Normal, for things with a lot of Digital circuitry, I think.
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@IanRobertM - Thank you for expanding above – and yes 135, 300, 500 models all have fans (and of course thermal protection trips). I recognise Naim’s speakers are 4/6ohm but, as stated, they are ‘stable loads’ in general terms.
I also recognise @CalumF 's observation, as having used a 250 to drive ‘inefficient’ 4-ohm Dynaudios playing rock at spirited levels, it got quite hot and the thermal trip cut in a couple of times after c25mins. And playing via a digital source was far more aggravating than an analogue source (TT). The Dyns even had the fan going on my 500!
Many years back, some 250 owners would have fans blowing over the casework and/or put cold packs on them.
I think I read that JV recommended standing them on their ends…!!!
Having used a 250 since 1985 (regularly serviced), and playing quite noisy music, I have - maybe sadly - never manged to get it to trip out… But I have never had any difficult or complex speakers…
(Speaker List - Tangent RS2’s, Linn Kans, Royd Doublets - and currently Kudos X3’s. All relatively easy. )
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Your Nova has an awful lot packed into a small space and those processor boards can get mighty hot, so as ever the casework is the heatsink.
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Which makes it interesting that power amp numbering is based on wpc into 4ohms. So a 250 is 125 wpc into 4 ohms. At least that’s my understanding, please correct if I’m wrong.
Roger
So playing the latest Def Leppard CD at 9 on the dial 250DR into NEAT Petite 30’s unlikely to cause damage ?
Originally, yes. Correct. When it was 70 W into 8 ohms. But now its 80 W into 8 ohms…???
Check the handbook for the speaker specs on max driven range – I suspect they will handle the stated ~125wpc of the 250 in to 4 ohms – edit: see @IainO 's post below for 250DR’s ‘real world’ specs.
As mentioned above, if the 250DR is pushed too hard (hot), the thermal trip should work – see your Naim amp handbook about this. IIRC, it cuts in at 70C.
Also, as above, this isn’t about wattage, it’s about how difficult to drive the speakers might be across their range. It’s been said before that with a line level digital input (e.g. streamer/CDP), a 250 is perhaps operating peaks at ~75/80% of it’s headroom at ~9pm on the dial. If you were to go to 10+ on the dial, I suspect you’d trigger the thermal protection (and don’t do this before checking what the speakers are rated at).
If you were to feed the amp with a lower input signal from say a vinyl set-up, you can often push to say 11pm/sometimes higher.
Google “Stereo Magazine NAP 250 DR PDF”
The original naim rating system of (125wpc into 4 ohms)x2=NAP(250) is slightly understimating the power delivery capability of the NAP250DR going by those measurements! Should it be renamed the NAP320DR 
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@IainO - As I said… the Naim listed rating for the ‘250’ is now 80 Wpc, not 70Wpc, which it was originally. So, its 4 ohm rating will be incorrect now…
The NAP300 is listed as 90Wpc (in to 8 ohms). In theory, it should be 150Wpc into 4 ohms.
I have always found the power of my 1985 CB/Olive 250 to be ‘enough’… 
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Can’t find a comparative measurement for the 300DR other than the PDF review on naim website (HIFI Critic) which suggests 300DR can do 165W sustained into 4 ohms.
Who knows how that was measured compared to the 162W quoted by Stereo Magazine for the 250DR.
Interestingly the 300DR is quoted as 325W pulsed into ONE ohm. Now I don’t know what kind of speaker would drop to 1 ohm. That is an interesting load which I suspect would cause many lesser power amps to cut out!
Only once have I got my olive 250 to thermally trip. It was a hot day and I was playing Tago Mago by Can. Maybe I should have known better.
Even playing Live in Japan by Deep Purple (“everything louder than everything else”) hasn’t tripped it, but it can run very warm. I now run a small external fan if needed.
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If you search for “707heavy” an article should pop up about speaker load and phase angles and their effects on an amplifier.
I didn’t find the 250 power enough. I play loud rock and techno music sometimes and was partying alot and the 250CB would cut out. No such problems with the 2 x 135s with their fans.
Not partying so much now but the 135s are fantastic amps.
It was 37 C outside yesterday. Windows opened. My 250 dr , after 2 hours playing on afternoon, was just warm. My tubes pre hot.
When I had a Unitserve, in same conditions, it was really hot. I could fry an egg on it.
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My 2012 SN2 doesn’t get hot either Mike, only just a bit warm. And I drive it quite hard at times with 70s rock and such. And my speakers are floor standing Kef R!07s which are 4 ohm, 90 db, with two 10 inch drivers in each.
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I also listen to music whilst cooking breakfast FR, but I usually do it in the kitchen in a frying pan …
My previous speakers, SF Guarneri Mementos, were 4 ohms nominal with a low(ish) 86dB sensitivity. These compact 2-ways lived in a large 60 sqm room at the end of a humble 200 for about a decade. Playing quite loud too, volume regularly around 10 o’clock. The 200 never got passed lukewarm.
My current Vivid Audio speakers are slightly less sensitive at 85dB but their minimum impedance is higher at 7.5 Ohms. The 200 seems to have a (even) easier life now.
My NDX gets much warmer than the NAP200…
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Same here