200a tpn for house?!

J.C.E · Sep 23, 2021

Can i scratch your head (bare in mind biggest i have done is 25mm 4core ?)

I am tendering for a big house new build

I think they already have 100tpn
(But in a kiosk 40m away from house)

And i think they are doubting its big enough now (they sent max demand table below)

So they are asking for a price for 200a tpn sub mains to house if they upgrade from 100a to 200a tpn.

Am i right that the swa from kiosk to house (40m through underground duct)

Will need to be 150mm 4c with a 120mm cpc

And its just bad design that the income is so far from the house and needs a swa sub main!

westward10 · Sep 23, 2021

I haven't calculated it but 240.0 sounds well oversized. For ease of installation parallel cables are often better and I would have thought a pair of 70.0s would suffice but it needs calculating using a 2% vd. The cpc size will need calculating and it must be of a csa to support the main protective bonding but again 185.0 seems oversized.

pc1966 · Sep 23, 2021

Is that table showing single phase loads or 3-phase?

If it is the sum of single phase then it is below 300A so with tolerable balance a 100A 3-phase feed sounds enough to me. Or have I missed something here?

In any case 200A 3-phase and 3% drop on-line calculator is coming out at 120mm, added CPC of 70mm to 4 core SWA might be OK but obviously need to look at Ze and end of cable Zs for 5s disconnection.

pc1966 · Sep 23, 2021

OK looking at some of the examples it looks like they are 3-phase loads so it is really adding up to that sort of value.

But if they use only 1/4 of the diversity-allowed current it is going to cost £125/day!

brianmoooore · Sep 23, 2021

Surely that's 189.9A diversified load across all three phases, 63A each? Also, if it's one dwelling, more diversity could probably be applied.

pc1966 · Sep 23, 2021

brianmoooore said:
Surely that's 189.9A diversified load across all three phases, 63A each? Also, if it's one dwelling, more diversity could probably be applied.

It is hard to read but the currents for, say, the 22kW EV charger look to match 3-phase calculation on that power.

But I agree, it looks a crazy total!

David Prosser · Sep 23, 2021

I don't think the diversity is anywhere near accurate. Induction hob GSHP water pumps and drainage pumps running all day ?
50% of sockets in use 66% light on.

J.C.E · Sep 23, 2021

some of the circuits will be three phase- like car charger, GSHP, ASHP etc

should you only divdide 189/3 if all circuits are single phase?

Am i right that the swa from kiosk to house (40m through underground duct) so ref method D for three phase cable:

Will need to be 150mm 4c with a 120mm cpc for a 200a sub
Or if they stick with 100a sub- a 50mm 4c with a 35mm cpc will be adequate?

pc1966 · Sep 23, 2021

J.C.E said:
some of the circuits will be three phase- like car charger, GSHP, ASHP etc

should you only divdide 189/3 if all circuits are single phase?

However you do it, do it consistently. Looking at some of the numbers (e.g. downlights) you seem to be doing it the 3-phase way (i.e. total power / (230 * 3) = 3-phase current) but check you really are consistent.

Also at some point plan the loads so they are fairly well balanced per-phase. The bigger ones are by design, but otherwise look at how they get allocated in the TPN board(s), or how you split out SP sub-boards, etc.

J.C.E said:
Am i right that the swa from kiosk to house (40m through underground duct) so ref method D for three phase cable:

Will need to be 150mm 4c with a 120mm cpc for a 200a sub
Or if they stick with 100a sub- a 50mm 4c with a 35mm cpc will be adequate?

Depends on supply Ze as well as volt drop. 35mm should be OK for 100A only on CCC but at 40m it is almost certainly going to be VD or R1+R2 that is your issue.

pc1966 · Sep 23, 2021

Looking at page 367 of the BBB, a BS88 fuse rated at 200A has 5s disconnection at 1200A. So your Zs for cold-checked cables and Umin is given by:

Zs = 230 * 0.8 * 0.95 / 1200 = 0.146 ohm.

You need to know Ze to allow for R1+R2 but assuming my example of 120mm & 70mm CPC it is:

R1 = 0.153 mOhm/m
R2 = 0.268

For 40m = 40 * (0.153 + 0.268) = 0.017 ohm, hence Ze < Zs - (R1 + R2) = 0.146 - 0.017 = 0.129 ohm

Pedants might query the use of DC values of R1+R2 instead of AC impedance, I leave that as an exercise for the reader...

WallaceP · Sep 23, 2021

Why have they used a p.f. of one?

David Prosser · Sep 23, 2021

WallaceP said:
Why have they used a p.f. of one?

Same reason as KVA is included ???

pc1966 · Sep 24, 2021

A quick check shows you can meet 3% voltage drop with 50mm at 200A so the above cable calculator values really are coming down to CCC.

So 120mm is going to meet even 2% VD as well as the CCC. Running a parallel 70mm CPC would meet Table 54.7 of course, but realistically 35mm is going to be fine on adiabatic for 200A fuse.

Looking at using a pair of cables and assuming grouping of 0.8 then you need 125A CCC per cable. For 70C limit you need 70mm 4-core (Table 4D4A column 7), for 90C it is 50mm (Table 4E4A column 7).

Running 90C is worrying to me as not all accessories will be happy or long lived at that, so I would go with 2 * 70mm just as @westward10 suggested. For the CPC another 70mm would then meet Table 54.7 as well, though if you check adiabatic and resulting Zs you might just find 35mm is perfectly good.

EDIT: In reality this is a house, not an industrial plant, so it is very unlikely they will hit 200A for any length of time, so specifying 50mm 90C design cables is not unreasonable if you check that the planned supply switch-fuse and main DB isolator are rated for 90C use and can cope with 2*50mm, etc.

pc1966 · Sep 24, 2021

Just to clarify the above, 2*50mm in duct looks to meet the CCC,

If you specify free air / perforated cable tray on the Doncaster cable calculator it comes up with this as suitable:

Single Core 90°C Thermosetting Insulated Cables, Unarmoured, With or Without Sheath

Tables Apply to: 6491B, 6181XY

Voltage: 400V

Load: 110.85kW / 200A

Length: 40m

Method of Installation: In free air or on a perforated cable tray

Cable Size: 50mm²

It was just a quick check on volt drop to see what would be possible without having the tables to hand.

J.C.E · Sep 24, 2021

I think I am going to price

for 200a a 4c 120mm 90deg +70mm CPC
for 100a a 5c 35mm 90deg

200a tpn for house?!

J.C.E

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