max load of 10mm and 16mm armoured

[adammid]

I am being told to run some 10mm and 16mm armoured cables. Both of which are protected by 100amp breakers.

The 16mm is three phase and the 10mm is single phase.

Both are feeding sub boards.

Firstly I said that a 16mm minimum should be used to feed a sub board and both cables are not capable of carrying 100amps.

I've basically been told that it's ok and do as I'm told.

Any thoughts guys. The cables are run on tray so therefore I understand they can carry more load but neither can carry 100amps right?

 

[ParkySparky]

Length to consider aswell but never the less seriously undersized! Clipped direct 16mm basic rating is about 83A!

Check your tables in the regs then throw it back at whoever told you this tosh and refuse the job!

 

[adammid]

It's the contractor that I'm working for. There argument is it's a 3 phase breaker rated at 100amps and the load is spread across 3 phases. Therefore the 3 cores of 16mm armoured will only have 33 amps each max running through it if all 3 phases are balanced.

I've told them that doesn't make sense. What if the phases are not balanced and one phase is loaded with 99 Amps or there is a fault lets say for arguments sake. The fuse won't trip and the cable won't melt.

I've said it needs to be at least a 25mm 4 core armoured.

Also a 100 amp 3 phase breaker is rated to take 100 Amps per phase right or is it 100 amps in total?

I always thought it was 100 amps per phase but I'm now starting to doubt. The guy is so confident in what he is saying. Arrggghhhh

 

[alpha spark]

Your right he's wrong is this guy a spark or some other trade

 

[adammid]

No he is a spark. Boss of a company with 15 guys working for him. They do some bigish jobs too. He acts as though he knows everything. It's only my first day with him and i think he thinks I'm a trouble maker already. Haha

 

[sb_89]

It's a 100A per phase, but you really should know that if your undertaking the work, are you just doing the run like a labourer or are u connecting and testing ? Because it doesn't sound like either of you should be designing or signing the job off

 

[adammid]

My previous post I put the cable WON'T melt. Bloody predictive text. It's meant to say the cable WILL melt.

 

[sb_89]

Your obv, doing the right thing and have enough about you to question what your told, I don't mean to be to harsh

 

[apprentice87]

Hi there, the contractor is a proper prat. A 100A breaker is obviously 100A per phase...

What about proper cable calcs???? Type of earthing too, as this COULD have a very large impact on cable selection...

john..

 

[DizzyMaskell]

Sound to me like the guy who told you this is not competent to undertake any jobs stick to your ground better to be sacked than cause fires !!!!

 

[sb_89]

If you gave the length of run and type of swa - PVC or xlpe , installation method it wouldn't take 5 mins to work out and earthing arrangements but I'm assuming it's a tn system

 

[ParkySparky]

I'd find a new job pal because I wouldn't be putting my name to that install! If your putting a 3 phase DB on end of swa then how the hell does he reckon it'l only have 33A per phase it could be added to surly?

Sounds like a right to55er to me!

I'd hate to see how many bits of 3 phase equipment he'd install with a hugely oversized mcb if he thinks the rating on them is total load not per phase!!!!!!

 

[Steve3948]

So it's a Sub-Main, what length is it.

It's hard to believe that this guy has an established business and thinks 100A is split across 3 phases and not 100A per phase, are you sure you are not confused and the design total load is 100A spread across 3 phases in which case he would fuse much lower than 100A.

Is there a 100A Mcb, I've only seen and used 63A Max, anything over that is MCCB.

 

[ProDave]

It's the contractor that I'm working for. There argument is it's a 3 phase breaker rated at 100amps and the load is spread across 3 phases. Therefore the 3 cores of 16mm armoured will only have 33 amps each max running through it if all 3 phases are balanced.

It's frightening that people designing 3 phase installs actually think like that. The guy obviously does not understand what he is doing, and as others have said, should not be in the position of designing and installing anything to do with electricity.

As others have already said, stand your ground and don't install it.

Some more information on what it's feeding, what loads it will supply and what distance will help. Then we might be able to advise what SHOULD be used, and you can go back and tell him how it should be done and earn the brownie points.

 

[sb_89]

Yeah Steve3948 makes a really good point, sure you haven't go your wires crossed ? That would be a reasonable explanation . The biggest I've ever used is also 63A 3ph

 

[wozz]

you say you are working with him and its your first day. Personally id go along with it and ask questions as to why things are done like that rather than downing tools. No where have you said you are I&T or energising it so its on his head be it. Of course presuming I have read it correctly.

Also as it probably a MMCB these are sometimes adjustable downwards.

 

[Riggy]

If this guy has 15 people working for him, and has some biggish jobs on the books, and assuming he's been in business for a while, then I think it's safe to assume you've got hold of the wrong end of the stick.

My advice would be to get off the forum, get on with your work and see what develops.

 

[adammid]

Right guys, a bit of an update.

I can confirm that this is a sub main. It is protected by a 100 amp MCCB. It is 16mm 70c multicore thermoplastic armoured. There are 3 circuits like it all 3 phase protected by a 100amp MCCB, all in 16mm armoured.

I know it's 16mm as it says on the drum that I'm pulling it off and from experience and other guys there it is 16mm.

The three circuits are 57meters, 65 meters and 75 meters. All three circuits have a design current of 60amps so yes I would assume the MCCB would be adjusted to 63 amps but it's not been as yet and some other circuits that should be down rated haven't been done as yet.

I have seen a copy of the cable calcs and it states in black and white that it should be 25mm cable, however this is def 16mm. That's the first f*#k up.

The other thing is on the Amtech cable calc sheet, it's states 3core armoured, which is obviously incorrect as its 4 core (3 phase and neutral). That's the 2nd f *#k up.

The DB schedule which has been fitted in the DB 10 mins ago states in black and white 16mm cable protected by 100amp MCB. So that's two f *#k ups there. First that it's actually an MCCB and not MCB as stated and that the cable and schedule do not match the cable calcs.

If the MCCB is being down rated then it shouldn't be too big a problem but the cable calcs do not match what had been installed and the MCCB has not been downrated yet.

I think perhaps what has happened is the calcs were issued for approval the the boss has decided that its unlikely the cables will be required to be loaded to 100 Amps so has decided to use 16mm instead and save some money.

The whole job is a farce.

This whole job is rough and I only going to stick around this week and then I'm getting out of here. Don't worry I will not be putting my name to any of it.

I'm not saying I know everything but I can assure you I am a qualified spark, I did a 4 year apprenticeship with JTL. I am a JIB approved electrician, have my 2391-10 inspection and testing and the 2391-20 design qualification. I am doing a bit of subby work as the money is good and will be starting uni next week for 1 day a week.

 

[Evans Electric]

A 10mm SWA cable, S/Ph is rated at 72A so why protect it with a 100A device.?

So how does this guy protect a 1.5mm SWA rated at 22A , does he use a 50A I wonder ?

 

[ParkySparky]

Yep defo sounds like he's decided to do it on the cheap! The length of run is the issue and if he does turn the mcb down to 63A it's not leaving alot of capacity on the cable! What size/number of way Sub DB are they feeding? 63A three phase sub DB doesn't sound very big to me, what type of building is it? Commercial? Industrial?