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S

smiffey

Good evening all,

OK so here goes-
I was asked to quote for a new dis board on a factory today with The Main feed being100amps going to a small 3 phase board in another building so-

-Main incomer 100a 3 phase
-50 meters of wire to a second board
-Second board with a 100a rccd acting as the main switch.

I automaticly thought 25mm 4 core swa clipped direct nice easy job as it would be under the volt drop and max rateing of the cable.
Then the customer tells me he has already bought the new board, main disconnector, and 50 meters of 16mm sy 4 core and im to use that?

Could someone give me a bit of a hand with the cable cals as i think the 16mm is to small due to my working out?
Design current 100amps
Supply voltage 415volts
Fuse rateing 101amps
Cable length 50 meters
ca 1.02 page 267 table 4b1
cg 1.00 page 262 table 4a2 refrence method c
cc 1.00
volt drop 7.37v (1.77%) pf 1.0
so the actual cable rateing is 124amps
where as on a 16mm
volt drop 23.40v (5.64%) so over the max 5%
and the actual cable rateing is 73amps!

hope i can sort it out as i have to give him an answer tomorrow.

cheers in advance Ashley
 
Cheers bigbaddave, does the maths look right tho?
I've not had very much interaction with three phase. After he asked I started doubting myself! For some reason I thought it was different as there are 3 phases not 1 so you could use smaller wire!
 
irrespective of the load, it's the front end fuse that determines the cable size. 100A fuses, 25mm cable sssimpless.
 
Yes 16mm is too small for 100A.
I dont like the idea of using SY cable from the DNO fuse to the dist board, unless is has sufficiant mechanical protection.
You could tell the customer that you would have to install a fused isolator neat the DNO fuse, fused to protect the cable, if he is determined that you install the 16mm. I dont think he would be happy with that.
I get a different figure for the 16mm VD. VD = Length x In x mV/A/m / 1000 = 50x100x2.4/1000 = 12V = 2.9%
 
Hi all thank you for your thoughts. The customer knows there will have to be a fused isolator next to db1. I've already explained that one too him and he has agreed but wants me to select it and he buys it, I think someone has ripped him off for parts at some point.

Db2 will probably pull 100amps like you said yellowvanman but the customer is already talking about adding things in.

With the 16mm I'd just put a 63amp main switch in to protect the cable. So if he refuses to buy the proper cable for 100 amps he will have to put up with the main switch going.

This customer is a battle and a half! Next issue is trying to work out how much to charge for the job! Any ideas anyone?
 
Hi all thank you for your thoughts. The customer knows there will have to be a fused isolator next to db1. I've already explained that one too him and he has agreed but wants me to select it and he buys it, I think someone has ripped him off for parts at some point.

Db2 will probably pull 100amps like you said yellowvanman but the customer is already talking about adding things in.

With the 16mm I'd just put a 63amp main switch in to protect the cable. So if he refuses to buy the proper cable for 100 amps he will have to put up with the main switch going.

This customer is a battle and a half! Next issue is trying to work out how much to charge for the job! Any ideas anyone?

I personally would walk away from this customer. If he doesn’t want you to make a profit he is no good to you in the long run.
Just do an estimate of how YOU feel the job should be done with a price and leave it at that.
If he wants to purchase the materials that YOU have specified fine, but explain that these would not be guarantied or warranted by you.(make sure you include screws plugs cleats etc) As for pricing if you are doing labour only, you should charge more per hour for the job.
I don’t know you but I would guess you are just starting out in the business (apologies if not)
What you must learn early on is how to charge for work.
PS .I personally would not use SY for this purpose SWA (not a great price difference) would be the way to go 25mm 4 core.
 
Hi all thank you for your thoughts. The customer knows there will have to be a fused isolator next to db1. I've already explained that one too him and he has agreed but wants me to select it and he buys it, I think someone has ripped him off for parts at some point.

Db2 will probably pull 100amps like you said yellowvanman but the customer is already talking about adding things in.

With the 16mm I'd just put a 63amp main switch in to protect the cable. So if he refuses to buy the proper cable for 100 amps he will have to put up with the main switch going.

This customer is a battle and a half! Next issue is trying to work out how much to charge for the job! Any ideas anyone?

You can't have an unqualified customer telling you what cable and protective devices to use. Your the electrician, it is you that should be either advising/stipulating what is required for this installation. For a start you can't in all honesty use this SY cable he has purchased, you'll either be without a neutral or an earth conductor, (the braiding does not conform to CPC requirements, and does not meet mechanical protection status). It's also stated by some manufacturers, not to be used as a supply/distribution means ..so that's a no, no straight off the mark!!


As others have stated if he's not interested in listening to the installation electrician walk away, or you're end up in trouble further down the line, when/if someone gets injured or killed. It'll be your name on the cert you leave behind!! lol!!!
 
Tell him to shove it unless he allows you to do the job correct, as at the end of the day if anything goes wrong and the proverble hits the fan it will come back to you for fitting it knowing it was not correct.
 
Thanks all, I've told him that I was correct when we first met and I seen the 16mm SY.
He was spitting and moaning about already buying the cable! He is going to try and take it back ( hehe good luck with that one )
I've given him a list of bits required including the 25mm 5 core swa, I've got a feeling he isn't going to like the price of that! I've rang my normal wholesales and they quoted me £650 on the swa.

As a few have said its going to be my name on the cert so I can't be doing a dog **** job!
It's up to him now if he takes me on for the job.

Funny thing was when I was leaving a car full of chaps from a un named Easter European place pulled up!
I bet they won't work out the cable and advise, Bish bash bosh thank you will be the answer.
 
Thanks all, I've told him that I was correct when we first met and I seen the 16mm SY.
He was spitting and moaning about already buying the cable! He is going to try and take it back ( hehe good luck with that one )
I've given him a list of bits required including the 25mm 5 core swa, I've got a feeling he isn't going to like the price of that! I've rang my normal wholesales and they quoted me £650 on the swa.

As a few have said its going to be my name on the cert so I can't be doing a dog **** job!
It's up to him now if he takes me on for the job.

Funny thing was when I was leaving a car full of chaps from a un named Easter European place pulled up!
I bet they won't work out the cable and advise, Bish bash bosh thank you will be the answer
.

Well if he doesn't get you to the job and lets them do the job, when his place burns down it will be up to him to do the explaining to his insurance company you can say well I told you so.
 
Good evening all,

OK so here goes-
I was asked to quote for a new dis board on a factory today with The Main feed being100amps going to a small 3 phase board in another building so-

-Main incomer 100a 3 phase
-50 meters of wire to a second board
-Second board with a 100a rccd acting as the main switch.

I automaticly thought 25mm 4 core swa clipped direct nice easy job as it would be under the volt drop and max rateing of the cable.
Then the customer tells me he has already bought the new board, main disconnector, and 50 meters of 16mm sy 4 core and im to use that?

Could someone give me a bit of a hand with the cable cals as i think the 16mm is to small due to my working out?
Design current 100amps

How have you arrived at a Design current of 100 amps?

Supply voltage 415volts

Supply voltage is 400 v

Fuse rateing 101amps

What type of fuse is this lol?

Cable length 50 meters
ca 1.02 page 267 table 4b1
cg 1.00 page 262 table 4a2 refrence method c
cc 1.00
volt drop 7.37v (1.77%) pf 1.0

Are you sure about the installation factors and the p.f?

so the actual cable rateing is 124amps
where as on a 16mm
volt drop 23.40v (5.64%) so over the max 5%
and the actual cable rateing is 73amps!

What about the final Circuits?

hope i can sort it out as i have to give him an answer tomorrow.

cheers in advance Ashley

I dont intent to be rude, your design is omitting a lot of information, id start at the beginning and design considering all the aspects of BS 7671.
 

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