I have about a dozen compact fluorescent light fixtures in a sandwich deli store kitchen area and about a dozen more in the front of the deli. When the vent fans over the griddle are turned on the compact fluorescent lights dim quite a bit and remain that way until about a minute after the vent fans are turned off. What would cause this and how do I correct the problem


It almost surely has nothing to do with the bulbs themselves but happens to all the devices. The bulbs are just what you see. Are you sure you don't notice it with any other devices? (fridges, microwaves, etc will most likely not show the effect because it happens too quick and not enough to turn them off).

It sounds like a common problem of simply overloading the mains. If you have too much stuff hooked up drawing too much power the mains will "sag".

Say it's 220V but with a bunch of stuff plugged in it might be 210V. BUT when large devices like motors kick in they draw a bunch of current at the start(They surge) which drops the voltage even more. Some devices can't handle this drop in voltage well.

This can happen also on a single circuit if it is heavily overloaded but it should trip the circuit breaker OR the wire is too small of a gauge(which means you would have to add a new circuit or increase the gauge of the wire).

The only practical solutions are:

  1. Put the lights on a different circuit.
  2. Increase the wire gauge on that circuit branch
  3. Increase power supply to building. (requires Elec. Co. to do it)

  4. Reduce the surge current to the fans(a soft start). This might be the easiest solution. Basically you buy a device that prevents the fans from coming on full speed at the start then slowly(over seconds) let them rev up. Motors generally draw the most current when they are not moving(so if they get jammed and you turn them on then they almost surely will burn up). If you don't need them to start instantaneously this seems like it would be easiest, and possibly cheapest solution. I'm not sure where to get such a device and it would depend on things like max current, max voltage, number of phases, etc...

a quick google search gives things like


which are the type of devices you want but you need to know more specifics about your motors. (note, this will solve your problem if it is only happening at turn on of the motors. This basically makes the motors come on real slow so the lights won't notice it. It is not necessarily the best solution because it could be due to the other 3 reasons and your just patching up a pothole)

These are not mutually exclusive and it may take all 4 to solve the problem as it might be a combination.

In any case, it being a business, you should contact an electrician to keep in code and possibly not kill someone(including yourself) or be sued cause someone got a tingle up there leg.

It could always be crappy lights too but probably not.


Here are two reasonably probable explanations.

The first one is less known of by many people.

(1) The temperature of a CFL bulb affects its light output and its efficiency.
Part of the warmup process is the bulb exsablihing thermal equilibrium with its surroundings. If you blow air over a CFL with a fan you will cool the glass envelope and the light output will decrease. If you cool part of the glass envelope - say shield one part and blow air on the other, the visible difference in output can be made very significant. To achieve significant brightness variations usually requires a 1.5:1 difference in brightness and usually 2:1+ TO be really sure, so the air flow / cooling is making a substantial difference.
In cold conditions CFLs become harder to start and in very cold conditions they may not start at all.

(2) As Uiy noted in part, line voltage variations can affect CFL output. The effect will not necessarily manifest in a similar manner with other devices.
eg a motor will not necessarily slow in proportion to mains voltage, and a thermostatically controlled fryer or other heating device will be affected either not at all if the thermostat works as it should or to a lesser extent than the line voltage variation for hings like 'simmerstats' which are not true thermostats and which try to "guess" the effect of voltage variations on heater performance.

In many cases line voltage variations due to adding a heavy load on circuits which are poorly designed and/or overloaded would be the most likely explanation. In this case, the fact that the lights take about a minute to "bright up" after the fans are turned off suggests that envelope cooling may be the major reason. Both effects MAY combine to some extent.

If the brightness decreases immediately that the fans are turned on then cooling will not be the cause as it takes a finite time for air flow to cool the envelopes.

If CFLs which are not air-flow affected dim at about the same level as the others then mains voltage variation is the most likely cause.

Measuring mains voltage before/during/after fan turnon would be useful. Ideally this would be measured on the lighting circuit but if the voltage is being substantially loaded then measuring at any power outlet may suffice.

If lighting voltage does drop when the fans are turned on but voltage at power plugs does not from and/or if turning on other heavy loads does not cause the effect then the fans and CFLs may share the same circuit and it may be wir4d with oo small gauge wire - to have this sort of effect the wiring would probably not meet wiring codes - which happens often enough. Plug a heater into any power point and turn i on. How does this affect the CFL's. Wire a power socket to a plug that will plug into a light socket. Plug heater into one CFL socket and turn on on low range. What happens?

All care, no responsibility:

WARNING: Making a std socket to lighting plug adaptor is not illegal and using one sensibly as described above is sensible. Whether you are allowed to do this depends on local wiring regs, your competence and local laws. (eg in my country it would now be legal but until about ?15? years ago an unregistered person was not allowed to do this by law). SO do only what you are competent and happy to do. Get a registered electrician to make you an adapter if you are not happy with doing so. Very quick nd easy and cost is probably worth it if other trouble shooting fails. IF this produces results it probably means CFL wiring has been done improperly in the past.

Note that heater on low = 1000 Watts = about 10 x 100 Watt std light bulbs of yore so a lighting circuit SHOULD be fused and rated for this. If not the heater MAY blow a fuse or trip a breaker but is unlikely to do so and is unlikely to do any damage.

To test the cooling theory, place an air shield around one or a sample of CFLs to minimise airflow over the glass envelopes and see if this causes selective dimming or different amounts of dimming. If so, a clear "shade around the CFLs which aims to minimise airflow, may assist and may even bright-up the CFLs compared to "normal". Do not allow them to overheat.

A quick and easy demonstration of how sensitive the CFLs are or aren't to the effect that I have described is to use a portable fan to blow air over a single CFL at a time.

  • \$\begingroup\$ I would imagine if it were issues with the fans blowing over them it would occur the whole time the fans are on. OTH, he does say it lasts for a few minutes and by this type the fans should be fully up and running(unless there is serious overloading going on). CFL's can be quite strange and have other startup problems too. The lights you have to "jump start"(hold in the button) can go into a dimmer mode if you do not hold it long enough. I guess CFLs have different excitation modes and it is another possibility. The surge may knock them offline and not allow them to fully come back. \$\endgroup\$ – Uiy Feb 28 '12 at 8:59

An easier solution than rewiring with an increased gauge is to find the end of the branch and loop it back to the circuit breaker, forming a ring main. This will reduce the volt drop seen by the lighting due to the two paths back to the source being in parallel. However, you need to check if this allowed in your local regulations and standards.


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