Cable rating/MCB rating

[wade88]

Morning guys,

Preparing to be shot down here, but im a little confused regarding a MCB rating on a piece of machinery at work.

Due to ongoing engineering projects that are taking place, i need to move on machine 15 ft or so furhter down the warehouse to free up space for some new ones coming in. The machine was installed before i began working here so i was not present to witness the install. The machine is rated @ 18.25KW's and is 3p, 415v. So i would have expected this machine to be on a 50amp breaker, but its on a C type 20 and has been wired with 6mm SWA.

If its only running on a 20amp then surely i dont need to buy anymore 6mm at cost to wire it back in, some 2.5 would suffice happily.

My knowledge of industrial controls and machinery is limited but slowly increasing, but is there something im unaware of with this type of machine. To the best of my descriptive powers, The machine is in two parts, a heated 500 litre capacity mixing vessel, with three band heaters run in zones around its circumference, a 1hp motor (0.75kw) so neglegible in the grand scale of things. This machine is then linked to a thermo regulator panel and control box for want of better terminology, that control the speed, temperature and which bands are on or off at any given time via switch control. i have recently replaced the inverter in this module and serviced the Omron control inside and re programmed it.

Apologies is anything i have said is total Bull, like i said, limited knowledge so doing my best to explain.


Long and short is, if its only on a 20amp now and running ok, then there really is no need for 6mm SWA and a 32amp commando to plug into?

Thanks chaps

Ben

 

[silva.foxx]

Hey up Wade88

Please may I suggest you download Lenny's cable calc template from http://www.electriciansforums.net/e...al-regulations/47059-cable-calc-template.html, fill it in then come back to us.

As this is a current install it'll be a good exercise for many on here, myself included, on real-life use of the template. Lets see how the result differs from what you currently have in place.

Most of us have done this in the past so it'll serve as a good refresher.

To answer your question...I certainly wouldn't downsize the cabling according to your MCB. You need to look at cables distances and volt drops. If the machine specs state 18.x kw then the supply needs to suffice this as a bare minimum.

 

[johnboy6083]

i would consider contacting the maunfacturers, an asking if diversity can be applied to their product, or is it possible that all of the machine can operate at once, for prolonged periods.
Have you run the machine at its maximum and clamped the feed?

 

[Richard Burns]

At the risk of also being an idiot:behead: is this equipment effectively 3 single phase heaters each drawing about 20A for a total rating of 18.5kW therefore only 20A per phase?

 

[wade88]

At the risk of also being an idiot:behead: is this equipment effectively 3 single phase heaters each drawing about 20A for a total rating of 18.5kW therefore only 20A per phase?

Well this is what i think. I havnt run the machine at its max Johnboy no because orders come in so thick and fast and this machine MUST be operational to accommodate such orders, if i start stressing it and something goes wrong im going to stop production for some time which will make me fairly unpopular. it runs under very strict parameters regarding temperature and speed to achieve the desired result of the mix, so i cant really mess about with it too much..

Silva ill take a look at that for sure, but the cable run from CU to machine is about 6ft so voltage drop is not an issue, the ambient temp of the room is between 15-20 degrees celcius so again negligable effect upon ratings. I think richard may have nailed it, the motor is .75kw or there abouts. It was installed by a friend of the family who technically speaking isnt a full time spark, more of a well rounded jack of all, and very accomplished metal worker, so i have a feeling him and his mate did no calcs at all and just figured, "the bigger the cable the better to cover our asses" when a 6mm is totally over the top for the demand.

 

[Richard Burns]

This will in no way be a balanced load if individual heating bands can be on and off at various times so what current are you anticipating in the neutral. This may well be the limiting factor for cable size.

 

[wade88]

I really couldnt say Richard, i frankly dont know. Generally speaking we run 2 of the 3 heaters at all times. the top band heater is rarely used.

This thread was a bit of a shot in the dark so i apologise, im not sure what sort of help i could realisticly asked you guys to give me with such vague descriptions so sorry for clogging the forum with this nonsensical rubbish. i tihnk ill just stick with 6mm cable and the brass will have to just pay the money. if it aint broke dont fix it comes to mind. im not guna fiddle with something i dont fully understand. ill just re wire it like for like.

cheers chaps

 

[Chr!s]

This will in no way be a balanced load if individual heating bands can be on and off at various times so what current are you anticipating in the neutral. This may well be the limiting factor for cable size.

Unless there are any harmonics then worst case will be that of as it were a single phase.

 

[Knobhead]

If the three heater bands are each on separate phases the maximum you will get flowing in the neutral is 24A. If as you say the run is only 6’ why are you worrying about the cable being oversized, I could understand if we were talking of 100+ foot.

 

[Chr!s]

If the three heater bands are each on separate phases the maximum you will get flowing in the neutral is 24A. If as you say the run is only 6’ why are you worrying about the cable being oversized, I could understand if we were talking of 100+ foot.

With a single neutral? What about the phase angle?

 

[wade88]

Im not so much worried about the cable being over sized at all, never have an issue with that, its just i have to move the machine some distance away from its current location , so the cable used at the moment will not reach its end destination, therefor i need to order some new cable to wire it with, i just didnt want to have to order 6mm at over twice the cost as 2.5 when i didnt need to....money isnt an issue really, i just like to save the company cash where ever possible however little.....you never know....do it enough times and MAYBE one day ill see some of that saving come my way....maybe....ha

 

[Knobhead]

With a single neutral? What about the phase angle?

In=√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))
How do you think I came up with 24A

 

[Chr!s]

In=√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))
How do you think I came up with 24A

Well you've got me posting the vector's lol, is this vector not assuming no reference to the neutral.

Three phase distribution, each phase connected to a single phase load, and a single neutral common to each load. The phase currents will be 120 degree apart with reference to the neutral.

So would the vector not be taking into account the the angles, so calculate as you would three phase.

 

[silva.foxx]

...If as you say the run is only 6’ why are you worrying about the cable being oversized, I could understand if we were talking of 100+ foot.

Exactly. I feel so stupid in assuming the OP needed real help :shame:

That reminds me... I need to build an extension on my house. Shall I continue it in brick or go with cardboard?

 

[Chr!s]

In=√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))
How do you think I came up with 24A

I used I (neutral) = I (0 degree) + I (120 degree lag) + I (240 degree lag)

This gives a worst case single phase, so Neutral CSA must be equal to Line CSA.

 

[netblindpaul]

What if the heater bands are 400V?...

 

[Knobhead]

No problem with neutral at the machine then. Balancing the load and neutral currents moves down the line to the DB and then on to the point of origin. Balancing loads is at the end of the day a juggling act. I’ve worked on systems with multiple 3MVA two phase loads supplied direct from the 11KV main boards. No problem with neutrals but a hefty financial walloping by the DNO! I was asked to find a way to balance out the loading using redundant gear we had on site, the company wouldn’t pay for it even though it would have saved thousands a month. I GAVE UP and walked away!