Contactor sizing

[RF Lighting]

I’ve got a job to install a photocell to control 3 existing 400W SON floodlights.

I’m just after some reassurance that I have sized the contactor correctly so that I won’t have any issues with inrush current welding the contacts.

If this were your job, what size contactor would you specify?

 

[bernardgreen]

Provided the worst case inrush current and its duration is less than the maximum current the contacts can carry when closing then it should be OK.

I recall in rush currents can be as high as 20 times the running current.

Do the lamps have a miminim time before a hot lamp can be re-lit (hot re-strike time ) ? If they do then you may need to add something to prevent the contactor re-closing before the lamps have cooled and ar safe to be re-lit.

 

[securespark]

According to my tables, for 3 x 400W HPS, a Type D 25A breaker with a contactor rated at 40A.

But it is a couple of decades since I did this sort of work!

 

[ban-all-sheds]

Ask the manufacturer of the contactor?

That would give you a better CYA than canvassing opinions on a DIY site...

 

[SUNRAY]

I’ve got a job to install a photocell to control 3 existing 400W SON floodlights.

I’m just after some reassurance that I have sized the contactor correctly so that I won’t have any issues with inrush current welding the contacts.

If this were your job, what size contactor would you specify?

I've always applied a rule of thumb of 3 times for the 3036/88 or 5 times for the B type MCB (25A) and tended to go up a size from there for the contactor. if the resistance allows go for a 2 times C type (13A or 16A depending on manufacturer).

 

[Robin0577]

I would agree with a 40A rated contactor. I generally use this data from Hager to size lighting contactors because it gives figures for most types of lamp. See pages 3 & 4 https://www.hagerelectro.com.au/files/download/0/30875_1/0/TECHINFO_CONTACTORS.PDF

 

[RF Lighting]

Thanks for this. It pretty much lines up with my calculations which was 10x the running current of the circuit.

 

[Lectrician]

I would of gone 40amp. The cost difference is negligible to smaller 20amp ones. AC3 rating is important. An AC3 rated 20amp contactor is designed to withstand a 100amp switch on surge. The bigger you go, the better though, and for the cost....

In theory you should go for an AC5 rated contactor, but trying to source one is a challenge, and then comes the cost of a rare item! Most would go with an AC3.

 

[JohnW2]

...AC3 rating is important. An AC3 rated 20amp contactor is designed to withstand a 100amp switch on surge. ...

As a matter of interest (i.e. 'for my education'), are they also required to be able to break that sort of current?

In context, I had in mind the possibility that if RF's 'photocell' had negligible hysteresis, the likes of a passing pigeon could result in a rapid on/off, hence possibly requiring the contactor to break the switch-on surge current.

Kind Regards, John

 

[Deleted member 18243]

What is the practice when installing outdoor LED lamps with photocells and timers? Most new outdoor lighting is LED, therefore much lower wattage. Is there a significant inductive or capacitive load or anything else to consider, and is a lighting contactor needed?

 

[bernardgreen]

if RF's 'photocell' had negligible hysteresis,

Hence the mention of some circuitry to prevent the contactor re-closing while the lamp was cooling down.

 

[JohnW2]

Hence the mention of some circuitry to prevent the contactor re-closing while the lamp was cooling down.

Yes, you did mention that, but it would be 'too late' to avoid the potential issue I mentioned ... I was talking about a situation in which the contactor opened whilst the lamp was still 'warming up'.

Kind Regards, John

 

[RF Lighting]

It’s only the initial magnetic charging of the ballast which causes the inrush current. It’s all over in a second even though the lamp may take several minutes to reach full brightness.

I don’t know if you remember my experiments to limit the inrush on my 10kVA transformer? The current curve is very similar to that of the lights in question, both being large inductive loads.

Standard photocell switches have circuitry built into them to prevent hysteresis and usually take 30 - 60 seconds after covering them to switch on the output.

 

[JohnW2]

....Standard photocell switches have circuitry built into them to prevent hysteresis and usually take 30 - 60 seconds after covering them to switch on the output.

Fair enough. That would certainly avoid the issue I was pondering.

Kind Regards, John