Overload Protection of "submain"

[Simon2021]

Hi can someone help me with this situation please.

The service head/meter is in a brick built housing box in the middle of a field, we are running power to a house some 40m away from it and im not sure of the most suitable method of protecting the 25mm SWA cable from overload. The DNO fuse is 100A and ideally would like to have 100A at the house. Can I safely use a 100A type C MCB in a small consumer unit at the origin? or is there a better way? I cant reduce the supply to 63A and go with a type B mcb as this wouldnt be sufficient.

Thanks

Simon

 

[Rockingit]

Hi can someone help me with this situation please.

The service head/meter is in a brick built housing box in the middle of a field, we are running power to a house some 40m away from it and im not sure of the most suitable method of protecting the 25mm SWA cable from overload. The DNO fuse is 100A and ideally would like to have 100A at the house. Can I safely use a 100A type C MCB in a small consumer unit at the origin? or is there a better way? I cant reduce the supply to 63A and go with a type B mcb as this wouldnt be sufficient.

Thanks

Simon

You're stuck between a rock and a hard place with this. From a purely electrical perspective the DNO fuse provides the overload protection, however there is the fabled 3m rule which says you need to provide a CPD and some discrimination. Your quick option would be an 80A KMF, there isn't really anything between that and 100A. If you want to get as close to 100A as possible just not quite (and I'd be intrigued on a domestic, but that's another story...) then you need to look at electronic adjustable MCCB's and then you're into big money.

 

[Darkwood]

Off the cuff I can tell you the cable is well undersized for your needs.

You need to establish the installation method, we already have the length and cable type so you then need to apply cable calc's to establish the recommended cable size for this run to ensure your VD is compliant.

What is in this house that demands so much electric? Most properties will happily run on a 60 or 80 amp fuse without incident.

PS - is this 1ph or 3ph?

 

[Rockingit]

Off the cuff I can tell you the cable is well undersized for your needs.

Well spotted, I didn't really look at that bit!

 

[Darkwood]

Well spotted, I didn't really look at that bit!

LOL - yeah it jumped out because (without checking) I believe its rated at 104amps surface mount meaning 40m and possibly buried might de-rate this by 40% but I will let someone else check up the figures, I'm winging the response without the tables here ?

 

[MJPD29]

100a at 40m in a duct could require 35mm, haven't checked though. Personally I'd go with the 80a fuse I doubt you'll ever need the 100a.

 

[Rockingit]

Personally I'd go with the 80a fuse I doubt you'll ever need the 100a.

Yes, I suspect someone hasn't done their diversity calcs

 

[Simon2021]

You're stuck between a rock and a hard place with this. From a purely electrical perspective the DNO fuse provides the overload protection, however there is the fabled 3m rule which says you need to provide a CPD and some discrimination. Your quick option would be an 80A KMF, there isn't really anything between that and 100A. If you want to get as close to 100A as possible just not quite (and I'd be intrigued on a domestic, but that's another story...) then you need to look at electronic adjustable MCCB's and then you're into big money.

Off the cuff I can tell you the cable is well undersized for your needs.

You need to establish the installation method, we already have the length and cable type so you then need to apply cable calc's to establish the recommended cable size for this run to ensure your VD is compliant.

What is in this house that demands so much electric? Most properties will happily run on a 60 or 80 amp fuse without incident.

PS - is this 1ph or 3ph?

Ok, so its buried underground, on the cable calculator I used, I admit I have forgotten all the manual calcs, 25mm came out as more than sufficient for 100a with a 2.7% VD... if buried! I am happy to be schooled on this because its been some time since school and I have never come across a situation like this.

I should add that this is a kind of temporary measure as a second house is being built on the land and the service head will be moved to the main house and a second supply put in to feed to the first house. Then we will be able to adhere to the 3m rule. ( which i thought had dropped to 1m now)

The house could survive on 80 Amps however they may want to put a kiln in it at some point. hence the higher requirement.

 

[Simon2021]

Sorry its 1ph

 

[westward10]

The rule of thumb for submain vd is 2% and based on 40m, 3-core cable and 80 or 100A you are looking at 35.0 minimum.

 

[Rockingit]

433.3.1 (iii) does actually give you an answer to this, if you can get the DNO to agree it

 

[Simon2021]

The rule of thumb for submain vd is 2% and based on 40m, 3-core cable and 80 or 100A you are looking at 35.0 minimum.

ok so its coming at 2.2% for 78amps across 40m is this not close enough to the rule of thumb? Also I wasnt aware that it was 2% for submains, can you reference that please?

 

[Darkwood]

Just to add here, temp' supplies still have to comply to the same regulations as permanent installs, the ability of the cable to carry the load and meet requirements of VD does not change because you are only using it for several weeks/months.

Can we also ask why such a huge demand for a house, is it a large state house?

 

[Simon2021]

Just to add here, temp' supplies still have to comply to the same regulations as permanent installs, the ability of the cable to carry the load and meet requirements of VD does not change because you are only using it for several weeks/months.

Can we also ask why such a huge demand for a house, is it a large state house?

oh I completely agree, but this seems to be on the edge of 25mm and 35mm cable from what earlier posts have said. As i mentioned it may have a kiln in it which has a large draw, but if they arent using it at the same time as the induction hob, oven, washing machine then im sure it will be fine at 80.

 

[Strima]

ok so its coming at 2.2% for 78amps across 40m is this not close enough to the rule of thumb? Also I wasnt aware that it was 2% for submains, can you reference that please?

It's not referenced but the volt drop is from the source of supply to the end of your final circuits. If you use 2.2% on your distribution circuit that gives you only 0.8% to play with for lighting.

What is the expected maximum demand for the new build?

 

[Simon2021]

It's not referenced but the volt drop is from the source of supply to the end of your final circuits. If you use 2.2% on your distribution circuit that gives you only 0.8% to play with for lighting.

What is the expected maximum demand for the new build?

Thanks, I take your point, just looking at a 60w bulb at the end of a 50m lighting circuit the vd is 0.2%, everything is led lighting so I reckon this will still be within.

well without the kiln... and with diversity.... 54 amps is the number i am getting to, which seems low so im not certain im calculating it correctly divide the total of mcb's by 1.4?

 

[Simon2021]

nope thats wrong, 102, being 144/1.4,

 

[Strima]

If this is a domestic dwelling 54 amps is pretty high.

How many rooms? Electric water heating etc? EV charging point?

 

[Simon2021]

If this is a domestic dwelling 54 amps is pretty high.

How many rooms? Electric water heating etc? EV charging point?

yes, it is just a 3 bed bungalow, no ev, no water heaters.

Ive just revisted my books and now have it at 80amps with the diversity.

Theres a 32a RF, 1x 20A Radial, 2 6A light circuits, 16A boiler, 32amp oven and 32 amp hob.

I followed the onsite guide method (making assumptions about the size of cooker and hob) and it comes out at 80 amps

 

[MJPD29]

yes, it is just a 3 bed bungalow, no ev, no water heaters.

Ive just revisted my books and now have it at 80amps with the diversity.

Theres a 32a RF, 1x 20A Radial, 2 6A light circuits, 16A boiler, 32amp oven and 32 amp hob.

I followed the onsite guide method (making assumptions about the size of cooker and hob) and it comes out at 80 amps

Even at that you'll probably rarely if ever be near 80a at any given time