quick calc question

[luke12345]

just a quick question. im installing 10 x 4 foot fluorescent fittings, each with 4 x 36w tubes.
I was under the impression that each fitting would use 144watts, so all 10 would be 1440watts, which would be 6.2 amps in total.
The problem is it keeps tripping a 10 amp breaker and when ive had a clamp meter on the cable it says its pulling 11 amps.
Theres no more lights on the circuit, am i calculating something wrong?

 

[Guest123]

If you multiply your 6.2A by the 1.8 factor for discharge/fluorescent lighting you'll arrive at 11.1A.


Is the breaker tripping on startup or after a period of time??

 

[ezzzekiel]

nail on head springs to mind

 

[luke12345]

it popping after about 5 mins. does the 1.8 factor just apply for the initial start up?

 

[GLENNSPARK]

The reason lenny was asking this was flourescent luminaires have ballasts on them which are inductives. This can cause a B type breaker to open.

 

[pushrod]

it popping after about 5 mins. does the 1.8 factor just apply for the initial start up?

The 1440W figure is the real power but the apparent power (VA) is higher because there is a power factor associated with lamps because they are an inductive load. This is giving the 11A+ figure that you are seeing on your clamp meter. This will be there the whole time and cabling and volt drop should be calculated on the apparent power. As Lenny said multiplying watts by 1.8 is usually accurate enough. However your 10A mcb should not be unduly troubled by 11A for 5 mins so the tripping is probably down to a combination of the inrush / startup current and the overload. The peak inrush for the ballasts should be in the manufacturers specs. Inrush can be countered by using a C type breaker ( if the max Zs is ok) but that still doesn't solve your overload. I reckon you probs need 2 circuits both on 10A mcbs

 

[ratpig]

I wonder how many people who install downlights know about inductive loads, and if they calculate for these loads??? Just a thought.

 

[Guest123]

The 1440W figure is the real power but the apparent power (VA) is higher because there is a power factor associated with lamps because they are an inductive load. This is giving the 11A+ figure that you are seeing on your clamp meter. This will be there the whole time and cabling and volt drop should be calculated on the apparent power. As Lenny said multiplying watts by 1.8 is usually accurate enough. However your 10A mcb should not be unduly troubled by 11A for 5 mins so the tripping is probably down to a combination of the inrush / startup current and the overload. The peak inrush for the ballasts should be in the manufacturers specs. Inrush can be countered by using a C type breaker ( if the max Zs is ok) but that still doesn't solve your overload. I reckon you probs need 2 circuits both on 10A mcbs

Good answer mate.

 

[luke12345]

Split the circuit into 2 and it works fine now. cheers for clearing that up for me.
I think I need to read up on cable calcs a little more.

 

[luke12345]

one more thing, anyone know any good books regarding the design side?

 

[ackbarthestar]

one more thing, anyone know any good books regarding the design side?

You could try this one - probably the best for electricians and associated trades

Electrical Installation Design Guide: Calculations for Electricians and Designers - IET Electrical

I hope I'm not infringing any advertizing on the forum :nonod:

 

[pushrod]

one more thing, anyone know any good books regarding the design side?

If it is a case of you knew it once and have forgotten then the IET's electrical installation design guide is ok and it has some examples, but it does go into a lot of detail and you might find it heavy going. If you are more of a beginner then On Site Guide is a good a place to start as anywhere. (appendix F in the new one)

 

[luke12345]

think ill order that book. cheers again