What is "cable derating"?

[bernardgreen]

Completely over the top. 1.0mm cable is the norm for lights on a 6 amp breaker even if it is buried in loft insulation.

How many metres before voltage drop becomes excessive and 1.5mm² is necessary.

Required Cable Size for 1.4 kW of lighting 30 metres from CU

Cable Size 1.5 mm
Voltage Drop 5.30 Volts.
Percentage Drop 2.3%
Current Load 6.09 Amps
Max Cable Load 14.5 Amps

Note that the cable is rated as being able to safely carry 14.5 Amps but if carrying 6 amps then the voltage drop is too high to meet the regulations.

 

[Lectrician]

Thanks @PrenticeBoyofDerry, same question as to ColJack - what does this mean in real life? Is something done to the circuit to derate it, or is the homeowner just told not to plug in 13amps?

You don't downrate a circuit, a circuit has a requirement in terms of load, you can't force a circuit to use less. You increase the cable size accordingly.

 

[PrenticeBoyofDerry]

Thanks @PrenticeBoyofDerry, same question as to ColJack - what does this mean in real life? Is something done to the circuit to derate it, or is the homeowner just told not to plug in 13amps?

As mentioned previously the circuit is designed to carry the design current and the rating of the over current protective device (fuse/mcb etc..).
Any electrician worthy of that title, would not leave you with a circuit that could potentially over load the current carrying capacity of the cable, without the fuse/breaker tripping.

 

[winston1]

How many metres before voltage drop becomes excessive and 1.5mm² is necessary.

Required Cable Size for 1.4 kW of lighting 30 metres from CU

Cable Size 1.5 mm
Voltage Drop 5.30 Volts.
Percentage Drop 2.3%
Current Load 6.09 Amps
Max Cable Load 14.5 Amps

Note that the cable is rated as being able to safely carry 14.5 Amps but if carrying 6 amps then the voltage drop is too high to meet the regulations.

Who uses 1.4 kW of lighting? Why would it be at the end of 30 metres of cable?

Typical domestic load with old incandescent lamps max 1kW if all on together. This is distributed along the cable length not all at one end.

 

[bernardgreen]

Who uses 1.4 kW of lighting? Why would it be at the end of 30 metres of cable?

Just accept that when selecting cable size voltage drop has the same ( or greater ) importance as current rating and resultantamt cable heating.

Consider the inrush current of a cold filament lamp ( typically 5 to 10 times the rated current ) and the effect the voltage drop could have of electronic drivers for other lamps on the circuit. Also consider that some electronic drivers have inrush currents.

 

[EFLImpudence]

If we assume your 1.4kW is equally spaced along the 30m circuit with an average of 0.7kW then the calculator comes up with this:

Required Cable Size 1 mm
Voltage Drop 4.02 Volts.
Percentage Drop 1.7%
Current Load 3.04 Amps
Max Cable Load 16.5 Amps

It has obviously halved the current load which still should be 6.09A.
Also, it goes to 51m before increasing to 1.5mm²

Why would anyone not consider that acceptable?

 

[EFLImpudence]

Back to the OP.

So if it is a 2.5mm power cable, derated at 50%, it must become 5mm?

Yes - if being used at its maximum current.

Of course, preferable is NOT installing the cable in that manner in the first place.

 

[ban-all-sheds]

Why would anyone not consider that acceptable?

Because they have other scenarios where they genuinely need 1.5mm² cable, and don't want to mess about "stocking" 1.0mm² as well in order to save a few pence per m?

 

[bernardgreen]

1.4kW is equally spaced along the 30m circuit with an average of 0.7kW

but the load is still 1.4 kW and the current from CU to first lamp will be 6.09 Amp. If it is a feed to lights on an upper storey then voltage drop between CU and first lamp could require a larger cable size.

 

[EFLImpudence]

Well, yes, it could - but rarely does.

Now you're talking about 1.4kW of lighting just upstairs.

 

[JohnW2]

but the load is still 1.4 kW and the current from CU to first lamp will be 6.09 Amp. If it is a feed to lights on an upper storey then voltage drop between CU and first lamp could require a larger cable size.

I presume that EFLI was talking about lights equally-spaced throughout the whole circuit, which obviously does not include a situation in which there is a very long run to the first light.

In any event, given that most people have a supply voltage at least 10% above the permissible minimum, and given the high tolerance of most lighting loads (except, perhaps for some fluorescent tubes) to voltage variation (and the very low loads usually present on lighting circuits these days), I find it extremely hard to get excited about VD on lighting circuits.

Kind Regards, John

 

[charliegolf]

Back to the OP.

So if it is a 2.5mm power cable, derated at 50%, it must become 5mm?


Yes - if being used at its maximum current.

In planning, wouldn't you always assume (worst case scenario) that the max current will be carried for some of the time? In other words, wouldn't all twin and earth in insulation be larger than 2.5mm? I know mine isn't.

 

[EFLImpudence]

Not necessarily. For example - A ring circuit must have conductors with a minimum CCC of 20A. However, the minimum size allowed (except MICC) is 2.5mm² T&E which has a maximum CCC of 27A. So a derating down to 20A is acceptable with no alteration.
In fact, one installation method has been 'fiddled' so that this can be the case.

A circuit is designed by determining the load, say, 30A, then the nearest MCB size above this, 32A, then the nearest cable above this, 4mm² 37A.
So, you see there is a certain tolerance before actual increase in size is required.
Plus - there is also a time factor which may be taken into consideration before the cable becomes (too) hot.

As I said above, whilst 50% is the maximum derating for cable completely surrounded by thermal insulation, this does not mean it will ever happen. Install the cable so that it is not necessary.

 

[JohnW2]

In planning, wouldn't you always assume (worst case scenario) that the max current will be carried for some of the time? In other words, wouldn't all twin and earth in insulation be larger than 2.5mm? I know mine isn't.

Strictly speaking, yes, but only if a significant length of the cable were totally buried in thermal insulation (requiring the greatest, 50%, de-rating) - which is why designers will do all they can to avoid that 'worst-case' installation method for sockets circuits.

Bear in mind that the cable in a 32A ring final circuit is only required to have a current-carrying-capacity of 20A, and the same is obviously also true of a 20A radial circuit. That means that up to about 26% de-rating (from 27A) is possible in such circuits without 2.5mm² cable becoming non-compliant - and, in practice, that means that 2.5mm² cable is OK for such circuits except when they have the 'worst' installation methods (e.g. totally buried in thermal insulation) - which, as above, is something one tries to avoid

Kind Regard, John
Edit: (fractionally) too slow typing again!

 

[charliegolf]

Thank you both for your illuminating replies.