Inrush currents

[soulman]

I need a bit of advice with inrush current on lighting. I have been reading through the IET circuit design guide & i cannot seem to find any answers to my questions.
1. Should the cable be rated at the running current or the inrush current? I was taught at college that the cable should be rated at the inrush current including any accessories, such as switches. The inrush should be taken as 1.8 x the running current.
But inrush current on certain lighting can be many times higher that 1.8.
It would appreciated if someone could point me in the right direction.

Cheers

 

[telectrix]

cables should be rated to thre flc of the load. the inrush is onlt for a few mSec, and the cable will withstand the overload for such a short time. however, you may need to use a c curve breaker to cope with the inrush.

 

[Marvo]

As per what Tel said, although circuits are primarily designed around running currents you may need to accommodate inrush currents in your design with regard to volt-drop and the curve of the OCPD etc.

 

[soulman]

Thank you Marvo & tel, that's much appreciated.

 

[Marvo]

It's also worth pointing out that with certain types of lighting the start up current could be as long as several minutes so don't class this as 'in-rush' current because it isn't.

 

[Darkwood]

Your confusion is coming from the need for switching to be rated for the inductive nature of the load, this means its can cope with the large spike as you turn the lights off, in the domestic realm you really don't come across any real design issues with lighting etc unless you have an excessive amount of inductive lighting.

When in commercial and Industrial the ball game can change greatly and you may need to use contactors for effective switching of the larger lighting circuits found, the cable, switches etc flc is compensated when you use the 1.8 factor for inductive lighting in your calcs, what will be your biggest issue on larger circuits is nuisance tripping of incorrectly spec'd mcb's.... manufacturers of the circuit breakers will provide you with a lamps per amp chart that gives the max no' of various inductive lighting types and this is the first step before any calculation is done as this with be key to how the circuits are divided and spread across the consumer unit without the risk of nuisance tripping.

Now inductive motors is a whole new ball game but thats for another thread.

 

[soulman]

Thank you darkwood, that's really interesting regarding the circuit breaker charts. So the 1.8 multiplier should be used for inductive lighting loads and this will be the flc to which switches etc are rated.