Large electrical ufh install

[sclarkin]

Hi all any advice welcome.
Long story but at the last moment I have been asked to advise on the possibility of installing electric ufh in a large yoga room as bums are cold on the floor.
Room is 90sqm but my thinking for easy install supplier does 12sqm mats so perimeter left out that is 2x mat per phase giving 72sqm of load at 150w psqm. Not looked at 3 phase loading for a while but instead of taking 3x single phases to the plant room was thinking of a 3 phase 6mm rcbo supply(distance) from consumer unit to local isolator then contactor and single supplies out to 2x mat per cable/phase. Load would be balanced but still been domestic for so long thinking about the neutrals.
Sorry post got a bit long, hope you get the gist thanks

 

[James]

N should be same size as phase cables.

 

[sclarkin]

N should be same size as phase cables.

I appreciate that but maybe I'm thinking too much, I was thinking of 6mm 3 phase to the plant room then 4mm te out to a pair of mats. Therefore in my mind it's balanced so the original 6mm neutral would be fine for all the single phase loads

 

[davesparks]

N should be same size as phase cables.

I though it was still permitted to run a reduced size neutral for balanced loads?

 

[davesparks]

I appreciate that but maybe I'm thinking too much, I was thinking of 6mm 3 phase to the plant room then 4mm te out to a pair of mats. Therefore in my mind it's balanced so the original 6mm neutral would be fine for all the single phase loads

Why 4mm?

Personally if I was doing this I'd probably run a TP supply to a small enclosure with seperate protection for each mat, take a supply from there to feed what ever control system is required and control the mats via a contactor.

Protecting each mat seperately makes things easier when they start to fail.

 

[sclarkin]

Why 4mm?

Personally if I was doing this I'd probably run a TP supply to a small enclosure with seperate protection for each mat, take a supply from there to feed what ever control system is required and control the mats via a contactor.

Protecting each mat seperately makes things easier when they start to fail.

4mm l(ength of run) , my thinking originally was 3 phase to the contactor then out in 4mm te each phase to an enclosure with 20amp protection for each set of two mats. Stat to control the contactor

 

[James]

I though it was still permitted to run a reduced size neutral for balanced loads?

I think it is, however when choosing between 4 and 6mm cable, why would you bother?
also, if you are splitting the load up into 3 identical single phase loads, you can’t guarantee they are ballenced, turn off one or two breakers and your undersized N might be in trouble?

 

[sclarkin]

Why 4mm?

Personally if I was doing this I'd probably run a TP supply to a small enclosure with seperate protection for each mat, take a supply from there to feed what ever control system is required and control the mats via a contactor.

Protecting each mat seperately makes things easier when they start to fail.

I think it is, however when choosing between 4 and 6mm cable, why would you bother?
also, if you are splitting the load up into 3 identical single phase loads, you can’t guarantee they are ballenced, turn off one or two breakers and your undersized N might be in trouble?

Thanks both, 6mm is oversized for 3 phase but was thinking of it possibly being unbalanced. So now thinking 6mm swa 3 phase to an isolator, then out to single phase protection for mats / control circuit then to single phase contractors instead of a 4 pole one via stat.

 

[davesparks]

Thanks both, 6mm is oversized for 3 phase but was thinking of it possibly being unbalanced. So now thinking 6mm swa 3 phase to an isolator, then out to single phase protection for mats / control circuit then to single phase contractors instead of a 4 pole one via stat.

What has it being unbalanced got to do with oversizing the cable?

 

[sclarkin]

What has it being unbalanced got to do with oversizing the cable?

More load? No

 

[davesparks]

More load? No

No, being unbalanced doesn't create more load.
If you design the circuit for six heaters balanced across 3 phases then removing one of the heaters will reduce the load not increase it.

 

[sclarkin]

No, being unbalanced doesn't create more load.
If you design the circuit for six heaters balanced across 3 phases then removing one of the heaters will reduce the load not increase it.

Yes but I was thinking it would increase load on a particular phase causing more heat.

 

[sclarkin]

Yes but I was thinking it would increase load on a particular phase causing more heat.

More heat on the cable. Thanks for your posts

 

[davesparks]

Yes but I was thinking it would increase load on a particular phase causing more heat.

How on earth could removing one of six loads increase the load? It will reduce it!

 

[sclarkin]

How on earth could removing one of six loads increase the load? It will reduce it!

Yes total load decreases but the imbalance increases, as I say not looked into the phase for many moons. Just remember we had to balance it best as possible

 

[James]

A balanced 3 phase system the N load is zero, reducing the load on any phase will increase the N current.
worst case Is all fully loaded on one phase and nothing on the other 2, the N current will then equal the phase current.

 

[davesparks]

Yes total load decreases but the imbalance increases, as I say not looked into the phase for many moons. Just remember we had to balance it best as possible

Yes it will become imbalanced but that doesn't increase the load!
With an unbalanced load there will be some neutral current, but with simple resistive loads the neutral current will never exceed the line current no matter how far out of balance it is.

 

[sclarkin]

Yes it will become imbalanced but that doesn't increase the load!
With an unbalanced load there will be some neutral current, but with simple resistive loads the neutral current will never exceed the line current no matter how far out of balance it is.

Didn't explain very well but I was thinking of the neutral current.

 

[westward10]

You can only have a neutral of reduced csa if your line conductors are of a minimum csa of 16.0, copper conductor.

 

[davesparks]

You can only have a neutral of reduced csa if your line conductors are of a minimum csa of 16.0, copper conductor.

Interesting, I haven't really looked at it since I encountered 3 1/2 core SWA used for TPN quite a few years ago.

Can you still get 3 1/2 core SWA?