After running my NAC282 and NAP250DR for an hour with the volume set at 12 o’clock, the 250 becomes extremely hot and powers off.
It returns to life after 20 minutes of fan-cooling.
It’s sitting on the top shelf with no obstruction to the surrounding air.
Is this to be expected? 
How big is your room ?
What speakers are you driving ?
How loud is it ?
Congratulations on shutting down your 250.2DR it’s a very small clue to belong to.
In a word - Yes.
At 12 noon (it must be loud?), you will be using most, if not all the amp’s output headroom.
ATC SCM19’s.
Volume at 12 o’clock.
Room is 3.5m x 6.5m with a sofa at 3.2m away from the speakers…
Rest of the kit listed in my profile.
The technical reason is that the 250 (of whatever vintage) is a regulated amp and is prone to heat build up and unlike Olive/CB 135s (essentially 1/2 of a 250), it doesn’t have a fan to aid cooling.
If you are using a line-level (digital?) source, 12-noon is very high on the volume dial - I’m surprised it lasted an hour.
My olive 250 runs hot. It has only once tripped out, in similar circumstances to those you have described. It was during hot weather.
I now run a fairly quiet fan to cool it during long sessions. I certainly can’t hear the fan with the dial at 12 o’clock!
So… as long as I keep the amp cool (entertaining the neighbours) it will run perfectly happily for long periods at that volume?
Not quite.
An amp will take an input signal and (obviously) amplify it. Phono-inputs (even with many phono amps) are relatively ‘low’ and below what is described as ‘line level’ (IIRC this is 2.1v for Naim’s CD players).
So, if you are using a digital line-level source, you are pushing quite a high voltage signal in to the 250. And most amp’s really shouldn’t go >12 noon IME. I had this with my 4-ohm Dynaudios - the 250 cut out after c.25mins at 11o/c from a CDS2/3.
Simply put, you’re pushing the amp very hard and at the outer edges of its performance envelope and, eventually, it overheats.
Mike - it can do this. Many people have fans stationed to blow air over the top of the casework. Being in Spain ain’t helping 
How much space is there between the 250 and the shelf above Mike? The 250’s case acts as a heat sink, so if there isn’t much clearance it will get hot fast.
Sounds like we have the answer as to why, but I have to know what your listening to?
Iron Maiden
Metallica - kill em all LP?
Or are we thrashing to Joni Mitchell or Van Morrison? 
Sorry, trying to be funny, but I am running 282/250 DR with PMC25.26 and never get about 10-11 o’clock on the dial, with an LP12 and low output MC Krystal.
Scott
As a further response to this:
When you play music loud, you often explore the impedance curve of the speakers and many, even the best quality ones, have quirks and foibles when they get to deliver bass (as the big(ger) woofer kicks out more energy and needs more control). Plus, the speaker cabling needs to be considered too.
In many speakers, a drop in impedance (ohms/resistance), is not good for an amp, as it literally sucks current from the amp and the amp thereby has to work hard to recover and keep-up = in the 250 series this means heat generation.
Yes - recognising what level of input you are working with.
Strange but true.
So, out of interest, why (spoiler alert - possible numpty question) have a volume control that enables more volume than the amp can take?
Because, back in the day of low-input phono-usages, the volume dial could be/needed to be turned up a tad higher.
I know it seems strange. I was advised that with a CD/other digital line-level input, one shouldn’t be going >11 o/c as, at that level, something like 80/90% of the amp’s capability was being exercised.
FYI, I’ve had my LP12/Superline source >12 noon a few times - no harm done given the low/modest input.
I have the 250 dr and 9 o’clock is already loud. I listen at 10 o’clock if I want to play very loud sometimes. Something doesn’t work properly. 12 o’clock is very very loud normally.
So that clears that up, then. Thanks for all the replies.
As you now know, it’s shutting down due to the thermal trip. Hot weather and high volume level will get the amp nicely warmed up! One of the things that can exacerbate shut down due to excess heat is inappropriate speaker cabling &/or speaker connectors, so get these checked out. Also inefficient or current hungry speakers can cause it to happen. And combine all these things and it’s probably a dead cert.
One of the old tricks back in the day of the original NAP250s trying to drive Isobariks was to use them tipped on their sides so the long part of the top and bottom of the sleeve was vertical - this cooled a bit better, but wasn’t particularly stable, so you had to be careful. The other trick was to run them with (quiet) fans pointing at the casework. Needless to say, many were relieved to upgrade to the NAP135s, which came with internal fan cooling…
Cabling is two 4.5m lengths of Chord Epic with gold-plated banana plugs. Not a problem, I would have thought…
I assume that running a fan to keep the amp cool will enable 12 o’clock volume usage?