Why Naim recommends 100A RCD isolator for Hi-Fi dedicated mains if there is only one 32A MCB in the chain?

I have my dedicated mains installed tomorrow to compliment my new upgrade from Nova to SN3/NDX2 (Fraim to follow :slight_smile:) and I realised that the electrician ordered VML402AH Consumer Unit (CU) which has 40A current rating as opposed to 100A as recommended in previous threads in the forum. I would only have a single 10mm SWA radial with a 32A MCB so I don’t think the 40A CU should be a problem since anything above 32A will trip the MCB anyhow and I’m not planning to use the second bay of the CU. Do you guys see any issues with that?

Many thanks in advance.

Dear ngivanov,

I am an electrcian in Australia. Please excuse my ignorance. Could you please tell me what 40A CU is, and what country you are in, please? I assume it is some sort of current conditioner?

Kind regards,


Consumer Unit AKA fuse box (probably less correctly)

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Hi @Gigantor it is a Consumer Unit and I’m based in the UK. See this for more info: VML402AH | Hager UK

I am off to bed now. Though will have a read of the datasheets.

Kind regards,


Looking at the details, it does have 100Amp Terminal bars, but is fitted with a 40Amp RCD, so I don’t see any issues. I hope it goes well, and please report back your findings.

Many thanks @GadgetMan. Will certainly report my experience once the dedicated mains is installed.

The 100A rcd main switch will have larger contact area inside and larger cabling. This should make the hifi sound better, otherwise the 40A version can potentially act as a bottleneck. You are having 10mm cable so I would insist on a 100A rcd.

I also realised that Russ Andrew is selling the same consumer unit on their website: Upgraded RCD Consumer Unit Metal case | Mains installation | Russ Andrews Accessories Ltd

Wouldn’t the 32A MCB be the bottleneck as opposed to the 40A main switch? I’m struggling to understand why Naim recommend 100A CU with one 32A MCB and a single radial? I’m probably missing something obvious and would welcome some insights.

Many thanks in advance

Its surprising that any of this makes the difference it does when you consider the equipment only draws a couple of amps, but it really does.

Ive tried 2.5mm, then 6mm, then 10mm radial circuits and with each increase in size the sound got better. I now have multiple 10mm radials which sound best of all.

I got the supply company to change the main incoming fuse from 60A to 100A and this made a noticeable difference too.

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It’s not magic. The larger conductors offer a lower resistance path. And while that matters not for most things, transients that drain and trigger recharging of reservoir caps can be measured is microseconds. It’s not about power consumption. It’s about transient recovery time.

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Thanks for the clarification @feeling_zen. Wouldn’t the lower resistance path offered by the larger conductor of the 100A RCD Isolator (vs the 40A RCD Isolator) not be limited by the 32A MCB in the chain?

:man_shrugging: You’d have to ask a sparky. The lower resistance is certainly the reason for using the thickest wire between mains socket and CU.

There are low resistance audiophile CU plugin RCD switches for sale though.

Thanks, I realised about transient peaks for power amps and lower circuit impedance but it applies to every piece of kit thats been connected.

When I was still experimenting with mains some years ago I even noticed that my meridian transport and dac sounded noticeably better using a 10mm radial each rather than sharing one regardless of their probably more gentle flow of current with little, if any transient’s.

I must say that installing the mains system to my kit has been a really worthwhile upgrade and paid dividends sonically.

The idea is to achieve the lowest impedance up to the point of connection. Regardless of the 13A plug fuse and the 3A equipment fuse this still gives great sonic results.

I would definitely use the 100A rcd as this will almost certainly be better than the 40A one sonically and for only a few pounds more.

Also don’t forget that the new mains circuit will take a while to settle down and burn in. You will hear this as the sound will change a bit for a few weeks, gradually getting better.

Thanks @Mrhappy. I will speak to the electrician shortly and try to replace the 40A RCD Isolator with a 100A RCD Isolator.

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Apparently the industrial 10KA MCB’s (rather than the default 6KA) have larger connectors, and may offer an advantage, but I never managed to get hold of one. Remember also your MCB needs to be of Curve C type.

G’day ngivanov,

Busy day here with no water due to contamination in the town’s water supply. Not flood-related. The floods are over 2,000 kilometers to the north.

I have had a look at the data sheets and things are done a little differently here in Oz. But all in all the VML402AH as you know is rated at 40 amps. I look at the other items in this range are of larger capacity in amps and the units are physically larger to allow installation of more devices which is unnecessary unless you wished to install surge protection.

I do not know how much room is in your Fuze Box or as we call in Oz a Switchboard.

I think your electrician has chosen the correct items. 40A CU with a 32A MCB all makes good sense. Right size for the job with a small footprint. I am sorry for replying so late. Dead mouse and keyboard then the whole computer locked up.

I do not appear to have the same noise issues as you may be experiencing. I do not know if my music room is on a separate feed. Though I will expect so. I might give it a try out with Shelley with a couple of 477MHz CB radios tomorrow.

Warm regards,


The only reason I can see for the 100A CU is for future expansion. Possibly on a new build.