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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> maximum load on a ring main? https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1103057725 Message started by neabitts on Dec 14^th, 2004, 8:55pm

Title: maximum load on a ring main?
Post by neabitts on Dec 14^th, 2004, 8:55pm

hi.
i have a sunbed that i would like to use at home, it is a powerfull one and i am not sure if i can connect it to a outlet on the ringmain.
what is the maximum wattage i can connect?

Title: Re: maximum load on a ring main?
Post by Lectrician on Dec 14^th, 2004, 8:57pm

13 amp. approx 3kW (2.9 really)

I doubt your flourescent tubes equate to that?? Assuming this is the type we are talking of??

Title: Re: maximum load on a ring main?
Post by neabitts on Dec 14^th, 2004, 9:07pm

hi.
it is 4.8kw with 100w tubes or 7.8kw with 160w.
to reduse power consumption i was going to use 100w tubes,
it has 160w ballast's in it, would using 100w tubes reduce power usage or would i have to change the ballast's to 100w

Title: Re: maximum load on a ring main?
Post by Lectrician on Dec 14^th, 2004, 9:11pm

How many tubes does it have??

Is that all that takes juice - ie - no heaters, pumps or motors etc??

Title: Re: maximum load on a ring main?
Post by Beanzy on Dec 15^th, 2004, 9:47am

This thread is danger of becoming as useful as a chocolate fire-guard to 'neabitts' folks. ;)

Maximum rated Amps for the unit is as per the spec/ manufacturers info., otherwise you won't be installing it as per BS7671, and I wouldn't recommend doing anything outside the scope of the regs unless you've got the paperwork to back it up.

So you're looking at two options. 4.8kW (21A) or 7.8kW (34A) at the point of delivery, plus you need to calculate the effect of voltage drop on the ccable you choose, and de-rate the cables for effects of heat or running in insulation through your loft etc..

What route would be possible for the new circuit? How long would that be? Will it run through conduit or loft insulation? will it have to be run outside and back indoors?

Can give you more feedback once we get a better picture of the circumstances.

Title: Re: maximum load on a ring main?
Post by neabitts on Dec 16^th, 2004, 11:56pm

hi.
it will be about 3-4 meters from the fuse box.
i will be in pvc trunking indoors back to the fuse box.
it also has a large fan in the top and a 60w spot light, i don't know the spec of the fan.
what size cable will i need?
it will only be used for about 10 min per day,
so it will not be in use for long periods at a time.

i have just started my pt1 2360 so i am fine wiring just not done much regs yet.
cheers

Title: Re: maximum load on a ring main?
Post by L.Spark on Dec 17^th, 2004, 12:07am

Hey Neabits

I would bet money you will find alot of different types of wiring you didnt expect to see and different ways of going about wiring in your course.

You won't get involved with any of the regulations in-depth on your course, unless you decide to move on to the next level.

Quote:

so it will not be in use for long periods at a time.

That wouldnt matter, Cables must be able to carry the steady current for a consitent ammount of time.

Title: Re: maximum load on a ring main?
Post by Beanzy on Dec 17^th, 2004, 7:03am

Oops! forgot to ask what the Supply characteristics to the consumer unit were, so I can sort out the disconnection times properly. Would need the earthing type, TT, TNC-S, TN-S or what not. If it's TNC-S we won't need you to measure the Ze as the Distributor should keep it below 0.35 ohms.

Assuming TNC-S; I'll give you a 'rule of thumb' option;
PVC T&E cable in trunking at 75% de-rating to cover most eventualities we can't see on here. This is probably well OTT. At least with 4metres voltage-drop will not really be a factor.

Demand 7.8kW (never mind what bits are on it it's the manufacturers max load rating for the whole unit you should use). So you need to supply 34amps to the load. Its in trunking on a wall so method 3 would be used. 6mm T&E will only do 38 amps max which if de-rated on a rule of thumb would be 28.5 amps. So the rule of thumb would give next size up at 10mm/52amps derated to 39amps. You may find it a bit difficult to get a breaker to suit this.

Now you need to see if working it out for the actual load would be better. If we can find out that a 40amp breaker would protect the cable ok, then the shopping gets easier.

I'm going to assume it's not runnning anywhere hot, such as wire behind heating pipes or sharing a cavity with them or a boiler flue etc. So correction factor Ca will probably be discounted at 30deg, it could even be a plus at 1.03 but I'm not assuming that here without measuring it. Ci doesn't apply either as you don't mention any insulation. Cg grouping factor won't play as you've only got the one T&E cable. You mention a "Fuse Box" but you may just be using the term to cover a consumer unit. I'll give you options for both.

The semi-enclosed fuse (BS3036) factor would kill our rating game and send us back to dividing by 0.725. So our only other option would be to leave out the trunking and clip the cable direct. This would give us Method 1 for calculating the capacity of the cable. Now we can take it as 63amps x by our worst cable correction factor of 0.725 or 45.6amps, well protected by a 40amp fuse wire (if you can find one) 45amp (1.25mm) is more common, but would still protect our cable. Now you know that a 10mm cable if clipped direct and not enclosed in trunking for more than 33cm will be capable of supplying the required current to your load. (can't remember wher that length comes from, so maybe one of the others can remind me).

If you mean a consumer unit then you'll be back in the calculation game again.

It'll be a simple method 3 with voltage drop and power factor corrections as per pages 208/209 of your regs book. (I ain't bothering here it's coffee time)

A rough calc from Table 4D2A against voltage drop may get us away with using a 6mm cable if you can find a 34amp MCB and leave out the trunking again. But for the sake of pennies difference and margin for error I wouldn't even go there. 10mm cable in trunking, not running anywhere over 30degrees, on a 40amp MCB will supply up to 52amps so you know you're there with that.

Why have I bothered..... well you'll be doing this soon so it's worth having a look at before you go full on into it. Also in the DIY forum it's no harm to show people that there's alot more to installing electrical circuits than just smacking in a bit of wire that looks big enough and hoping for the best. Not that they shouldn't do it, I reckon they should but do it right and do it well if you're gong to DIY.
:-*

PS that's just the current carrying capacity. Next you've got to see if the load will disconnect in time if there's a fault.............. 'back on your heads sinners! tea break's over'

Title: Re: maximum load on a ring main?
Post by ban-all-sheds on Dec 17^th, 2004, 9:26pm

wrote on Dec 17^th, 2004, 7:03am:

Also in the DIY forum it's no harm to show people that there's alot more to installing electrical circuits than just smacking in a bit of wire that looks big enough and hoping for the best.

;D ;D ;D

Title: Re: maximum load on a ring main?
Post by L.Spark on Dec 18^th, 2004, 11:34pm

Quote:

Also in the DIY forum it's no harm to show people that there's alot more to installing electrical circuits than just smacking in a bit of wire that looks big enough and hoping for the best.

And that’s why we must keep this post as a reminder to all ;D

Quote:

Not that they shouldn't do it, I reckon they should but do it right and do it well if you're gong to DIY

Yes, BAS does electrical work he's a DIY expert and so no one is incapable just because it's not there trade ;) ::) ;D

Title: Re: maximum load on a ring main?
Post by Beanzy on Dec 18^th, 2004, 11:57pm

Just realised the end of the 10mm bit reads like pants as I stuck in "with a 40A mcb" That's just thrown in to give the full circuit spec not trying to say a 40A mcb would magically let 52 amps through.........

::)

Title: Re: maximum load on a ring main?
Post by L.Spark on Dec 19^th, 2004, 12:14am

I didnt actualy read all of that so never noticed ::)

Modify if you want to correct it Beanzy

Title: Re: maximum load on a ring main?
Post by ban-all-sheds on Dec 29^th, 2004, 12:54am

wrote on Dec 18^th, 2004, 11:57pm:

That's just thrown in to give the full circuit spec not trying to say a 40A mcb would magically let 52 amps through.........

::)

Well, it will for a while - 52A falls between I₁ & I₂ for a 40A breaker.....

Title: Re: maximum load on a ring main?
Post by L.Spark on Dec 29^th, 2004, 1:21am

wOoOo, welcome back ranger! ;D

Title: Re: maximum load on a ring main?
Post by ban-all-sheds on Dec 29^th, 2004, 9:44am

Sorry chums. I have been popping in, but it's been a combination of busy at work for me, and fairly quiet here....

Shame there isn't a way to get emails every time anything is posted to the forum, rather than having to remember to tick the box on every individual topic...

Title: Re: maximum load on a ring main?
Post by L.Spark on Dec 29^th, 2004, 10:15am

There is, it's called 'notify of replies'

try it, it does work, I don't use it here, but I do on other forum's I don't visit frequently if there's something I really want to keep an eye on ;D

Title: Re: maximum load on a ring main?
Post by ban-all-sheds on Dec 31^st, 2004, 1:04pm

But it's a manual option, that you have to select on each and every topic. Ideally there should also be the following:

1) Automatic notification of replies to any topic I've created or posted to.

2) Automatic notification of any posts or new topics to the entire DIY Electrical Questions forum...

Title: Re: maximum load on a ring main?
Post by L.Spark on Jan 1^st, 2005, 4:46pm

lol, well you can always find something, just have to come on if you want to keep up-to-date & help out.

I wouldn’t bother notifying of topics unless it was something like on D!YN0T that was particularly interesting, which is practically never so ::)

Title: Re: maximum load on a ring main?
Post by ban-all-sheds on Jan 4^th, 2005, 10:59pm

wrote on Jan 1^st, 2005, 4:46pm:

I wouldn’t bother notifying of topics unless it was something like on D!YN0T that was particularly interesting, which is practically never so ::)

There are at least 3950 interesting posts on D|Ynot..

PS - still got this ridiculous feud going I see - thought that was now sorted?

Title: Re: maximum load on a ring main?
Post by L.Spark on Jan 5^th, 2005, 12:29am

wrote on Jan 4^th, 2005, 10:59pm:

There are at least 3950 interesting posts on D|Ynot..

That many? :o

Quote:

PS - still got this ridiculous feud going I see - thought that was now sorted?

Which one ;)

Title: Re: maximum load on a ring main?
Post by ban-all-sheds on Jan 5^th, 2005, 9:52am

The one where each side automatically censors the other's name.

Title: Re: maximum load on a ring main?
Post by L.Spark on Jan 5^th, 2005, 11:30am

here and D!Y place yea, sh!t happens ::)

Title: Re: maximum load on a ring main?
Post by ban-all-sheds on Jan 7^th, 2005, 10:19am

FFS call a truce.

Title: Re: maximum load on a ring main?
Post by L.Spark on Jan 7^th, 2005, 12:55pm

nothing to do with me ::)