Current carrying capacity and volt drop calc

[sparky05084]

Hey all.

Hope that this post is relevant to this section. Got a call this morning from my colleague who is on a large industrial job at the mo. He has asked me to calculate and come up with a suitable cable size that is forming part of a distribution circuit. I also need a suitable fuse size which I believe will be > Ib?

A SWA will be run to a new three phase board 140M away clipped direct to fabric of building. Maximum demand has been quoted at 100A per phase. This I think is worst case scenario. No diversity has been applied.

Well, I’ve had a look in the regs and OSG and my mind has gone to marmalade

I will give you the info. This is all I have and I’m hoping this isn’t too vague. If I am missing vital info or this doesn't quite make sense please do inform me.

Thanks

Sparky05084

Cable: SWA four core insulated PVC
Length of run: 140M
Max demand: 100A per phase
Grouping: N/A
Insulation: N/A
Ambient temp: 30C
Op temp: 70C
Ref Method: clipped direct (basically to fabric of building with cleats + cable on its own not touching etc)

Would also appreciate a volt drop in here as well.

 

[Lizard King]

First of all, the overcurrent device should be 100A TP&N.

Next, work out if any de-rating factors should be applied....

Ca (ambient temp) - 1.00 for most types of cable/insulation
Cg (grouping factor) - 1.00
Ci (thermal insulation?) - 1.00
Cc (protective device / installation method) - 1.00 (not using semi-enclosed fuse and cables not buried)

So, no de-rating factors to worry about then...!

Therefore, the current-carrying-capacity of the cable should be >100A

From Table 4E4A, Ref Method E, choose a 4c 25mm2 cable (capacity 131A)

BUT, must also check volt drop...

for 25mm2 cable, highest VD is 1.9mV/A/m

Therefore, VD = (1.9*100*140m)/1000 = 26.6V, which is 6.65% of 400V so > the 5% limit set by the regs. Therefore, should look at the next cable size up , the 4c 35mm2.

for 35mm2 cable, highest VD is 1.35mV/A/m

Therefore, VD = (1.35*100*140m)/1000 = 18.9V, which is 4.7% of 400V so < the 5% limit set by the regs. Therefore, a 4c 35mm2 cable should suffice.

Being a consultant I'd then be tempted to up this to a 4c 50mm2, given the 4.7% VD, but then I'm not paying for the cable...! Also, if concerned about VD approaching the 5% limit, it's worth bearing in mind that most UK DNO's still operate at 415V so you have a little extra bounce to play with...

Of course, if your sub-main is a fair bit 'downstream' from the origin of supply to the facility, with a series of sub-boards inbetween, you'll also need to consider the likely total VD experienced by the infrastructure up to your sub-main point. In this case it's definitely a good idea to look at the 50mm2 or even 70mm2 cable...

Hope this helps... Sorry if there are sections here that are stating the bleedin' obvious but always best to cover all the bases....

LK

 

[m4tty]

^^^^^^^^^ what a reply ...... Nice one mate cheers

 

[acat]

Apart from what time to brew up your post covers it


Chris

 

[Spa Key]

Fantastic post Lizard. Clearly explained.

If you don't mind me asking how did you get into consultancy? Is it the sort of thing that requires a degree?

 

[Lizard King]

Well, I'm glad my reply seems to have well-received...!

As for consultancy Spa Key, I do happen to have a couple of engineering degrees but they're in Product Design and Building Services rather than pure Electrical Engineering...

I worked for an Electrical Contractor doing as-fitted drawings and coordination for a few years and they decided to let me have a go at some design. A few years later I decided to look for a job closer to home and consultancy was what I was doing anyway (by another name) so I decided to give it a bash.

So - in another long-winded response - not something you absolutely need a degree for (there's a couple of guys in my office who have HNC and that sort of thing) but a sound training in general engineering certainly helps, allied to some good practical experience. Similar to acceptance as a member of an Engineering Institution I guess, either via qualifications or experience...!

LK

 

[mechelec]

(Quote) First of all, the overcurrent device should be 100A TP&N.

Can you tell me why it needs to be TP & N. Most sub mains are fed via an MCB or MCCB or switch fuse and none of these switch the neutral. As far as I can see there is no requirement to switch the neutral in this situation

 

[Addlight]

Here's me, doing my first post (hello everyone!) and doing some my own first calcs for SWA to be run down someone's garden...

So very handy to read your answer on voltage drop as I'd just had to do it for my job, as was surprised that 4.0mm2 was a bit too close for comfort.

Very clearly explained - thanks!

 

[Lizard King]

(Quote) First of all, the overcurrent device should be 100A TP&N.

Can you tell me why it needs to be TP & N. Most sub mains are fed via an MCB or MCCB or switch fuse and none of these switch the neutral. As far as I can see there is no requirement to switch the neutral in this situation

Er, TP&N is generally just a description to identify the breaker as "triple pole and neutral". If we were switching the Neutral as well then this would be a "4-pole" breaker.

So basically, you're right enough - a TPN MCCB / Fused Switch will only make/break the line conductors and the neutral will be a solid connection.

Be careful though, as this all assumes a basic upstream infrastructure with a single connection to a TN-C-S source. If more complicated arrangements exist (such as standby generation or multiple transformers) then 4-pole breakers may well be required, to ensure connection to only 1 neutral point at any given time.

Hope that clarifies....

 

[mechelec]

Er, TP&N is generally just a description to identify the breaker as "triple pole and neutral". If we were switching the Neutral as well then this would be a "4-pole" breaker.

So basically, you're right enough - a TPN MCCB / Fused Switch will only make/break the line conductors and the neutral will be a solid connection.

Be careful though, as this all assumes a basic upstream infrastructure with a single connection to a TN-C-S source. If more complicated arrangements exist (such as standby generation or multiple transformers) then 4-pole breakers may well be required, to ensure connection to only 1 neutral point at any given time.

Hope that clarifies....

Be carefull, quoting a TP&N MCCB at the wholesalers will get you a four pole device whether you wanted one or not. A TP&N switch fuse will have three fuses and a neutral link. Correct terminology is important. Protective devices are usually either single, double, three or four pole devices, again there is no need to switch the neutral in this instance as no mention of secondary supplies is mentioned by the OP so only a 100A three pole protective device would be required.

 

[sparky05084]

What a superb answer!

You know its at times like this I am glad that I joined such a good forum. I didn't expect such a reply and it is very thorough indeed.

Much effort has gone to research my query and to post back. No matter how good you think you might be in this life or how much you may know, there is always someone who knows that little bit more and one should be gracious in approval and acceptance of this.

Thankyou Lizard King

*takes a bow*

 

[GKSElectrical]

Hi i was just reading your replys. just wanted to point out if i have read it correctly. you are installing a three phase fuse board 140mtr away from the supply. If this is correct i would like to point out that the volt drop is the total amount from the supply to the end of the circuit. so if you have a submain cable feeding a fuse board and the standard circuits from this fuse board. you will need to make sure that the cable to the board dont have to great a volt drop or the circuit to say the socket will have to be increased. i no which one is easyest to increase. calculate the fuse board loading and maximum volt drops on the circuits this will give you an idea of how much vold drop you have to play with.