ASHP Current Draw

[SBMS]

We are in the process of getting mains electric to our site and are deciding between single phase and 3 phase.

 

I am trying to work out the max current draw of our designated ASHP - a Nibe F2040-12.  It has in its technical docs a max operating current of

 

23 Amps - Heat Pump

22 Amps - Compressor

 

Does this mean the unit could draw 45 Amps or am I mis-interpreting?

[PeterW]

Where in the UK are you as Western Power now install 3Ph as a matter of course for the same cost. No problem putting a single phase meter on a multi phase supply and only using one of the phases to begin with. 
 

for reference the F2040-12 requires a C-Curve MCB rated 25A from the technical guide so the maximum current would be the 23A for the entire unit. 

 

[SBMS]

Thanks @PeterW. In North west - Electricity North West. £1500 for single phase connection, £2500 for three phase ?

[PeterW]

Car charger is another consideration - some are 3Ph only and some you will need 50-60% of your feed on 1Ph so be aware of overall load. 

[SBMS]

6 minutes ago, PeterW said:

Car charger is another consideration - some are 3Ph only and some you will need 50-60% of your feed on 1Ph so be aware of overall load. 

Yes this has been my consideration.  If i turned on:

 

Oven: 13A

Hob: 32A

ASHP: 23A

Car Charger: 32A

 

then boom... I've maxed out my 100A single phase connection.

 

[redtop]

1 minute ago, SBMS said:

Yes this has been my consideration.  If i turned on:

 

Oven: 13A

Hob: 32A

ASHP: 23A

Car Charger: 32A

 

then boom... I've maxed out my 100A single phase connection.

 

If you are not having gas I would def go for 3phase

[Ajn]

38 minutes ago, SBMS said:

Electricity North West. £1500 for single phase connection, £2500 for three phase

but what supply rating were you getting? Single or three phase does not mean anything without knowing how much you can take from it or what you requested.

[Carrerahill]

22 minutes ago, SBMS said:

Yes this has been my consideration.  If i turned on:

 

Oven: 13A

Hob: 32A

ASHP: 23A

Car Charger: 32A

 

then boom... I've maxed out my 100A single phase connection.

 

Doesn't work like that, you must apply diversity, if all supplies were calculated like this we would have colossal electrical networks and switchgear. 

 

Actual demand after maximum diversity, or ADMD is how the DNO's work out how many buildings they can actually supply, most class a 2 bed house with gas heating as 2.5kVA - which is basically 2500W at 230! They still fuse it at 60/80/100A depending on what/where etc. but they don't actually allow for all properties to use the max or the grid would fall over, which it will do soon enough with all the EV chargers. 

 

 

Edited February 22, 2022 by Carrerahill

[SBMS]

56 minutes ago, Ajn said:

but what supply rating were you getting? Single or three phase does not mean anything without knowing how much you can take from it or what you requested.

I’ve been told single phase is 100A and 3 phase is 80A per phase for a total of 240A

[SBMS]

40 minutes ago, Carrerahill said:

Doesn't work like that, you must apply diversity, if all supplies were calculated like this we would have colossal electrical networks and switchgear. 

 

Actual demand after maximum diversity, or ADMD is how the DNO's work out how many buildings they can actually supply, most class a 2 bed house with gas heating as 2.5kVA - which is basically 2500W at 230! They still fuse it at 60/80/100A depending on what/where etc. but they don't actually allow for all properties to use the max or the grid would fall over, which it will do soon enough with all the EV chargers. 

 

 

Sorry are you saying that in my scenario with those 4 appliances operating they wouldn’t draw 100A off the supply? Or are you saying the DNOs capacity calculations wouldn’t see this as a valid scenario?

[SBMS]

FYI the DNO offer a scheme where if you can demonstrate an insufficient single phase supply owing to the installation of green appliances (ASHP, car chargers etc) then they will upgrade the supply to three phase free of charge. This is only relevant for an existing single phase supply. 

Would my scenario demonstrate this?

[Carrerahill]

30 minutes ago, SBMS said:

Sorry are you saying that in my scenario with those 4 appliances operating they wouldn’t draw 100A off the supply? Or are you saying the DNOs capacity calculations wouldn’t see this as a valid scenario?

I am saying that the chances of all those electrical loads all simultaneously drawing full current is unlikely. Ovens and hobs modulate most of the time they are on, so they don't often pull that current for long periods of time. Even if you did go and load up your electrical system the chances are that the full load may only last 15minutes or so then start to fall off. You also need to consider normal use.

 

In my house just now the WM is on a 40° wash, the oven is on too, this laptop and some lighting. Then there are the parasitic loads, so peak power draw right now is probably 25A. I could add an EV charger and ASHP and still only be at 80A, however, as said above, the oven and washer load will modulate, so might peak at 80A but realistically sit at 70A. 

 

For the record, a 100A fuse won't blow at 100A, see green curve below:

Without going into the technical details of it all, basically even if you did pull 100A for say 10-15minutes it would be fine, not ideal but it would be fine, the actual BS88 fuse would take 100A for over 1 hour, seems odd you may think but incoming fuses are there to do multiple things, one is to protect downstream and upstream cabling. Yes, that fuse also protects the DNO's incoming cable before your fuse, because that cable will probably be jointed to a much bigger cable in the street which is on a 1250-5000A fuse at the other end, but that cable coming into your house sure isn't capable of carrying even 250A. Cable protection is based on time. A cable will not go 1A overcurrent then explode into flames, it will just warm up quicker than intended and may then tip over its maximum rated operating temp and at a point the cable will be damaged, twin and earth is rated at 70° - so if the cable goes overcurrent for say 30 minutes and got up to 50° it would be fine. 

 

It is quite common for submains distribution and switchgear to allow periods of time where the rating may exceed for example the cable rating, but it is controlled with MCCB's (big fancy, sometimes micrologic controlled breakers) or even a BS88 fuse like the DNO will fit. 

 

The weak point in your system might be the incoming isolation switch(s) therefore a board with a main isolator rated at 125A is a good idea. I personally always spec a higher mains isolator than the supply ought to deliver. 

 

 

 

 

[Carrerahill]

46 minutes ago, SBMS said:

FYI the DNO offer a scheme where if you can demonstrate an insufficient single phase supply owing to the installation of green appliances (ASHP, car chargers etc) then they will upgrade the supply to three phase free of charge. This is only relevant for an existing single phase supply. 

Would my scenario demonstrate this?

Do it if they offer it, but I think you would cope.

 

Say you have ASHP and EVC and also electric cooking (which is meant to be green - ha!).

 

I would think so.

[SBMS]

34 minutes ago, Carrerahill said:

I am saying that the chances of all those electrical loads all simultaneously drawing full current is unlikely. Ovens and hobs modulate most of the time they are on, so they don't often pull that current for long periods of time. Even if you did go and load up your electrical system the chances are that the full load may only last 15minutes or so then start to fall off. You also need to consider normal use.

 

In my house just now the WM is on a 40° wash, the oven is on too, this laptop and some lighting. Then there are the parasitic loads, so peak power draw right now is probably 25A. I could add an EV charger and ASHP and still only be at 80A, however, as said above, the oven and washer load will modulate, so might peak at 80A but realistically sit at 70A. 

 

For the record, a 100A fuse won't blow at 100A, see green curve below:

Without going into the technical details of it all, basically even if you did pull 100A for say 10-15minutes it would be fine, not ideal but it would be fine, the actual BS88 fuse would take 100A for over 1 hour, seems odd you may think but incoming fuses are there to do multiple things, one is to protect downstream and upstream cabling. Yes, that fuse also protects the DNO's incoming cable before your fuse, because that cable will probably be jointed to a much bigger cable in the street which is on a 1250-5000A fuse at the other end, but that cable coming into your house sure isn't capable of carrying even 250A. Cable protection is based on time. A cable will not go 1A overcurrent then explode into flames, it will just warm up quicker than intended and may then tip over its maximum rated operating temp and at a point the cable will be damaged, twin and earth is rated at 70° - so if the cable goes overcurrent for say 30 minutes and got up to 50° it would be fine. 

 

It is quite common for submains distribution and switchgear to allow periods of time where the rating may exceed for example the cable rating, but it is controlled with MCCB's (big fancy, sometimes micrologic controlled breakers) or even a BS88 fuse like the DNO will fit. 

 

The weak point in your system might be the incoming isolation switch(s) therefore a board with a main isolator rated at 125A is a good idea. I personally always spec a higher mains isolator than the supply ought to deliver. 

 

 

 

 

Thanks this is really useful and I’ve definitely learned something tonight. The person at the DNO did say they’d only had 3 properties in a year that could justify the upgrade to 3P so maybe it’s be ok?

[SBMS]

Out of interest @Carrerahill what sort of a load would justify 3 phase?

[joth]

19 minutes ago, SBMS said:

Thanks this is really useful and I’ve definitely learned something tonight. The person at the DNO did say they’d only had 3 properties in a year that could justify the upgrade to 3P so maybe it’s be ok?

This is a weird statement.

 

Generally people hope a new house is going to last many decades. Generally people hope the mains supply won't need digging up and re-laying just because a boiler or heat pump of EV charger or PV install or etc etc changes over those many decades.

So the justification for putting 3ph is less about what you're going to have in it on day 1, and more about what is plausible to be needed in it in the next.. 20+ years?

 

Some of the common justifications are EV chargers (more than one car? so you may want 2 chargers at some point). If it's a forever home, maybe a stair lift or elevator.  Will you have PV solar panels  - going over 4kW is much easier on 3 phase.   Heat pump already discussed. Etc.

 

[SBMS]

1 hour ago, joth said:

This is a weird statement.

 

Generally people hope a new house is going to last many decades. Generally people hope the mains supply won't need digging up and re-laying just because a boiler or heat pump of EV charger or PV install or etc etc changes over those many decades.

So the justification for putting 3ph is less about what you're going to have in it on day 1, and more about what is plausible to be needed in it in the next.. 20+ years?

 

Some of the common justifications are EV chargers (more than one car? so you may want 2 chargers at some point). If it's a forever home, maybe a stair lift or elevator.  Will you have PV solar panels  - going over 4kW is much easier on 3 phase.   Heat pump already discussed. Etc.

 

You can apply the same logic to every single item you might put into a self build. PV? Battery wall? Solar water heating? Rainwater harvesting in case water rates go up? MVHR? 150mm insulation? 250mm? 500mm? It’s pretty much an endless list of options. With an unlimited budget - sure, I’d go for every single one. Unfortunately I don’t; so every single decision that we make weighs up the cost versus risk versus return on that capital investment. In isolation it’s probably easy to say ‘it’s only 1k’. But they add up and this was why I was asking those more experienced in electrical current loading whether this particular investment would make sense. 

[joth]

24 minutes ago, SBMS said:

You can apply the same logic to every single item you might put into a self build. PV? Battery wall? Solar water heating?

 

The difference is it's generally much cheaper to do the 3ph upgrade now than retrofit. If you do it later you'll pay the costs all over again. Battery costs about the same to do now or later. (in fact probably costs less to do it later as prices are dropping)

That said the in your case the single phase is fairly cheap (ours was quoted over £4k) so it makes the upgrade looks proportional larger I agree

Edited February 22, 2022 by joth

[TonyT]

For £1k Difference to get 3 phase it’s money we’ll spent.

 

more PV

Faster EV charger.

no drama with any ASHP.

[Marvin]

3 hours ago, Carrerahill said:

I am saying that the chances of all those electrical loads all simultaneously drawing full current is unlikely. Ovens and hobs modulate most of the time they are on, so they don't often pull that current for long periods of time. Even if you did go and load up your electrical system the chances are that the full load may only last 15minutes or so then start to fall off. You also need to consider normal use.

 

In my house just now the WM is on a 40° wash, the oven is on too, this laptop and some lighting. Then there are the parasitic loads, so peak power draw right now is probably 25A. I could add an EV charger and ASHP and still only be at 80A, however, as said above, the oven and washer load will modulate, so might peak at 80A but realistically sit at 70A. 

 

For the record, a 100A fuse won't blow at 100A, see green curve below:

Without going into the technical details of it all, basically even if you did pull 100A for say 10-15minutes it would be fine, not ideal but it would be fine, the actual BS88 fuse would take 100A for over 1 hour, seems odd you may think but incoming fuses are there to do multiple things, one is to protect downstream and upstream cabling. Yes, that fuse also protects the DNO's incoming cable before your fuse, because that cable will probably be jointed to a much bigger cable in the street which is on a 1250-5000A fuse at the other end, but that cable coming into your house sure isn't capable of carrying even 250A. Cable protection is based on time. A cable will not go 1A overcurrent then explode into flames, it will just warm up quicker than intended and may then tip over its maximum rated operating temp and at a point the cable will be damaged, twin and earth is rated at 70° - so if the cable goes overcurrent for say 30 minutes and got up to 50° it would be fine. 

 

It is quite common for submains distribution and switchgear to allow periods of time where the rating may exceed for example the cable rating, but it is controlled with MCCB's (big fancy, sometimes micrologic controlled breakers) or even a BS88 fuse like the DNO will fit. 

 

The weak point in your system might be the incoming isolation switch(s) therefore a board with a main isolator rated at 125A is a good idea. I personally always spec a higher mains isolator than the supply ought to deliver. 

 

 

 

 

Would the meter also take over 100amps?

[TonyT]

Standard direct connection meters will take 100amp per phase all day.

[Carrerahill]

18 hours ago, SBMS said:

Out of interest @Carrerahill what sort of a load would justify 3 phase?

A diversified load which demonstrates sustained current draw well over the 100A mark. 

 

When I do a load assessment, usually for a commercial/industrial development (residential, until now has generally just been taken as a 100A fuse) I take all the main plant loads, lighting loads, small power, lifts, water pumps, EVC, etc. and add them all onto a table.

 

Small power for an office for example is based on office type and W per meter  sq. So that gives an estimate of "uncontrolled loads" i.e. plugged in items. In your house you could work this out, by working out how many of you there are and what do you all do. Laptops, TV's, do you run washes a lot, cook a lot etc. etc.

 

Lighting can be much more accurate as you spec the lighting and know the connected loads and switching arrangement so you can work out burn time.

 

Then we apply diversity to the other loads. Lighting might be 80% if you assume stores, WC's etc. and occasional use rooms are not always lit, general power I usually take at 100% because I have already worked out estimated loads and I don't like to reduce it more if I am sure the estimate is good. Lifts in a busy office maybe 70-75%. Vent 90% Water pumps, 60% and so on.

 

At the bottom of the speadsheet it spits out a amps per phase figure, then I add 30% for expansion. On this office I am doing just now the figure was 215A per phase before the spare 30%. So clearly this will be a 3 phase supply, but had that figure rolled up at 16A per phase and I knew that we could get single phase kit, I would propose a 25kVA single phase supply which is about 100A. 

 

If you PM me your email address I will send you the spreadsheet and you could tinker with it. I will leave it populated so you can see how it works. 

 

 

 

 

 

 

 

[ReedRichards]

@Carrerahillclearly knows vastly more than I do but I can tell you that the man who came to change my mains fuse assured me that these are very slow blow and you could exceed the nominal current rating of the mains fuse for some time without harm.

[SteamyTea]

I was taught that fuses are designed to have a life, the higher the current, the shorter the life.  Was not a straight line but an exponential curve.

This is why, a low amp fuse i.e. may suddenly blow after years of sterling service for no apparent reason.

Also why we have quick blow fuses.

[Carrerahill]

8 hours ago, ReedRichards said:

@Carrerahillclearly knows vastly more than I do but I can tell you that the man who came to change my mains fuse assured me that these are very slow blow and you could exceed the nominal current rating of the mains fuse for some time without harm.

Indeed, see my graph posted above, that is the fusing curve of a BS88 (what they fit in the fuse carrier of just about every fused supply in the land) 80A and 100A. To get a 100A BS88 to fuse within 1 second (which is slow) you would need nearly 800A. They will of course fuse within 0.1s at about 1000A which is basically instantaneous short circuit protection.

Edited February 25, 2022 by Carrerahill