Why does my breaker trip occasionally when I power up my 250

I’ve had this problem since I bought this 250dr new in January, it doesn’t trip the breaker every time, just now and again. It did it from day one and the first day I couldn’t get the amp to power up without tripping the breaker, which I put that down to moisture in the amp as it was delivered on the coldest day of the winter here and there was moisture visible on the outside of the case.
I recently had a new consumer unit fitted with current building regs etc and still have this problem. It only happens when the 250 is switched on.
I usually leave it powered except when I’m going to be away from home for a while.

What type of circuit breaker do you have on that circuit?
Sounds like it’s the wrong type to deal with the surge required by your power amp.

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Are you talking about the mcb (fuse protecting just that circuit) or an rcbo (fuse with rcd trip combined) or the main rcd (trip covering every circuit).

It’s normally the wrong type of circuit beaker. You need type C as opposed to type B which is standard.

The 250 has a large transformer and when turned on you will get a large inrush of current which trips the breaker.

I have type B and get the same problem when the Supercap 2 is switched on.

The 135s, 555PS and XPS2 are okay when turned on.

I will change the MCB at some point.

I wouldn’t worry too much about it. Just turn the MCB back on with the 250 still on and you should be okay.

I don’t believe it will damage the 250 but I will stand corrected!


Just leave it powered up……as it was designed to be used?


If you are in the U.K., a Type C breaker is what you want. And once on, don’t turn the 250 off, other than for going on holiday, if lightning is likely, or for the annual system clean and rebuild.


Looking at the fuse it appears to be type B, so what’s the difference between a c and a B?

I don’t like leaving on when I’m away from home and besides Naim put the on off button on the front as opposed to the rear on previous Naim gear I’ve owned except the Nap140

It will be on now until Sunday evening and then off as I’ll be away.
I was more interested why it keeps tripping, faulty amp or breaker/fuse.

I suggest you ask Mr Google if you want a technical explanation. It’s all about inrush current or something like that. But get a Type C and it will almost certainly make the problem disappear. It’s what I use and it’s what most people on here, who have big power supplies, use.

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The difference is described here:
Type B devices are designed to trip at fault currents of 3-5 times rated current (In). For example a 10A device will trip at 30-50A.
Type C devices are designed to trip at 5-10 times In (50-100A for a 10A device).

(mainly about equipment fuses, but the part about the inrush current applies to the mains breaker similarly)

The equipment fuse is fine, I know as I swapped the spare fuse that’s located in the fuse holder at the back of the amplifier.
I recently had a new consumer unit installed and wonder why the sparky didn’t use a c fuse over a B fuse.

I understood that, my point was that the inrush current that can trip the equipment fuse can also trip the mains breaker if it’s of the fast-blowing kind. The concepts in the FAQ are still applicable and useful

Type B is more standard. Just get a C.


Type C is used for more inductive loads such as transformers. As others have said, fit this type. This will fix the issue.

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Type B is standard for domestic uses and an electrician would fit them in pretty much any domestic installation. Type C would be more common for industrial use, but the high inrush current when you turn on a Naim box with those outsized toroidal transformers can be too much for a type B. Get it changed and I very much doubt you will ever trip it again.

Looks like C it is then, I’ll get the sparky to swap it over.
Thanks all.

That’s exactly what I had to do. The the 300, SC2 and current 145 & 200 could/would all trip it. It could take 30 mins to switch on the system if it had been off for a more than a few hours.

I wouldn’t have stood any chance trying to power up the 500 on the Type C.

Yes… anything with a very inductive reactive load such as a big motor or a largish efficient transformer will likely need a category C MCB …
In short the more inductive the load, the more it resists change so has a large initial surge current… as the load briefly resists being energised.

The other consideration of highly inductive loads is power factor and power factor correction… a poor power factor ratio can cause issues, add distortion harmonics into the mains and even cause higher electricity bills… I never got to the bottom of whether Naim power factor correct their larger transformer circuits.

I have also had this problem from time to time,
And it can happen with any of the power supplies as well as the power amp.
I got into the habit after switching off everything, of unplugging everything and then pressing
the power button on each of the power supplies while unplugged.

Tripping might also be a voltage issue. My NAP 300 worked fine off a 20A type B MCB for years, then one day it tripped at switch on, (I turn my system on and off every time I use it). After this It tripped the MCB 3 or 4 times that month. It was only then that I noticed that the ground floor ring circuit was on a 20A type B. I suspect the mains voltage has been raised a little by the utility Co. over time.

I had the MCB replaced with the correct 32A type B MCB and it hasn’t tripped it at all.