poor 3 phase setup and responsibility

[johnmf]

hi, opinions please.

got a circuit to add to a busbar chamber but want some opinions as im a bit concerned over the current setup, which is......

mains tails go into a old isolator which has 3x 300A bs3036 fuses in it. from there a xlpe swa 4 core 120mm cable feeds the chamber. the swa is just laying on a wall, the run is only a few metres.

cant remember the exact ratings of the cable when looked up but i think it was underated.

am i only responsible for the addition or have i got to refuse to do it as the customer doesnt want to touch anything else. opinions please.

cheers

 

[Spark123]

If the supply you are connecting to is inadequate for the load then you don't do it.

 

[ricicle]

Are you sure the fuses are bs3036 (rewirable) I'm sure you mean BS88 fuses 120mm² is fine with 300A if it is XLPE (subject to de-rating factors) but a little underated if its 70°c pvc or bs3036 backed up

 

[johnmf]

hi,

a recent PIR states 3036's. although the guy might have just guessed as he never shut any power off.

the isolator looks old, its made by a company called royal and is silver coloured with the on/off lever on the bottom right hand side.

i wasnt aware they made 300A fuse wire!

 

[Big_Spark]

If it's Royal then it is 90% likely to be BS3036.

120mm² XLPE SWA is rated at 335A for a single 3 phase multicore clipped direct, but you need to account for the BS3036 protection. A method I have used for years is to simply derate the cables by multiplying the current capacity by 0.8, in this case it reduces the cable to 260A. In this case the fuses should not exceed 250A.

Just because the SF Isolator is rated at 300A, I would not assume that the fuse wire in them is 300A, it may well have been rerated at some point, the only way to ascertain is to arrange a power down and inspection of the devices.

 

[ricicle]

. A method I have used for years is to simply derate the cables by multiplying the current capacity by 0.8.

The correct factor is 0.725
Royal was a range made by Bill

 

[johnmf]

hi again guys,

bit more advice if thats ok, checked the isolator and its 300A bs88's which is lucky! so the cable is rated at 335A which is fine but would you say i had to use the ratings for pvc cable as i cant confirm if the mains isolator and busbar is suitable for temperatures above 70 degrees?

the busbar chamber is mem and is not that old, probably 15 years or so but the isolator is 30 years old at a guess?

also, any advice on assessing diversity in this kind of setup?

ive been monitoring the most loaded phase with a clampmeter recording maximum over set time periods and the average seems to be 140A so plenty spare for my new 3 phase machine (taking 80A per phase) would just be interested to no how others assess and approach this kind of thing?

cheers guys.

 

[Big_Spark]

. A method I have used for years is to simply derate the cables by multiplying the current capacity by 0.8.

The correct factor is 0.725
Royal was a range made by Bill

I know that, but not all circumstances require it to be this extreme...I should have clarified my point....This is a short cable run and full derating it unlikely to be necessary.

If the cable were a full size sub-main then I would simply do the calculation properly, taking into account ALL factors necessary for the cable..and certainly recommend that consideration be given to replacing the SFU..

 

[Big_Spark]

What worries me, and I do not mean to cause offence, is that someone working in this industry feels the need to come to a DIY forum to ask advice on a system that DIYer's cannot legally touch and would have little knowledge of...I think it shows a marked drop in training quality...

 

[Spark123]

On the other hand I would prefer he ask than to jump in with two feet and make a dangerous situation.

 

[ricicle]

. A method I have used for years is to simply derate the cables by multiplying the current capacity by 0.8.

The correct factor is 0.725
Royal was a range made by Bill

I know that, but not all circumstances require it to be this extreme...I should have clarified my point....This is a short cable run and full derating it unlikely to be necessary.

If the cable were a full size sub-main then I would simply do the calculation properly, taking into account ALL factors necessary for the cable..and certainly recommend that consideration be given to replacing the SFU..

Big Spark can you tell me whereabouts in the regs it says that length of cable run determines wether or not you can use the 0.725 factor for BS3036 fuses

 

[johnmf]

hi again guys,

in reply to big spark, no offence taken mate but most of my work has been domestic and sometimes if your unsure i always think its better to ask.

if you look at the IET forum they have far more basic questions on there every day from sparks....ill hold my hands up and say i dont no everything and sometimes its nice to have a second opinion.

and this isnt just a DIY forum in my opinion, theres some good sparks on here by the looks of things and why not use there knowledge to further your own.

 

[ricicle]

any advice on assessing diversity in this kind of setup?

Sounds like you've done the best way of assessing diversity by actual monitoring of the load itself.A couple of things to look out for are:-
1)This might be the running load but sometimes there is a start up load eg several machines with heavy heating loads which when have been switched on for some time will have their heating loads being controlled in and out so you will rarely see all connected load on together, but when first switched on together all controllers will be calling for heat until their setpoints are reached
2)Motor starting loads can typically be 3.5 x F.L.C for star/delta and 7 x F.L.C for D.O.L starting.This needs to be allowed for if you are adding new machinery to an installation which is near full capacity, although BS88 fuses will take surges it is worth checking especially if there are frequent starts

Hope this helps

 

[industryspark]

trying to calculate or estimate diversity in this kind of setup is a problem ive struggled with before, soooo wish there were some written rules but sadly there isnt.

agree with ricicle, best method ive found is to spend some time watching a clampmeter, you can usually get a fair idea of demand.

unfortunetly though its experience mixed with some guesswork as only you will have an idea of whats going to be used at anyone time, what the shift patterns are like, etc.

i always leave a small marin for error just incase.