Cable Calc Confirmation

[NozSpark]

Guys,

I've done some cable calculations (from the book) and tried to confirm them using an online calculator and they've come out slightly different so I'd appreciate it if someone could double check me.

It's a 3 phase TNS supply with a Ze of 0.39ohms, I want to run a 3 phase SWA submain 32m from this protected by 100A HRC fuses. I've calculated the demand per phase to be 78A 79A & 86A (after diversity),,,, having domestic type final circuits including 2 x 50A submains a further 10m and 16m away supplying separate accommodation. Part of the submain (first one) will be underground, the other two will be T&E clipped direct.

I've calculated that the first submain should be 50mm (according to BRB) and the online calculator (Doncaster cables) came up with 25mm but with a volt drop of something like 6.5V:O

 

[steptoe]

Im not doing your homework for you!

:slap

in other words, I cant be A**** getting me books out at this time of night, cos I doing some certs and Im fed up already........

 

[NozSpark]

Oh cheers (not)

It's a long time since I was at college mate

 

[NozSpark]

Cough

 

[Sidewinder]

Noz,

If you still need to check this, poke me later if they let me out of prison I'll check it for you!

 

[NozSpark]

Cheers, will do. I could do with it TBH

 

[bryanh]

cant do a print out but cable calc on my mobile says 25mm is ok vd is only 2.7volts ,if I was you I might up it to 35mm but 50mm is way over the top

Worth getting its only £1 https://play.google.com/store/apps/details?id=org.coopersoft.Cablecheck&hl=en

 

[ProDave]

Can I throw my spanner in.

The BRB (or BGB) tables, I have always understood includes the volt drop in both L and N cores of the cable.

Fine for single phase.

But the 3 phase column has a very similar (only slightly lower) volt drop figure.

Now, in your case, the phases are very nearly balanced, so the ACTUAL neutral current is going to be very small so much less volt drop in the N core.

So I would expect the real world total volt drop to be a lot lower than the tables suggest.

Discuss.

 

[NozSpark]

Here's what I get on my calc software,, my own calcs are identical

Cable sizing calculated on iPhone using -Capable-Cable type: 70 deg.C Multi (armoured)

Fixing method: In ground (direct)

Protection device: BS 88 (gG)

Protection rating: 100 A

Cable size: 50 mm

CPC size: 25 mm

Current carrying capacity of cable in this instance: 116 A

Final circuit: No

Ambient temperature: 20 deg.C

Number of cables grouped: 1

The cables are spaced

Voltdrop: 0.56 %

Allowable voltdrop: 3.00 %

Length: 32.0 m

Calculated R1+R2: 0.036 ohms

Calculated Zs: 0.43 ohms

Maximum Permitted Zs: 0.34 ohms

Type of system: TN

Supply: Three phase at 400 v

Ze: 0.390 ohms

Fault current: 0.54 kA

Applied correction factors:

Ca: 1

Cg: 1

Ci: 1

Cc: 0.9

---------------

Ct: 0.9

Zs above maximum value for protective device! Your Ze value is over the quoted value for a TN-C-S system! If you have TN-C-S, contact your supply authority.

RCDs must be incorporated into any circuit where the cables are installed less than 50mm from the surface without earthed metallic sheathing, on socket outlets in domestic and similar situations and in some special locations (ie. bathrooms) You have agreed to abide by the terms and conditions of this app. It is your responsibity to confirm the results calculated here. C.Smith Software takes no responsibility for loss or damage caused by any errors in this app.

-Capable- Copyright C. Smith Software 2011

 

[Sidewinder]

Noz,

Forget the cable calc software mate.

You have an allowable R1+R2 of 0.07 hms or 0.01 hms based on design values regardless of anything else.

You are going to need a HUGE cable to satisfy this at 32m!

You can work this backwards if you want and based on copper for both conductors you will need 0.035 hms per conductor which at 32m means 0.0011 hms /m.

TBH going on the table in the OSG, which is on its limits of application you ARE looking at 50+50 for R1+R2 TBH!

TBH I'm amazed that a TN-C-S supply with that sort of Ze is "stiff" enough to provide an additional 100A of supply?

Have you estimated the VD at the origin for the supply based on the Ze = "ZL-N"?

Anyway, taking 35+35 would give you 1.048 hms x 10-3/m = 0.03668 hms /m, which is actually too high.

So taking 50+50 you would have 0.714 hms x 10-3/m = 0.229 hms /m, to 3 significant figures, which is just in.

So your idea of 50+50 is the way to go, but this is pushing the boundaries.

This is not a rigorous proof, just a quick calc from the OSG.

Ring me quickly if you want to discuss, as I'm off to bed soon I'm in one of HM's hotels again early tomorrow!

 

[NozSpark]

It actually a TNS supply, but the calc software only differentiates between TT and "not TT" IYKWIM

 

[Sidewinder]

Doubtful it is a "real" TN-S TBH, do a ZL-N & a Ze & see what you get first!

 

[NozSpark]

When I'm there next I'll check it out,,,, which won't be this week

TBH I'm glad you're quick calcs are similar to mine.... I did do my own calculations BTW but they were sooooo far off from the online calculator (Doncaster cables) that it got me second guessing myself,,, IIRC my first go even came out at 75mm