So I decided to test some of my 18 watt 2ft tubes out and I used a 20w ballast rated for 0.36A, after about half an hour the ballast started humming and started smelling like enameled copper wire, it overheated somehow. The lamp is rated for 0.355A, I do t think 5mA mismatch can make the ballast burn like this. Any clues?

Any objective measurements? Tube current, ballast temperature? Latest ballast have some high-temp insulation, and may run close to 100C temperature even in normal mode. Look at Tw and deltaT marking at the nameplate.

Tw is 130 degrees, delta t is 60 degrees, but it is a no name brand and on top of the smell it also started humming which probably isn't good right? The current is 0.37A warm which is correct, I can't really measure the temperature. Maybe it's just a poor quality ballast.

I like a good ballast hum but not when it's overheating. 

I like a good ballast hum but not when it's overheating. 

It is brand new so it shouldn't hum, to be fair it didn't hum at first, only when it got really hot.

I've got a 50w mv ballast in a nos suburban which hums very loudly but a well used 1980s 80w mv ballast in an optimum which is very quiet. Both are atco.

Well, single tube 230V 18/20W ballasts run at unfavorable rate of lamp voltage to line voltage (57V vs 230V) so they all run rather hot having some quite shameful losses, especially modern 'optimized' types. It is not unheard for a new ballast to smell hot paint for some time.

On the other side, modern ballasts generally do not hum by themselves until mounted close to some thin magnetic material sometimes.

If it’s a VS it’s perfectly normal!, I’ve got 2 VS chokes in my old fluorescent fittings from the kitchen, and they have always run hot! 🥵

If it’s a VS it’s perfectly normal!, I’ve got 2 VS chokes in my old fluorescent fittings from the kitchen, and they have always run hot! 🥵

I'd figured, just switched to electronic ballast since it is fixed on a piece of wood, it is very hot indeed.

On practically each transformer/ballast is a rating figure named "deltaT", "dt" or something along the lines. This means how much warmer/hotter that thing is expected to run versus the ambient (the tempoerature of the environment the ballast is in, so within the ballast compartment/fixture) temperature. So if a ballast is rated "dt=70degC" and the ballast compartment uses to warm up to 30degC, the expected operating temperature is up to 100degC.
If the real temperature is below that, the ballast/whateverthing is by itself good.

Now there is also a second rating, called "tw" (winding temperature) or "tc" (marked control point temperature; usually used for electronic things), which says how hot the running temperature is allowed to be.

Sometimes one or the other is replaced by "Ta" rating, which means the maximum ambient temperature where the thing is placed.

Now if both the temperature difference, as well as the operating temperature are below their rated limits, everything is working as designed and the (seemingly hot) temperature is normal. Many vballasts are designed to operate around or even above 100degC (=boiling water), which may seem quite high for an electrical device, but it is designed to be that way. Yes, it may have an impact on the reliability of such designed product, but so has the selection of materials. The main consequence for you is to make sure the enclosure where it is used can handle these temperatures long term.

If "Ta" is exceeded or the "deltaT" is OK, but the "Tw" is exceeded, the thing is being operated in a too hot environment, so you should fix/modify the place where the ballast is to get better cooling.

If "deltaT" is exceeded, there is something wrong with the ballast (short between turns,...) or the circuit it is operating in (lamp rectifying,...)

Not sure if this comment is pertinent to your situation, but anyways here it goes. I've had a metal halide ballast run hotter than I liked. It was a HX autotransformer for North American 120V, and stated an optional power factor correction capacitor, which I omitted from the circuit. I then installed the cap as per the wiring diagramme (in parallel to the mains), and the ballast heats less. I realize that adding power factor correction is not really necessary in small domestic applications, but in my case it lowered the ballast operating temperature a bit. Might be worth a try.

The PFC capacitor is neded for autotransformer ballasts with multivolt primary, where it allows to distribute the reactive power over the full (or majority) length of the primary (the whole 240V or so primary). When PFC not connected and operated at 120V input, all the reactive power needs to be handled by just the 120V section, so it wastes more heat.

With series inductor ballasts (what this thread is about, if I understood well), the PFC capacitor does not influence the ballast at all, it only influnces how the ballast is seen by the mains connection, so how much current it draws through the input wiring (with larger installation it has an impact on the required wiring rating).

@Medved : Thank you for the explanation ; I wasn't 100% sure it made sense, but now I know.