interesting find on cable selection

I was running through all the cable cals for my house today and made an interesting find on the cable requried for a light on a relay.

now correct me anywhere you feel I am wrong in this - I am open for discussion as I believe it is an area that is not very well understood.

This is based on a 12ch relay, with a single 2c+e for every channel running out to lights in a domestic installation (ie 12 cables from the relay).

I would assume that most poeple would use a 16A RCB/MCB to protect the circuits (I have done so in the past).

Assuming a max load of 10A as this is what the relay is rated for (although this is a mute point in the end)

This is the killer - the de-rating of the cable. As we have all 12 bunched together, they need to be de-rated by 0.7 (T22C14 item 3 is best I can work out for this installation)

A 1.5mm cable satisfies the current carrying capacity for the install as 12.6A derated

To comply with voltage drop you have a max route length of 119m (actually loop impedance will dictate this and reduce it to 53m with 1.5mm & 89m with 2.5mm)

This is the one that ruins it all. Overload protection. To comply with AS3000:2.4.3.2, the cable that needs to be used is a 2.5mm cable becasue after de-rating it (0.70 remember) you get a cable that is capable of 18.2A. Apply this de-rating to a 1.5mm cable and you get 12.6 - which is less that 16A of the MCB. (for those that dont want to look up the standard Ib<In<Iz, where Ib is 10 A, In is 16A for the MCB and Iz is the continous current capacity after derating)

This gives 10<16<12.6 - this means it does not comply

So it would appear that any install with a relay that is protected by a 16A or greater MCB will requirer 2.5mm cable. This does not affect the dimmers as we tend to use a 10A RCD/MCB for them and that is less than the 12.8 derated value of 1.5mm cable.

You can reduce the relay protection down to 10A, but it may trip on overload - My real life relay has a max of 16.3A load - not on at once, but with a remote on feature this can happen.

Also note that once you hit 12 cables the de-rating stops decreasing, hence 0.7 is the figure used for all installs.

This makes a good argument for using SDI (albeit 2.5mm SDI) and looping the neutrals at the lights.

Mick
PS - no I did not forget the short circuit performance - It was not necessary to put across this point

 

Hi Mick - very similar to what we discussed in this previous thread :

http://www.cbusforums.com/forums/showthread.php?t=2127&highlight=derating+cable

I think that this whole issue is the reason that the Euro Standards are saying (implying) that people should have output protection on the outputs of the relays.

Because, if you use a 10A MCB on each relay output you only need to cable for 10A including de-rating - so following your example above the 1.5mm T&E (at 12.6A de-rated) will comply.

-------------------

You mention the "bunching" of load cables is the reason for the de-rating. Is there a way of separating them out into (say) 3 groups, so that the de-rating doesn't apply? I could probably look it up myself, but I am much more an expert at AS1680 than I am at AS3000

John

 

Yes - I had forgotten about that discussion - probably because it was about dimmers. It has some valid points regarding this issue.

This does make a good argument for output protection, but you have to balance up the costs of cable v MCB.

12 MCB's at ~$130 over the cost difference between 1.5mm and 2.5mm cable over a variable length for every install. Not to mention the extra din rail space required to mount the MCB (or fuses - but dont foget to de-rate them as well)

so basically the output protection is not there to protect the c-bus device, but to reduce the cable size you are using.

As for spreading out the load cables - you could do it, but there is still a distance effect (0.3, 0.6m etc) so de-rating still exists and even small bunces will be de-rated, just not as far. YOu also have to be able to fit them in the board with other bunches from other dimmers and relays.

Mick

Edit
Thought about this a bit overnight and I think if you use a 16A RCD/MCB with two 10A MCB's below it, you will comply. One MCB feed the top 6 channels and the other feed the bottom 6. This means you need to load them accordingly. You may not have the best discrimination, but that should not be a big issue for a domestic install. That should only cost you two MCB's as well and save a lot of cabinet space. - I think that is the way I will go

 

there is the issue of one RCD protecting many circuits. While feasable, do you want a light to take out that many lights - thats a specific design issue really (and as you indicated up to 4 din rail units is probably the most you would want to do), but yes you could do that.

If your relay is feeding loads that are say 1a or less then you can use a 10A MCB and 1.5mm cable - again a specific design issue. Sure a remote on may cause the breaker to trip on thermal - but 2A over will take a very long time to trip (if at all)

 

Is this not ok since the maximum demand on the whole bundle of cable is limited by the breaker supplying the relay or dimmer. So all the cables in the bundle wont be taking 16 amps at once?

Edit - typo

 

not sure what you are trying to say here

Maximum demand is not typically dictated by the breaker protecting the circuit (it can be when loads are unkown or variable). The designer determines the MD from AS3000 or from calculations.

You can be drawing theoretically up to 120A through a relay if all channels are on and the load is 10A/channel. The remote on/off makes it too easy to trigger all of them on at once.

If the 16A feeds two 10A breakers that feed the relay (or only some channels) then the only cable tha needs to handle the 16A is the cable between the 16A breaker and 10A breaker. After the 10A breaker the cable needs to be rated to carry a minimum of 10A continous

They wont be taking 16A at once, but if a fault occurs on one cable alone then the breaker will let through its full let thorugh current (16Ax7.5 for type C). The cable has to be able to handle this without out getting hot and it has to allow the breaker to trip in under 5s (or 0.4 if designing for that time). The RCD will ususally act before this though as most faults are earth faults.

 

Interesting stuff - please note that I am not a licenced electrician, but I do have lots of experience from working in the industry over the past 25 years or so. Thus my comments are "observations" only...

I think the biggest problem is that electricians think that just because it is C-Bus, it is somehow different to normal wiring. In actual fact it is exactly the same as normal wiring, except that "switches" are all remote, jammed inside a enclosure under the stairs ...

So, whatever normal design practices and standards that apply to a normal installation will also apply to the C-Bus wiring. Just because it's a Lighting Control System, then that doesn't mean you get exemptions (if the job's in Australia) to AS3000 You still gotta work out all the loads, do your cable calcs, then work backwards and determine how to group circuits and protect them, and split them across phases, and balance them all nicely, just like a normal "hard-wired" installation.

You cannot hang (say) 40A of loads off a 16A breaker in a normal installation, so why do people think that you can do so in a C-Bus installation? If you are pulling 10A through a certain relay channel, then really you should be protecting that particular circuit UPSTREAM of the relay, anyway. That's why they are Voltage-Free, so you can do that easily.

Maximum demand calc's are one thing, but as znelbok points out you must also allow for the cable de-rating issues. And a typical C-Bus installation generally is going to have a HEAP if individual loads, so the overall electrical design (ignoring the C-Bus for a moment) is going to be hugely more complex than a standard 3 bedroom spec home that the average wire-jerker is used to working with.

Another aspect (that I always carry on about) that affects the cabling calculations is the trend to put all the C-Bus in one location. This means extremely long runs of (switched) cables, and lots of cable bunching. And apart from costing heaps and being an absolute bitch to wire up (on a large project), it also means that cable de-rating is almost always mandatory.

I guess I keep going on and on about this because I come from a commercial background, and there's no way that you'd have only 1 switchboard on a commercial site of any magnitude. You wouldn't have one main board, then run 1 x TPS out to each and every load - primarily because it would cost too much in cables! So I thought that all electricians understood the concepts of having "sub-boards" around the place - well, that was until I started getting involved with domestic sparkies trying to design and wire up C-Bus

Anyway, back to the cable - I realise that Project Home builders use 1mm solid for lighting circuits to save the extra 5c per metre (of whatever it is) over 1.5, but in my own experience (I just paid for all the cables to have my house rewired) I've haven't found 2.5 to be all that much more expensive than 1.5 stranded - in Sydney at least. I think I paid like $60 vs $50 for a roll, perhaps 10c / metre difference?

Just my (probably mis-informed) 2c worth, JC

BTW - I don't think I've ever seen a C-Bus installation where the remote on/off is actually even connected ! I know that I always fold those 4 wires back and wrap 'em around, they never get stripped or terminated on my jobs Is that what you were referring to by "remote on/off", Mick ?

 

yeah thats what I meant by the remote on/off

While I have never used them other than testing their functionality, mine have gone off a few times.

I'll admit that it may be exceptional, because mine was due to mice getting into the nice warm board to nest (they will find any hole you forget about) and then peeing on the control side of the unit which then shorts something out and all of a suddden all your lights go on for no reason.

 

The idea of bunching of cables is based on the number of circuits. Since you only have one circuit, you will not need worry. As RICHMS said, all of the cables won't be taking 16A at once and so long as you can prove that the loadings of each cable is minimal, then you need not worry.

I can's find it, but I remember seeing a clause that said if the run was shorter than 2 meters, then it was not applicable. Otherwise every switchboard in the country would not comply

The best way to wire relays and dimmers it to bring SDI's or Twin Active cables back to the channels anyway - this eliminates the need for de-rating. With the introduction of the new dimmer range, bringing Twin Actives back will be the way to do things in the future!

 

I was never taught about the 2m rule. I was taught the opposite - you should consider it (but most dont). Just because its a short run doesnt mean that it doesn't heats up.

I dont agree with your first paragraph. Each cable is a circuit and must be capable of taking the energy let through from the breaker under a fault condition - otherwise you will burn the house down.

I dont see how you dont have to derate the SDI if you use that. THe standard specifically covers SDI and it is still a cable that has to withstand the energy let through

You are a little misguided in your comments (no offense intended)

 

I am no Standards expert, but consider the following note to table 22 of AS3008.1.1

4 These factors are applicable to numbers of circuits comprising the following:
(a) Groups of two, three or four single-core cables.

It is an ambiguous statement in it's current wording without context and can be interpreted in a few different ways. Many people will have different views of this statement. I am not saying who is right and wrong here.