Sub main question

[t0m87]

Hello, ive recently been to a job where a new electricity supply is being installed. I need to run a 5 core 16mm swa 85 metres. will this be ok on a 100amp switch fuse with 80amp fuses? all the cable calculators ive used say yes and have been told on the phone, however im thinking its close and 63 amp the better option. Thoughts please. Supply is on its own in a duct, duct existing. 4 way tpn board on the end supplying a few floodlights. Maybe an outbuilding fed off it in the future. Thanks in advance

 

[Simonslimline]

What do your calcs say?

 

[Darkwood]

It would depend on whether you need to achieve discrimination here, you would normally be looking at a 160amp sub supply to give discrim' on your standard mcb's although some brands do a lower rated 125amp mccb that gives the same discrimination (Schneider) when used upstream of their boards.

If the supply coming in is lower than these values then you cannot achieve it and you cannot guarantee a fault on any final circuit will not take out the sub' OCPD but it is better to over rate on this occasion and fit the highest one you can IE - 100amp which afford a better partial discrimination and is less likely to fuse on a fault.

 

[Richard Burns]

16mm² TP SWA in a duct is only rated to 64A.
The volt drop over 85m even on only a 20A load will exceed the recommended level for lighting.

Since you will therefore only be drawing 20A at most on the supply then a 63A fuse protecting the sub main should not be a problem.
Though I am sure I have missed something obvious here.

 

[Darkwood]

Best ask the question here, is this domestic or industrial/commercial, my previous reply was worded for the latter and I should have asked the question first.

 

[davesparks]

I need to run a 5 core 16mm swa 85 metres. will this be ok on a 100amp switch fuse with 80amp fuses? all the cable calculators ive used say yes and have been told on the phone, however im thinking its close and 63 amp the better option. Thoughts please.

My thoughts are that you have carried out these calculations completely wrong!
You appear to have started with a cable size and worked backwards towards an In
You need to start with Ib from which you can establish In (taking in to account diversity etc) and then apply de-rating factors before selecting the cable size from the tables.
Then calculate the volt drop and the predicted Zs.

Why have you selected 16mm 5core?

 

[t0m87]

New supply / meter etc has not been put in yet, however there is a small rigid duct running g the 85 metres with a 10mm 3 core swa. This needs to be taken out. Maximum cable to be used is 16mm 5 core, due to duct size. Appreciate the discrimination views.

 

[telectrix]

work out your cable size as davesparks suggests. then determine if a larger duct needs to be dug in.

 

[Spoon]

The volt drop over 85m even on only a 20A load will exceed the recommended level for lighting.
.

I may be reading this wrong mate but the volt drop on a 16mm armoured cable is 2.4mV/A/m (table 4D4B). Which is 85x20x2.4 / 1000 = 4.08V (1.02%). This is less than the 3% for lighting.

According to my calcs the biggest load you can have is 58A without going over 3%.

 

[GEOFF S]

I may be reading this wrong mate but the volt drop on a 16mm armoured cable is 2.4mV/A/m (table 4D4B). Which is 85x20x2.4 / 1000 = 4.08V (1.02%). This is less than the 3% for lighting.

According to my calcs the biggest load you can have is 58A without going over 3%.

Hi

Not sure about 58A, maybe more like 40 Amps.

Cheers

 

[Spoon]

Hi

Not sure about 58A, maybe more like 40 Amps.

Cheers

This is what I get. 85 x 58 x 2.4 / 1000 = 11.832V (2.958%)

 

[telectrix]

5% spoon. you've used 400 as the voltage. should have used 230.

 

[GEOFF S]

I may be reading this wrong mate but the volt drop on a 16mm armoured cable is 2.4mV/A/m (table 4D4B). Which is 85x20x2.4 / 1000 = 4.08V (1.02%). This is less than the 3% for lighting.

According to my calcs the biggest load you can have is 58A without going over 3%.

Hi Spoon

Okay i see your using 400 V, apologies. I tend to work to 230 V, especially when loading isn't known etc. The three phase mV/A/m accounts for no neutral current, this sound unlikely in the ops case.

Cheers

 

[Spoon]

The OP mentioned 5 core. I presume that its for 3 phase, N & E. Yes it is also presuming the load is balanced. I seem to be presuming a lot....

 

[GEOFF S]

The OP mentioned 5 core. I presume that its for 3 phase, N & E. Yes it is also presuming the load is balanced. I seem to be presuming a lot....

Hi

Lol, i think we all do a lot of presuming, and yours is a fair one. Utilising the 400 V, at 58 Amps cable temp is 63 C, now utilising 230 V this limits the cable temp to 40 C. So less energy dissipated, allows more headroom for harmonics etc and takes into account worst case neutral current. That said a lot of text books follow your example, i suppose the reasoning being in real life Volt drop is often far less than calculated.

Cheers

Cheers

 

[Richard Burns]

I may be reading this wrong mate but the volt drop on a 16mm armoured cable is 2.4mV/A/m (table 4D4B). Which is 85x20x2.4 / 1000 = 4.08V (1.02%). This is less than the 3% for lighting.

According to my calcs the biggest load you can have is 58A without going over 3%.

Whoops! I thought I had missed something obvious; such as using a percentage rather than a voltage!
Sorry about that.
Even so a 58A or 40A load will be fine on a 63A protective device.

 

[Spoon]

I'm sure the OP will get back to us to clarify things.