Hi There,

I've got a greenhouse type application where the growth chambers each have a ton of lighting load (i.e. compressor on top of chamber might have 30 FLA of current while interior lighting load is 21 amps!). The lights are 6 or 8 lamp T8 fluorescent type. Which such an extensive amount of lighting load in aggregate between the growth chambers, might these lights, not necessarily the compressors, be drawing the inrush current sufficient to trip the main?

I know HID lighting has an inrush but I haven't had to try and model it before. Not finding much for recommendations or comments on google and my analysis software doesn't have HID lighting as a load you can model. Any advice or directions you could send me are appreciated!

Thanks,
Brian

If the facility is existing, then obtain a high speed power analyzer / recorder and measure the inrush. There are some single phase units that work but the best option would be a three phase unit similar to Fluke 1750.

I agree with measuring it. However just recognize that although an electronic ballast might be a bit different a magnetic ballast is no different then an inductor...ie, transformer-like inrush. So I'd expect somewhere around 1000%-1200% during startup. The real trick is that if you have an HID lamp that goes bad, it STAYS at that same level of current until something trips out and can and has actually damaged other ballasts on the same circuit from undervoltage causing overheating and damage if it doesn't trip out the overcurrent protection quickly. However this is for HID lighting (metal halide, mercury, sodium vapor, high pressure sodium). Fluorescent fixtures such as the ones that the OP described are a little different...they rely on high frequency pulsed arcs to cause emission of photons so there really isn't inrush but the X/R ratio is generally very low so it's more like a very capacitive circuit. In itself this is relatively easy to trip on but causes a bunch of consequences due to the tendency for restrikes and other overvoltage effects.

Thank you for the responses guys.

I had a suggestion from a colleague on how to approach this HID lighting inrush question - the HID inrush current is that of the ballasts, and those are transformers. So what we essentially have is a large bank of small transformers. One coul estimate the total kva and model it as one transformer with some assumed multiple for inrush. Inrush on any one particular transformer could be 4x to 10X for a few cycles.

Seems reasonable enough to me, do you agree?

At the end of the day the main breaker at the start of this system that is nuisance tripping is seeing X amount of motor inrush along side Y amount of inrush from HID ballasts. If we simply lump the motors into one aggregate load at the main bus as well as lump the HID lighting ballasts (transformers) into one aggregate at the main bus we can then compare to existing and proposed breaker setting adjustments via the TCC plot.

This is the path we'll take unless someone calls me out as crazy.

Yes you can lump model ballasts. The standard value is usually to look at inrush at 0.1 seconds as about 12X but with small transformers like ballasts it's closer to 14X and on medium-large transformers (2 MVA+) it's more like 6-10x. Inrush is driven by internal core effects and not impedance but it is also affected by loads and since your "loads" are switched on immediately unlike say starting up a distribution transformer, you need to err a little on the side of the higher multipliers, if you have any influence over what your modelling software does at all.

It's similar to "lump" modelling small motors. Currently the rule of thumb is to ignore anything under 25 HP but the old rule of thumb of 6x FLA only applies to medium size motors, not very small ones where it is higher. Then again with today's energy efficient motors (and transformers), core losses have been reduced to the point where I've actually measured Siemens and Toshiba motors to name a couple brand names at around 22x FLA as an INRUSH. This is very typical of IEC motors that are rerated for North American markets and flies in the face of the Code requirement of no more than 17x FLA for "instantaneous" settings. With modern microprocessor controlled circuit breakers, they will trip on this sort of thing no matter what you do. The only recourse is to slow them down a bit and add a delayed tripping to "instantaneous", if available.