Hey guys. I have a 400W MV ballast that originally took two 13.5uf capacitors, with a total of 27uf. I came across a 28uf capacitor with the same voltage rating (330VAC). Would one uf make a huge difference? I know it's safer to go down a uf rather than up, but is +1uf still considered safe or could one extra uf be  a death sentence for the ballast? Thanks a ton guys.

I've heard that adding a uf would cause the ballast to overwork and eventually burn out. But I'm not 100%.

If the capacitors are for PFC, I wouldn't think so.

The capacitor is in series with the lamp, so it's a constant-wattage type ballast. Yes, the +1uf will make the ballast work harder but would it be severe enough to fry it?

If the ballast has good ventilation, then likely it will be OK.
If the ballast is known to heat up or is in a low ventilation area, you might want to do it with caution.

Not sure how hot the fixture gets. It's a 1964 GE M-400 with an OEM ballast. The capacitors were removed from the light when it was taken down in 1994. The light actually belongs to my friend Marco (I got it for him) but I have a capacitor and didn't know if it would work or not. I'm hoping Ash, Don, or Medved sees this post since they know a lot about ballast specs.

The uF value is fine

The capacitor type however.. What is the intended application of the capacitor you want to use ? Motor run would be fine. Motor start probably not. and PFC - only if it can handle the lamp arc current (if the capacitor is in series with the lamp)

The current that PFC capacitor is made to handle is :

(rated voltage V) * (capacity uF) * 10^-6 * 6.283 * (frequency Hz)

As a quick indicator, the capacitor should be about as big as the 2 originals together. Smaller capacitor means thinner either plates or isolator

The capacitor will have ~3% lower impedance than originally rated for the ballast, but that does not mean 3% higher current or so. The complete ballast topology compensates some of the current variations (coming either from the mains variation, or the component parameters), but in the worst case (no compensation whatsoever), the current may be up to 6% higher than originally designed. That would mean about 12% of the power dissipation.
In any way I would expect the extra loading of the ballast will correspond to about 6% rise in the mains voltage. I think that should be well within the ballast designed tolerance, of course unless your mains voltage is already at the top side.

And other aspects: The capacitors are manufactured with certain tolerance and so the ballast design should have taken that into account. I don't know what is the standard tolerance for such use, but I won't expect it to be any tighter than the +/-5% or so. Even your capacitor is subject to some tolerance range, so hypothetically it may be the 3% above what the ballast is designed for. But for that you have to have really bad luck - the rated tolerance means at least +/-3sigma of the statistical spread of the capacitance plus some margin for the tolerance of the end of line measurement. Because the makers most likely do want the srap rate to be low, I would expect even more "sigma's". That all means in reality with a 5% tolerance rating more than 90% of the production will be actually within way less than 2% tolerance band.
So if you have that possibility, try to measure the real capacitance.

If I'm understanding this correctly, capacitors are placed in series with the lamp on the hot side to accomplish lamp light output reduction or current draw from the ballast?

If the capacitor is placed in parallel before the ballast, this is used for PFC?

Thanks for the help, I've gained a great amount of knowledge by joining this site!

Yes this is a constant wattage ballast with the lamp and capacitor in series. So 1uf won't be detrimental to the ballast then? Good to know. Thank you for your help guys!

Also, the capacitor in question came from a 250W HPS ballast. The ballast was 208V single tap so it was replaced. (Came from Jake's M-250R1). Capacitor is a dry film no PCBs and GE branded.

If I'm understanding this correctly, capacitors are placed in series with the lamp on the hot side to accomplish lamp light output reduction or current draw from the ballast?

In this case you can not separate the capacitor from the rest, it is just a part of the ballast. The magnetic part won't work without the capacitor at all. Without the capacitor it may ignite the lamp, but then the current may be either too high (so overheat it, frying the coils), or too low to underdrive the lamp (that used to be an intentional design feature to protect the magnetic part from overheating and so even reduce the risk of fire in case the capacitor fails short circuit; but it means higher cost and higher losses).
In order to work, the capacitance really has to match what the ballast is designed for.
Even when the HX vs CWA ballasts looks very similar (and they are even made using common component types, like bulk core, winding bobbins,...), they work in a complete different way and they do not work in the place of the other one. The main difference is mainly in the magnetic shunt (a magnetic insert between the primary and secondary coils): With the CWA it is designed to partially (softly) saturate at the rated current (to provide, in cooperation with the series capacitor, the required current regulation) during normal operation, in the HX transformer the shunt is designed to never saturate during normal operation (it forms the only ballasting impedance), so usually it either has larger cross section, or the ballast uses more turns on both windings.

If the capacitor is placed in parallel before the ballast, this is used for PFC?

If it is parallel to mains (could be directly, or indirectly like e.g. across 240V tabs even when the ballast is supplied from 120V). That s the case for either series reactor ballasts (99% of the 230V-area ballasts, some 240V-only and the lower wattage HPS in the US), as well as the HX (auto)transformer ballasts.

But do not mix this up with the "magnetic regulator" ballast (featuring three coils, separated by magnetic shunts). There the capacitor is connected parallel to one of the windings (the regulator one) and it's function is to form a resonance circuit with the impedances formed by the rest of the ballast. There the exact value should be used as well, otherwise the ballast just won't work at all (the capacitor is needed to form the resonator to provide sufficient OCV for the lamp start).