Lighting Question

[Lc1]

We have a 3 ph DB. I am a little confused with the cable sizing that has been carried out and would like to verify it for my self.

A 4core 16mm (3 ph and N) goes from this DB to a jb in the field, through an underground cable route. At this JB the cable splits. One cable goes off and feeds 14 400W HQI lights and another cable feeds 12 400 W HQI Lights and three street lights.

From a sizing point of view is this treated as one three phase circuit with 29 lights on it?

Thanks

 

[Leesparkykent]

We have a 3 ph DB. I am a little confused with the cable sizing that has been carried out and would like to verify it for my self.

A 4core 16mm (3 ph and N) goes from this DB to a jb in the field, through an underground cable route. At this JB the cable splits. One cable goes off and feeds 14 400W HQI lights and another cable feeds 12 400 W HQI Lights and three street lights.



From a sizing point of view is this treated as one three phase circuit with 29 lights on it?

Thanks

Its 1 circuit as they are all on one OCPD. From a sizing point of view the 16mm cable before it splits would obviously have to be sized to cope with the load of all 29 lights.

 

[HandySparks]

Its 1 circuit as they are all on one OCPD. From a sizing point of view the 16mm cable before it splits would obviously have to be sized to cope with the load of all 29 lights.

...taking into account how the various loads are distributed across the three phases.

 

[Lc1]

Thanks for the replies. I am struggling a little with this. We are not sure at this stage what lights are on what phase so to verify this cable size I will assume there are 10 per phase.Does anyone have any worked examples of this type of calculation?

thanks

 

[Andy78]

Thanks for the replies. I am struggling a little with this. We are not sure at this stage what lights are on what phase so to verify this cable size I will assume there are 10 per phase.Does anyone have any worked examples of this type of calculation?

thanks

Measuring current draw at the TP MCB with all lights activated would be my starting point. If possible I would be doing the same at the JB so I could verify the load on the smaller cables and look at their size and suitability.

 

[Lc1]

Does anybody have any worked calculations that have a 3ph source that splits into single phase loads such as the lights above. Just to I can get my head around it. Am I missing something really simple?

Thanks

 

[Richard Burns]

If each light is single phase then the load on each phase is P/V = I where P is the total light wattage on that phase.
The maximum current on a phase is then the highest loaded phase.
With the lights as evenly distributed as possible (9*400W + street light) then the load would be ~16A per phase.

 

[Lc1]

ok thanks. I understand the lights on single phase from the jb. I am not sure of the sizing from the db to the jb which is 3 phase. Do we just treat this a 3 single phases?

thanks again

 

[telectrix]

ok thanks. I understand the lights on single phase from the jb. I am not sure of the sizing from the db to the jb which is 3 phase. Do we just treat this a 3 single phases?

thanks again

yes, because you need to know the load per phase. that's what the cable size and OCPD ratings are determined from.

 

[Lc1]

Good morning,
to follow on from my question above.
The 29 lights mentioned above are on way 1 of a 3 phase db
way 2 has a further 3 400 w lights
way3 is spare
way 4 is double poled and feeds a further 3 400 w hqi lights
way 6 feeds a further 4 400 w hqi lights

I have been told that these are controlled by a photo cell and a contractor. So when the photo cell activates can the lights on these ways energise at once ? After speaking to a colleague they say there is no way this can happen. I am not quite sure as to why ?

thanks

 

[snowhead]

If it's a single contactor fitted on the incomer to the 3 phase DB then yes 1 contactor could control all lights.

 

[Lc1]

Thanks Snowhead,

would it it be possible if it was a 3 phase contractor on the incomer to the Db ?

thanks

 

[ipf]

Thanks Snowhead,

would it it be possible if it was a 3 phase contractor on the incomer to the Db ?

thanks

Is this 3 ph contractor self employed?

 

[snowhead]

Is this 3 ph contractor self employed?

He'd be on a good earner turning the power on and off every day if he was.

 

[snowhead]

Thanks Snowhead,

would it it be possible if it was a 3 phase contractor on the incomer to the Db ?

thanks

Yes a Single 3 phase contactor on the incomer OR multiple contactors or relays on the outgoing feeds to the circuits.

 

[Gnasher]

When calculating the cable size don't forget to multiply the load on each circuit/phase by 1.8 to take into account the starting current of discharge lighting. This is the current you need to cater for for your cable and protection device. On top of this you need to take into account correction factors such as installation method, grouping factors, thermal insulation, and any other relevant correction factors.

The calculation would be the highest of the 3 phases. Number of lights on each phase

Number of lights 10 multiplied by wattage 400 multiplied by 1.8 divided by 230 this will give you your load.
You will then need to take into account the correction factors that will ultimately determine your cable size.

Next step is confirm volt drop and calculated Zs value for selected protection device.

Would be interesting to see a photograph of termination method and any local fusing as you may have a large value fuse at the origin of the circuit.

 

[HandySparks]

When calculating the cable size don't forget to multiply the load on each circuit/phase by 1.8 to take into account the starting current of discharge lighting. This is the current you need to cater for for your cable and protection device.

If the 1.8 multiplier was to allow for power factor, then yes, it needs to be allowed for when calculating cable size. If it's for short term starting current, then surely you just need to check that the volt drop during starting won't be so great that the lights fail to start.

 

[Gnasher]

The 1.8 is not for power factor, it is used to calculate the maximum current requirements for the circuit being designed regardless of it is only under those conditions for a short time. This calculated current will be used in calculating the volt drop when starting. Amongst others.
The problem is that on normal domestic installs standard circuits tend to be used. 2.5 with a 32A MCB for a ring. 1.5 with a 6A MCB for lighting. On the whole this is fine, but during install the installer should be aware of the correction factors and reasons for them. For instance if the cable passes through thermal insulation all around the cable for more than 500mm the current carrying capacity is reduced by a huge amount.
The lighting circuit in question is not a standard domestic circuit.
So the correct way is a full design. That includes using all the correction factors stated in BS7671. The outcomes of thes calculations can be supprising.
Don't forget that on an EIC there is a requirement to sign for design not best guess.

Sorry this is a not giving a full design example but there are so many variants that it is not an easy thing to do in a few words, but correct design is often overlooked or not fully understood. I can understand this as it is not always easy and is a subject in itself. One course worth doing is the City and Guilds design and verification this is a 10 evening course with a four hour written exam. Not easy but well worth it. Not my intention to insult or upset anyone on here just trying to help and share my experience.

One book worth getting hold of is the IEE Electrical installation guide if they still do it it is the same format as the onsite guide.

 

[Darkwood]

Q - How many 400w metal halide lamps can I have on 1 6amp(b) breaker, the lamps will be switched on together with one switch, do you calc's with and without the correction factor and you will start to understand why its neccessary to apply to inductive loads. There are also added issues with my example question that colleges just don't teach anymore that can lead you to make a very expensive design flaw if you can guess it.

 

[Pete999]

We have a 3 ph DB. I am a little confused with the cable sizing that has been carried out and would like to verify it for my self.

A 4core 16mm (3 ph and N) goes from this DB to a jb in the field, through an underground cable route. At this JB the cable splits. One cable goes off and feeds 14 400W HQI lights and another cable feeds 12 400 W HQI Lights and three street lights.

From a sizing point of view is this treated as one three phase circuit with 29 lights on it?

Thanks

This JB in the field, is it just a joint or is there some kind of feeder pillar, whereby the cables going to the lighting circuits are fused down?