current carrying capacity

[meercat]

can anyone help please,
I am testing a new installation for another contractor, lighting circuits are wired in 1.5mm pvc singles in 4inch metal trunking, protected by 16amp type c mcbs, is this overated considering correction factors applied? thanks

 

[shanky887614]

A 2.5 radial goes on a 16amp breaker so

 

[trev]

A lot could depend on what derating factors you think should be applied

 

[Richard3009]

borderline 17.5A

 

[meercat]

possibly grouping, as there are about 18 circuits in trunking,

 

[meercat]

maybe need to add grouping factor? , think its .82, no book in front of me, brings it down under 16am anyway?

 

[trev]

Make a note and check it out in the book tomorrow mate. It's frida neet bonny lad

 

[meercat]

I know, its been annoying me all week, just thought id throw it in, I party on sat anyway, cheers

 

[trev]

Lightweight!

 

[davesparks]

All week? How long does it take to do the cable calculation?

 

[Hawkmoon]

Next time you're at the job, you should stand up on your hind legs and have a bloody good look round!

 

[Richard3009]

Down rate the breaker and sleep well

 

[wozz]

18 circuits! there maywell be a bit of derating there! unless all 18 are rather lightly loaded! which i doubt.

 

[davesparks]

This might be a daft idea but if you are carrying out initial verification could you not consult the installation design paperwork? You do of course have this as you are verifying the installation against this as well as bs7671

 

[Lucien Nunes]

there maywell be a bit of derating there! unless all 18 are rather lightly loaded!

Or unless there isn't a total of 288A supply capacity, in which case you can't run them all at full whack anyway. They might be C16s to avoid tripping on inrush, but only loaded to say 5A with a fixed load. In which case it might not be as bad as it looks...

 

[GLENNSPARK]

Down rate the breaker and sleep well

oh aye?

so...you know what the I B is here is do you....caus i havn`t seen it yet...

or have they gone for 16A MCBs in a vain attempt to get over start up currents....

 

[Richard3009]

ayyyyyy ?????

 

[micknew]

ayyyyyy ?????

Yorkshire for yes.
You must get out more lol.

 

[wozz]

Or unless there isn't a total of 288A supply capacity, in which case you can't run them all at full whack anyway. They might be C16s to avoid tripping on inrush, but only loaded to say 5A with a fixed load. In which case it might not be as bad as it looks...

7671 allows for this with the paragraph below the grouping table. Although they do have to be very lightly loaded to avoid being included in the calculation, hence my comment.

 

[GMES]

What's with all the bloody eh's eeeh's arghhhhhs and ohhhh's then

Can ya lot nay speak proper english like eye yused to did

 

[7029 dave]

Nay nay.

 

[Lucien Nunes]

Although they do have to be very lightly loaded to avoid being included in the calculation, hence my comment.

There are two separate points here interestingly, and I was focusing on the other one to yours. As you say, if a circuit has a fixed load much lower than its ungrouped capacity, it can be discounted because it will never dissipate much heat. OTOH if heavy diversity has been applied, even where the circuits can each be heavily loaded, it might not be possible to load more than a few at a time. E.g. twelve 10A circuits each with 10A load, but all running on one 60A supply. It's only possible to load half of the circuits fully, and this will always be a worse case for cable heating than the same 60A divided between more of the circuits*, so only six contribute to the grouping.

*Seems obvious to me and can be proven in three lines of math, but I had the devil's own job convincing some sparks of this once.

 

[wozz]

yes i was focusing on 0.3CgIt, rather than the simultaneously loaded conductors route. thanks for the reminder, that had nearly fallen out my head.

 

[Lucien Nunes]

It's a situation that often occurs in stage lighting. E.g. 96 circuits all protected by K10 MCBs, but running on a total of 3x60A. Only 18 can be fully loaded at once, but in practice it's more likely to be 41 circuits loaded with 4.4A, which requires less derating even though they are above 0.3CgIt.

Not that this applies to normal commercial or industrial lighting though.

 

[davesparks]

And then Mr self proclaimed LX expert comes along and types 1 thru 96 @ full enter

And then the lights come on, and then they go off again in a rather terminal fashion thanks to Mr MCCB esquire.