Derating power circuit in insulation.

[andreas]

I have been told that as I am pulling up all flooring, I have to add insulation. I know that running a cable through insulation means that you have to derate it. 

I have done the maths on all the lighting circuits, even derating the cables 50%, and assuming a worst case scenario of max lamp wattage per fitting possible with worst power factor, the lighting circuit has a massive safety margin, max current would be half the derated capacity.

What I don't understand is the power circuit - how do you work that out? In theory in pretty much every house I've been in, it's possible to overload the *cable's* current capacity if you were stupid enough to plug a 2kw heater into every outlet. 

I don't want to have to replace all the perfectly good power circuits that are in place just because I want to take the floor up and have to add insulation as a result. Best way to work around it?

Edited December 19, 2023 by andreas

[MikeGrahamT21]

Which floor are we talking? Assuming first floor if you are on about lighting circuits?

[andreas]

11 minutes ago, MikeGrahamT21 said:

Which floor are we talking? Assuming first floor if you are on about lighting circuits?

 

All floors throughout. 

[JohnMo]

How do you safely derate the circuit, what stops the wife or future owner plugging a 3kW kettle or hair dryer?

 

What rules mean is you have to install bigger cables, not put a sticker (for example) on an outlet.

 

Just move the majority of cable to above or below the insulation, then it's not insulated.

[andreas]

26 minutes ago, JohnMo said:

How do you safely derate the circuit, what stops the wife or future owner plugging a 3kW kettle or hair dryer?

 

What rules mean is you have to install bigger cables, not put a sticker (for example) on an outlet.

 

Just move the majority of cable to above or below the insulation, then it's not insulated.

That is my point - as it already stands, the majority of houses I've seen have enough outlets per circuit that if you plugged a high draw device in every outlet, it would surpass the rated current carrying capability of the circuit even without insulation being a factor.

I'm not saying to derate the circuit with stickers, I'm asking how you're supposed to calculate it nowadays. As far as I am aware you can still have as many outlets on a circuit as you want, unless modern houses that are fully insulated have only got a couple of double sockets per circuit?

Very few of the cables can be moved easily, to the degree I think it would be easier to rewire, but that seems a ridiculous requirement just so floors can be lifted. Also laying cables over the insulation but under the floor would mean it's almost touching the underside of the floor.

Edited December 19, 2023 by andreas

[JohnMo]

When you insulate between floors you don't full fill, you part fill with maybe 50mm of insulation. It's there to the drum effect.

 

Who is saying you have to add accoustic insulation?

[andreas]

1 minute ago, JohnMo said:

When you insulate between floors you don't full fill, you part fill with maybe 50mm of insulation. It's there to the drum effect.

 

Who is saying you have to add accoustic insulation?

BC said "more" insulation between floors needed, though waiting for clarification on how much, and it needs full insulation below ground floor. BC isn't that helpful. Ground floor cabling was all obviously done on a budget that involved saving every possible mm of cable so they'd be too short to reroute. 

I am just trying to understand better how they calculate power circuits now with all the insulation required everywhere. 

[SteamyTea]

Is there a diversity formula for ring circuit?

[andreas]

2 hours ago, SteamyTea said:

Is there a diversity formula for ring circuit?

This is what I was trying to work out when I asked the question. 

[saveasteading]

3 hours ago, andreas said:

3 hours ago, JohnMo said:

When you insulate between floors you don't full fill, you part fill with maybe 50mm of insulation. It's there to the drum effect.

 

Who is saying you have to add accoustic insulation?

Expand  

BC said "more" insulation between floors needed

I didn't know this!

It seems that the easy options are 1. Single layer plasterboard ceiling and 100mm mineral wool on it.

2. Double layer plasterboard  no insulation.

 

Floor chipboard  has to be a certain density, as does the pb.

[Mr Punter]

I don't think this is one to lose sleep over.  As long as the cables are not buried in the middle of insulation you are unlikely to have an issue with them overheating.  They can be better (cooler) clipped to joists.

 

Unless BC have requested derating it or you have used cable smaller than 2.5mm2 I would just ignore this.

 

There is no requirement to fit thermal insulation other than on the bottom floor and none to fit acoustic insulation in an existing house.

[saveasteading]

8 minutes ago, Mr Punter said:

I don't think this is one to lose sleep over.  As long as the cables are not buried in the middle of insulation you are unlikely to have an issue with them overheating.  They can be better (cooler) clipped to joists.

 

Unless BC have requested derating it or you have used cable smaller than 2.5mm2 I would just ignore this.

 

There is no requirement to fit thermal insulation other than on the bottom floor and none to fit acoustic insulation in an existing house

I thought that until this discussion.

 

[Mr Punter]

6 minutes ago, saveasteading said:

I thought that until this discussion.

 

 

I understood that part E only applies to new build, change of use and conversions.  I think that @andreas is working on an existing house.

[SteamyTea]

1 hour ago, andreas said:

3 hours ago, SteamyTea said:

Is there a diversity formula for ring circuit?

This is what I was trying to work out when I asked the question

Ask @ProDave

[Mattg4321]

Diversity is not usually applied to a single ring final circuit. It’s not really relevant. 
 

The circuit can’t be overloaded, because if the current surpasses 32amps for any length of time the breaker will disconnect. Ie if you plug in enough 2kw heaters. 
 

Each point on the ring is supplied with 2x cables. The key requirement being that each cable must be rated at 20amps or greater. Table 4D5 is the one to use here for twin and earth cable. Reference method 100 is probably the most likely result of acoustic insulation between floors. This gives a current carrying capacity for 2.5mm2 of 21amps and is thus acceptable. 

[andreas]

25 minutes ago, Mattg4321 said:

Diversity is not usually applied to a single ring final circuit. It’s not really relevant. 
 

The circuit can’t be overloaded, because if the current surpasses 32amps for any length of time the breaker will disconnect. Ie if you plug in enough 2kw heaters. 
 

Each point on the ring is supplied with 2x cables. The key requirement being that each cable must be rated at 20amps or greater. Table 4D5 is the one to use here for twin and earth cable. Reference method 100 is probably the most likely result of acoustic insulation between floors. This gives a current carrying capacity for 2.5mm2 of 21amps and is thus acceptable. 

Thank you, that was interesting reading and very helpful I think!

A question if I may - if the cable is clipped along the joists so only has insulation on one side of it, would you then use reference method 102? I know the end result is the same I just don't want to try and argue it and not have things right in my head. Would you also have reference to where >20amps is acceptable just so I have everything to back it up?

[Mattg4321]

23 minutes ago, andreas said:

Thank you, that was interesting reading and very helpful I think!

A question if I may - if the cable is clipped along the joists so only has insulation on one side of it, would you then use reference method 102? I know the end result is the same I just don't want to try and argue it and not have things right in my head. Would you also have reference to where >20amps is acceptable just so I have everything to back it up?

Look at Table 4A2 for more detail on installation reference methods. It’s a judgment call though really. 
 

Regs book is out in the van, but think it’s Regulation 433.1.204 in BS7671. 

[Roger440]

2 hours ago, Mattg4321 said:

Diversity is not usually applied to a single ring final circuit. It’s not really relevant. 
 

The circuit can’t be overloaded, because if the current surpasses 32amps for any length of time the breaker will disconnect. Ie if you plug in enough 2kw heaters. 
 

Each point on the ring is supplied with 2x cables. The key requirement being that each cable must be rated at 20amps or greater. Table 4D5 is the one to use here for twin and earth cable. Reference method 100 is probably the most likely result of acoustic insulation between floors. This gives a current carrying capacity for 2.5mm2 of 21amps and is thus acceptable. 

 

A good point. 

 

Ive been pondering the effects of beads in a cavity that has the ring main running in it. It not as bad as i feared. As you say, 2 cables feeding any given poinr.

 

 

 

[Mattg4321]

The reality is that most rings these days are carrying a very light load, with the exception of kitchens, but even here the load will be sporadic. 
 

Very few people plug in electric heaters nowadays. 
 

The number of burnt out connections on a ring circuit that I’ve seen is very low compared to cookers/showers/main switches/neutral bars in consumers units where it’s very common

[SteamyTea]

7 hours ago, Roger440 said:

Ive been pondering the effects of beads in a cavity that has the ring main running in it.

That can give you other problems.  Polystyrene can degrade the PVC insulation.

 

[Mattg4321]

4 minutes ago, SteamyTea said:

That can give you other problems.  Polystyrene can degrade the PVC insulation.

 

This isn’t a definitive answer, but I’m not sure it’s something to lose sleep over. I’ve come across loads of cables stuck to polystyrene, but in no cases did the cable sheathing appear brittle, although obviously some sort of reaction has taken place for them to become stuck. It does seem to happen more frequently and seriously with older polystyrene and older cables, but that could just be due to them being in contact for a longer period of time. 
 

I’ve never come across a measurable fault caused by polystyrene and cables touching. That’s not to say it doesn’t happen, it just must be uncommon. 
 

 

 

[saveasteading]

17 hours ago, Mr Punter said:

E only applies to new build, change of use and conversions.  

That seems to be right.