Wiring over 13A questions.

[Gareth J]

Have a 3hp compressor that is at the limits of a 13A plug.

Checked the basics and has fresh oil, it's just a bit big and fairly old.

Not a problem though, I have a 32A supply I want to use anyway. It's perfectly located and the switch is in a very convenient place.

So I've put a 32A commando on it and all is well...

...but, the only thing that's protecting it now is the trip switch in the fusebox. No fuse.

I feel like I should have some weaker break point in the system. Shouldn't I?

If it's ok, then that's fine. If not, can you get a fused 32A commando plug? Or I could take a supply off the 32A socket to a fused spur or something...

 

[slim_boy_fat]

I don't 'do' electrics [well, beyond absolute basics], but I've read about RCDs?

 

[daleyd]

The 32 amp breaker is really only protecting the wiring so as long as the wiring is up to it then it should be fine. (Presumably it will be ok if fitted by a competent sparky??)

Had the compressor got an overload to protect the motor?

I guess if your concerned you could always down rate the breaker to a 20 amp.

 

[skotl]

The 32 amp breaker is really only protecting the wiring so as long as the wiring is up to it then it should be fine. (Presumably it will be ok if fitted by a competent sparky??)

Had the compressor got an overload to protect the motor?

I guess if your concerned you could always down rate the breaker to a 20 amp.

All of the above. Thing is, if the compressor fails in such a way that it's going to trip a breaker it's probably gone dead short and will trip a 32A breaker just as happily as a 16A or 20A one.

 

[dobbslc]

The flex on the compressor probably won't be rated at 32A, I'd downsize the breaker feeding the socket to 16A.

 

[Gareth J]

Yes, ultimately it will all be protected by a c40 breaker.

There is no overload button on the motor.

The big wiring was fitted by an electrician, the compressor will be done by me.

So, to downrate or not...?

 

[Gareth J]

The flex on the compressor probably won't be rated at 32A, I'd downsize the breaker feeding the socket to 16A.

Sorry, just posted before I saw it.

Downrating the breaker isn't an option. It needs to be hefty for my big welder.

But you're right, the compressor wire will only be rated for 15-20a (need to check)

 

[daleyd]

You can perhaps fit an adaptable box close to the compressor (might even be able to be mounted on the compressor itself?) with a smaller fuse or breaker in if you are concerned about the wiring, as long as it’s clearly marked I can’t see it being a big problem.

 

[Richard.]

What you shouldn’t do is have your total Mcb protection to be higher than the rating of your main breaker. So if your main breaker is 100 amps you want to keep your mcbs in under that as a collective total. You can’t just up rate the main breaker either because the incoming cable will be specced to the 100 amp load.
I’d check the data plate on the item and feed it from a supply it’s rated to and no more unless you’ve got the extra capacity
A 32 amp supply wants fitting with 4mm csa cable from the breaker.

 

[lshigham]

What you shouldn’t do is have your total Mcb protection to be higher than the rating of your main breaker. So if your main breaker is 100 amps you want to keep your mcbs in under that as a collective total. You can’t just up rate the main breaker either because the incoming cable will be specced to the 100 amp load.
I’d check the data plate on the item and feed it from a supply it’s rated to and no more unless you’ve got the extra capacity
A 32 amp supply wants fitting with 4mm csa cable from the breaker.

Sorry Richard but that's just not true, look up diversity.

Essentially you only have to allow a portion of the rated current of each circuit.

 

[WorkshopChris]

You can perhaps fit an adaptable box close to the compressor (might even be able to be mounted on the compressor itself?) with a smaller fuse or breaker in if you are concerned about the wiring, as long as it’s clearly marked I can’t see it being a big problem.

That is also what i would do or fit a contractor starter and overload between the supply and compressor motor.
The toolstation ones are good value.
https://www.toolstation.com/shop/Electrical/d190/Motor+Control/sd3080/DOL+Starter/p56285
https://www.toolstation.com/shop/Electrical/d190/Motor+Control/sd3080/Thermal+Overload+Relay/p21443

 

[daleyd]

That is also what i would do or fit a contractor starter and overload between the supply and compressor motor.
The toolstation ones are good value.
https://www.toolstation.com/shop/Electrical/d190/Motor+Control/sd3080/DOL+Starter/p56285
https://www.toolstation.com/shop/Electrical/d190/Motor+Control/sd3080/Thermal+Overload+Relay/p21443

Yeah that’d do nicely

 

[Gareth J]

Thanks, some great ideas. And I'm sure I DO need to protect the wiring/compressor from the bigger amps, the 32A cabling can provide.

Had one thought which should be legit and is convenient; I have a spare single trip/breaker box here that I could hardwire into the 32A supply (or just put a 32a commando on to supply it) and run the comp. from that.

Compressor and it's wiring are protected and I can use the supply I intend to... Right?

 

[brightspark]

fit the lower breaker box on comp and run it from a 4.0mm cable to the 32 amp commando plug is 1 option/ easiest

 

[dobbslc]

That would work as long as the cables are restrained properly and it's not getting a good bashing.
Brightspark got in while I was typing!

 

[Gareth J]

fit the lower breaker box on comp and run it from a 4.0mm cable to the 32 amp commando plug is 1 option/ easiest

That would work as long as the cables are restrained properly and it's not getting a good bashing.
Brightspark got in while I was typing!

Yep, sounds perfect, it's safely out of harm's way and can be though of as a permanent fixture.

Suitable breaker size? In the days of fusewire, I would have though 15A ought to do it.

 

[brightspark]

it may have to be a 16 amp c type breaker if its starting a compressor up a 3 hp motor runs at 12 amp full load but starting current will be about 50 amps for a couple of seconds