

 

An alternative would be to deal with the diversity problem by fitting a unit to prevent the shower and water heater being used at the same time.  Might just get you inside the limit, depending on what else you have in the house.  Garo make a unit to do this, they are popular in Ireland, where 60 A supplies are fairly commonplace.

[Roz]

4 minutes ago, JSHarris said:

He's right, it exceeds the maximum allowable load under the diversity rule.  Best bet is to ask your DNO if you can have a 100 A supply instead of the 60 A one.  Whether or not this is possible depends very much on your local grid load, as often the supply cables used for a 60 A supply are the same size as those for a 100 A supply.  The DNO may ask you for a contribution towards local grid reinforcement, though, which might be costly.  Still worth asking, as sometimes 60 A supplies were put in before local grid reinforcement work was undertaken, so there may be spare capacity.

Thanks Jeremy. Do power showers of the same KW just not require the same A supply as inline heaters then? Do you know what amp I should be able to go up to?

[Jeremy Harris]

3 minutes ago, Roz said:

Thanks Jeremy. Do power showers of the same KW just not require the same A supply as inline heaters then? Do you know what amp I should be able to go up to?

 

 

It's a diversity problem, rather than an individual appliance maximum load problem.  If both the water heater and shower were on (neither of which can have diversity applied) then the installation would be overloaded.  The Garo unit that gets around this is this one: http://www.meteorelectrical.com/distribution-control/consumer-units-accessories-1/garo-priority-shower-board-choose-priority.html

[Roz]

4 minutes ago, JSHarris said:

 

 

It's a diversity problem, rather than a maximum load problem.  If both the water heater and shower were on (neither of which can have diversity applied) then the installation would be overloaded.  The Garo unit that gets around this is this one: http://www.meteorelectrical.com/distribution-control/consumer-units-accessories-1/garo-priority-shower-board-choose-priority.html

I don't totally understand how that would work if we only have one shower being fed from the inline heater in the studio. Then the shower in the house would be from the sunamp (probably). 

[Jeremy Harris]

15 minutes ago, Roz said:

I don't totally understand how that would work if we only have one shower being fed from the inline heater in the studio. Then the shower in the house would be from the sunamp (probably). 

 

Is the shower not electric as well?  As you'd mentioned the shower and cooker being on at the same time as the water heater I assumed it was.

 

The problem is that the maximum current that the supply can deliver is limited by the 60 A fuse on the incomer.  An 11 kW electric water heater is going to draw about 48 A, leaving 12 A for everything else running from that supply.  There's a thing called the diversity that is applied to the electrical loads which allows for not all of them being on at the same time.  For example, for a cooker it's assumed that it will draw 10 A plus 30% of the maximum rated load over 10 A (if the cooker outlet has no 13 A outlet as well).  For a cooker rated at 40 A (not unusual) then the cooker load would be 10 A + (30% x 30 A) = 19 A.  The sum of the cooker and water heater load alone (ignoring everything else on that incoming fuse) would now be 67 A, over the 60 A rating of the fuse.  The ring final and lighting circuits also need to be accounted for, and adding those as well increases the maximum allowable load even more.  Your electrician cannot sign off the installation if the total load exceeds that which can be safely delivered by the incoming supply.

[Jeremy Harris]

BTW, that priority shower board doesn't have to be used for electric showers, you can use it to prevent the water heater running whilst the cooker is on if needed.  This would then bring your installation back within the maximum allowable demand so it could be signed off as safe.  I know someone who uses one to allow him to have two electric car charge points, the priority switch stops both being use at the same time, which could overload his incoming supply.

[Roz]

6 minutes ago, JSHarris said:

 

Is the shower not electric as well?  As you'd mentioned the shower and cooker being on at the same time as the water heater I assumed it was.

 

The problem is that the maximum current that the supply can deliver is limited by the 60 A fuse on the incomer.  An 11 kW electric water heater is going to draw about 48 A, leaving 12 A for everything else running from that supply.  There's a thing called the diversity that is applied to the electrical loads which allows for not all of them being on at the same time.  For example, for a cooker it's assumed that it will draw 10 A plus 30% of the maximum rated load (if the cooker outlet has no 13 A outlet as well).  For a cooker rated at 30 A (not unusual, some are a fair bit more than this) then the cooker load would be 10 A + (30% x 30 A) = 19 A.  The sum of the cooker and water heater load alone (ignoring everything else on that incoming fuse) would now be 67 A, over the 60 A rating of the fuse.  The ring final and lighting circuits also need to be accounted for, and adding those as well increases the maximum allowable load even more.  Your electrician cannot sign off the installation if the total load exceeds that which can be safely delivered by the incoming supply.

Sorry, I meant because the studio shower will be heated from the water heater. So we have the water heater, then the plan was to go to a concealed mixer shower 

 

That description makes sense, thanks. When the electrician rang he mentioned using a smaller electric shower that only has 36 amps. I presume the same could be said of using a smaller inline heater if it was 36 amps too? Both would just produce a pretty low flow rate which will be annoying 

[Carrerahill]

52 minutes ago, Roz said:

I've contacted various electricians to get quotes and only one of them has said that our studios' indicated inline water heater at 11kw would not work for our supply. The water heaters at this level e.g. STROM need a 48amp supply, and apparently our whole supply from western power is only 60amp. As we're only on electric and no gas, the electrician has said that it's asking for trouble if we leave the water heater in the studio when the house is finished and people in the house are using the oven, other electical items, and then someone in the studio uses the shower. 

 

As we've only had one person say this, I thought I would get thoughts here. If we downscale the size of the water heater we will get a lower flow rate which would be annoying. I also really dont like the look of electric showers, hence going down this route to begin with. 

He is bang on - 60A - 48A = 12A - you could draw that 12A through your oven alone - so that is your service maxed out. 

 

He is probably the only electrician out of all of them I would now trust - he is clued up.

 

This is why a move away from natural gas over the next couple of decades and electric car charging is going to be problematic, the grid is not ready!

Edited July 29, 2019 by Carrerahill

[Jeremy Harris]

If you can list the circuits you have coming from your consumer unit, then I can have a go at working out what you might best be able to do to be able to retain the 11 kW inline heater with the other loads from that supply. 

 

At a guess I'd say you probably have a couple of ring finals, a couple of lighting circuits, a cooker circuit, the water heater circuit and maybe an immersion heater circuit (not sure how the Sunamp is being heated).  You may have other circuits as well, for example we have separate circuits for the garage, water treatment plant, borehole water pump, ASHP etc.  All have to be included when working out the maximum allowable load, and it should then be possible to pick the second highest  load and use that, together with a priority switch, to keep the maximum load below the 60 A limit.

[ProDave]

Do you KNOW you only have a 60A fuse?

 

We were only offered a 12KVA supply which you would think came with a 60A fuse, but it connects to the same 100KVA transformer as all our neighbours houses, with the same size cable, and I asked the DNO when they installed the supply, what rating fuse they are fitting and they said "100 Amp"

 

Happy days.

[Roz]

6 minutes ago, ProDave said:

Do you KNOW you only have a 60A fuse?

 

We were only offered a 12KVA supply which you would think came with a 60A fuse, but it connects to the same 100KVA transformer as all our neighbours houses, with the same size cable, and I asked the DNO when they installed the supply, what rating fuse they are fitting and they said "100 Amp"

 

Happy days.

No idea....it's just what the electrician said. this says 80A on it? 

[Roz]

3 minutes ago, Roz said:

No idea....it's just what the electrician said. this says 80A on it? 

oh it also says 100A around the side of this?

[Jeremy Harris]

Unfortunately they all say 100 A on the fuse carrier, it's the rating of the actual fuse fitted inside that matters.  This can't be accessed without breaking the seal, though.  That does look as if it's fairly new and has an 80 A fuse, rather than a 60 A one, though.

[Roz]

4 minutes ago, JSHarris said:

Unfortunately they all say 100 A on the fuse carrier, it's the rating of the actual fuse fitted inside that matters.  This can't be accessed without breaking the seal, though.  That does look as if it's fairly new and has an 80 A fuse, rather than a 60 A one, though.

If it was 80A would the 45A instant water heater be manageable? Recognise I might not be providing enough information

 

I should ask Western Power what they installed I guess

[Jeremy Harris]

Just now, Roz said:

If it was 80A would the 45A instant water heater be manageable? Recognise I might not be providing enough information

 

I should ask Western Power what they installed I guess

 

 

If you can give that list of circuits that are being run from this supply then it's five minutes work to tell you the answer.  It may be that your electrician has misread the main fuse as being 60 A rather than 80 A, but it's easy to calculate whether you can have the instant water heater connected without a priority switch.

[Roz]

13 minutes ago, JSHarris said:

 

 

If you can give that list of circuits that are being run from this supply then it's five minutes work to tell you the answer.  It may be that your electrician has misread the main fuse as being 60 A rather than 80 A, but it's easy to calculate whether you can have the instant water heater connected without a priority switch.

So I don't totally know the answer to this but I can list what we have going on

House:

3 areas of downlights (one room but two areas that you can switch on and off separately, plus landing/stairs)

Various plug sockets

Oven

Fridge

Dishwasher

2 bedrooms each with a pendant and plug sockets, one with wall lights too

Bathroom with shaver socket, extraction fan, main light

Air Source Heat Pump direct to underfloor heating

Sunamp for domestic hot water direct electricity overnight no water tank

Outdoor socket

 

Studio: (have been told it will have its own consumer unit but will be run off same house supply)

1 pendant light

2 pendant lights same switch

various plug sockets

washing machine

Inline water heater

 

Is that enough info? 

Edited July 29, 2019 by Roz

[Jeremy Harris]

6 minutes ago, Roz said:

So I don't totally know the answer to this but I can list what we have going on

House:

3 areas of downlights (one room but two areas that you can switch on and off separately)

Various plug sockets

Oven

Fridge

Dishwasher

2 bedrooms each with a pendant and plug sockets, one with wall lights too

Bathroom with shaver socket, extraction fan, main light

Air Source Heat Pump direct to underfloor heating

Sunamp for domestic hot water direct electricity overnight no water tank

Outdoor socket

 

Studio: (have been told it will have its own consumer unit but will be run off same house supply)

1 pendant light

2 pendant lights same switch

various plug sockets

washing machine

Inline water heater

 

Is that enough info? 

 

 

It's really the circuits coming from the consumer unit that are needed. 

 

You should have a set of circuit breakers that are labelled, one for each circuit.  Typically these would be something like two ring final circuits (maybe upstairs and downstairs) with 32 A MCBs, two lighting circuits, again upstairs and downstairs, with 6 A MCBs, a circuit for the ASHP (need the rating of the MCB, probably about 16 A I suspect), the cooker circuit (typically about 40 A), the water heater circuit (probably about 50 A), The Sunamp circuit (probably 16 A).

 

If you can check what you have in the consumer unit then it's a pretty quick job to do the diversity sums and see what the maximum load is.

[Roz]

16 minutes ago, JSHarris said:

 

 

If you can give that list of circuits that are being run from this supply then it's five minutes work to tell you the answer.  It may be that your electrician has misread the main fuse as being 60 A rather than 80 A, but it's easy to calculate whether you can have the instant water heater connected without a priority switch.

When the electrician rang he definitely said 60A but I have just seen a text from him that says 80A, so there's some confusion!

[Jeremy Harris]

Just now, Roz said:

When the electrician rang he definitely said 60A but I have just seen a text from him that says 80A, so there's some confusion!

 

80 A might still be too low to allow the water heater to be connected without a priority switch, best to check.  If you can take a photo of the consumer unit then that should show all the circuits well enough to do the sums.

[Roz]

6 minutes ago, JSHarris said:

 

80 A might still be too low to allow the water heater to be connected without a priority switch, best to check.  If you can take a photo of the consumer unit then that should show all the circuits well enough to do the sums.

We haven’t got our consumer unit yet as the main house isn’t done. We just have a little garage consumer unit I think so we have a power socket running off that. We are trying to complete the studio first so we have somewhere to sleep while we finish the rest

 

would the priority switch just give priority to everything else over the water heater to save things tripping? 

Edited July 29, 2019 by Roz

[Jeremy Harris]

37 minutes ago, Roz said:

We haven’t got our consumer unit yet as the main house isn’t done. We just have a little garage consumer unit I think so we have a power socket running off that. We are trying to complete the studio first so we have somewhere to sleep while we finish the rest

 

would the priority switch just give priority to everything else over the water heater to save things tripping? 

 

 

OK, so I'm guessing here that your electrician is planning ahead and has worked out that the sum of the loads, allowing for diversity, exceeds the 80 A fuse.  I've listed what I'm guessing you may have in the consumer unit (once it's fitted):

 

1 off ring final circuit at 32 A, allowing for diversity =  10 A + (50% of (32 A - 10 A)))  = 21 A (these are your 13 A outlets, I'm assuming there will be only one circuit as the house is a modest size - need the number of outlets to do a proper calculation, really.  If your floor area exceeds 100m² then you need a second ring final circuit)

 

1 off lighting circuit at 6 A, allowing for diversity = 6 A x 66% = 3.96 A (same assumption as above, just one circuit assumed)

 

1 off Sunamp circuit  at 16 A, allowing for diversity and assuming the heating element is rated at 3 kW = 13 A

 

1 off ASHP circuit at 16 A (need to check this, and need power rating of ASHP) = probably around 10 A (could be a bit lower)

 

1 off electric water heater at 60 A (48 A appliance load, no diversity allowable) = 48 A

 

1 off cooker circuit at 40 A (might be 32 A, or could possibly be 50 A, need the maximum power on the appliance(s) label(s)), allowing for diversity = 10 A + (30% x 30 A) = 19 A (add 5 A to this if there is a socket on the cooker outlet switch plate).

 

The sum of all the above comes to 21 A + 3.96 A + 13 A + 10 A + 48 A + 19 A = 114.96 A

 

This is way over the 80 A supply limit, and also way over a 100 A supply limit too.

 

Fitting a priority load switch so that the cooker and electric water heater share the same circuit (needs care over circuit protection devices to do this) would allow you to stay under the 80 A limit, but would mean that the cooker and the electric water heater couldn't be used at the same time.  You can choose which you wish to take priority over the other.

 

The alternative is to get rid of the water heater.  There isn't enough spare capacity (based on the rough and ready estimates above) to allow any reasonable size of instant water heater to work OK really.  Without the water heater the maximum demand comes down to about 67 A, well within the limit of the supply.

[Roz]

13 minutes ago, JSHarris said:

 

 

OK, so I'm guessing here that your electrician is planning ahead and has worked out that the sum of the loads, allowing for diversity, exceeds the 80 A fuse.  I've listed what I'm guessing you may have in the consumer unit (once it's fitted):

 

1 off ring final circuit at 32 A, allowing for diversity =  10 A + (50% of (32 A - 10 A)))  = 21 A (these are your 13 A outlets, I'm assuming there will be only one circuit as the house is a modest size - need the number of outlets to do a proper calculation, really)

 

1 off lighting circuit at 6 A, allowing for diversity = 6 A x 66% = 3.96 A (same assumption as above, just one circuit assumed)

 

1 off Sunamp circuit  at 16 A, allowing for diversity and assuming the heating element is rated at 3 kW = 13 A

 

1 off ASHP circuit at 16 A (need to check this, and need power rating of ASHP) = probably around 10 A (could be a bit lower)

 

1 off electric water heater at 60 A (48 A appliance load, no diversity allowable) = 48 A

 

1 off cooker circuit at 40 A (might be 32 A, or could possibly be 50 A, need the maximum power on the appliance(s) label(s)), allowing for diversity = 10 A + (30% x 30 A) = 19 A (add 5 A to this if there is a socket on the cooker outlet switch plate).

 

The sum of all the above comes to 21 A + 3.96 A + 13 A + 10 A + 48 A + 19 A = 114.96 A

 

This is way over the 80 A supply limit, and also way over a 100 A supply limit too.

 

Fitting a priority load switch so that the cooker and electric water heater share the same circuit (needs care over circuit protection devices to do this) would allow you to stay under the 80 A limit, but would mean that the cooker and the electric water heater couldn't be used at the same time.  You can choose which you wish to take priority over the other.

 

The alternative is to get rid of the water heater.  There isn't enough spare capacity (based on the rough and ready estimates above) to allow any reasonable size of instant water heater to work OK really.  Without the water heater the maximum demand comes down to about 67 A, well within the limit of the supply.

Gosh, thanks for that Jeremy! 

 

So even if we brought the water heater / electric shower down to 30 amps it still wouldnt be OK.

 

So the only option is to run them on the same circuit (starting to understand this, thanks), but can this be done if the cooker is in the house and the water heater is in the studio which has its own consumer unit (i think)? 

It's not an ideal situation anyway. 

 

I wanted an instant water heater to avoid the installation costs of an unvented cylinder, plus the studio will be used irregularly once we are in the main house. I'm starting to think we should have buried a hot water pipe between the house and the studio so it could just use the regular hot water from the house. At the time we didn't think this was worth it as the studio would be done first, but an installer told us we could have just installed the sunamp in the studio and then moved it over into the house when we were ready. Now a;; the ground is back in place so would mean re-digging (7 metres or so)

 

For installing a say 90L unvented cylinder  (?)  would we be looking at hundreds in labour?

[Jeremy Harris]

The maximum rating for any water heater (if the assumptions I've made above as to the loads is accurate - it may not be!) would be 13 A, just about enough for an immersion heater, but not enough for an instant water heater.  If you can get the supply uprated to 100 A, then you have about 33 A available, so could then install a 7.5 kW instant water heater.  This all depends on the actual loads you have though - if you have two ring final circuits and two lighting circuits the load will increase a fair bit.

 

Splitting a priority load switch across two locations is possible, but not ideal, really, as it would make things a bit messy.  A lot depends on the physical arrangement of the studio relative to the main house.  Also, if the studio has it's own power and lighting circuits on a separate consumer unit, fed from the same incoming main fuse, then these have to be added to the load calculation above, so will impact on the total load and may even take you close to, or over, the allowable limit with an 80 A fuse, without any water heater.

 

Do you know what the input power rating of the ASHP and the cooker is?  I've just guessed numbers for those, so may have over-estimated.  Also, do you know the floor area of the house?  If over 100m² then two ring final circuits may be needed, in addition to any circuits in the studio.

[Roz]

18 hours ago, JSHarris said:

The maximum rating for any water heater (if the assumptions I've made above as to the loads is accurate - it may not be!) would be 13 A, just about enough for an immersion heater, but not enough for an instant water heater.  If you can get the supply uprated to 100 A, then you have about 33 A available, so could then install a 7.5 kW instant water heater.  This all depends on the actual loads you have though - if you have two ring final circuits and two lighting circuits the load will increase a fair bit.

 

Splitting a priority load switch across two locations is possible, but not ideal, really, as it would make things a bit messy.  A lot depends on the physical arrangement of the studio relative to the main house.  Also, if the studio has it's own power and lighting circuits on a separate consumer unit, fed from the same incoming main fuse, then these have to be added to the load calculation above, so will impact on the total load and may even take you close to, or over, the allowable limit with an 80 A fuse, without any water heater.

 

Do you know what the input power rating of the ASHP and the cooker is?  I've just guessed numbers for those, so may have over-estimated.  Also, do you know the floor area of the house?  If over 100m² then two ring final circuits may be needed.

Yes I believe they are fed from the same mains fuse, I dont know this for sure but I haven't heard anything else is being installed in the meter cabinet. 

 

There's about 23 double sockets on my plan across the house and the studio. I dont know about the ASHP as we havent picked one yet, but we only need a small one. The house area is about 60-65 square metres split between the two floors, and the studio is 36 square metres.

 

Google seems to think I'm looking at £500-£1000 for an UVC ?

 

 

Edited July 30, 2019 by Roz