Compressor - 32 amp power supply

[Where'sMyBrew?]

Hi

although this question is strictly more to to with electrics rather than air compressors, this forum strikes me as a very helpful place to newbies, so here goes.

When I had my home garage re-wired by a professional electrician, I had separate 16amp and 32amp sockets wired straight from the consumer box. The 16amp now happily supplies my Portamig 210 MIG welder.

I had the 32amp supply installed in anticipation of eventually getting a meaty air compressor. I had no real knowledge of compressors at the time, other than vaguely being aware that a higher-rated supply might be required. A bit of forward-thinking for once in my life

Anyhow, the time for me to get a compressor had arrived.

From reading these forums, it seems that (up to) 20amp should be sufficient for most mid-range DIY compressors.

So to finally get to my question: what, if any, would be the risk of wiring a compressor requiring, for example, 20 amps, into a 30amp supply ?

To mitigate any risk, would it simply be a case of replacing the 32amp MCB with a lower rated one in the consumer unit ?

many thanks,
Gareth.

 

[brightspark]

Hi

although this question is strictly more to to with electrics rather than air compressors, this forum strikes me as a very helpful place to newbies, so here goes.

When I had my home garage re-wired by a professional electrician, I had separate 16amp and 32amp sockets wired straight from the consumer box. The 16amp now happily supplies my Portamig 210 MIG welder.

I had the 32amp supply installed in anticipation of eventually getting a meaty air compressor. I had no real knowledge of compressors at the time, other than vaguely being aware that a higher-rated supply might be required. A bit of forward-thinking for once in my life

Anyhow, the time for me to get a compressor had arrived.

From reading these forums, it seems that (up to) 20amp should be sufficient for most mid-range DIY compressors.

So to finally get to my question: what, if any, would be the risk of wiring a compressor requiring, for example, 20 amps, into a 30amp supply ?

To mitigate any risk, would it simply be a case of replacing the 32amp MCB with a lower rated one in the consumer unit ?

many thanks,
Gareth.

down rate the mcb to a 20 amp from a 30 and put a c type in in stead of a b type as they have a higher starting current charactaristic

 

[pedrobedro]

There is no risk in running a 20 amp machine on a higher rated circuit. It is like running a light bulb on a 13 amp circuit, you are not going to overload the circuit so it is no risk.

 

[3oh6]

Circuit breakers are in place purely to protect the cables in the circuit, so as long as the cable size was correct for the 32A supply, then you should be fine. Although I'd do as brightspark said and replace the 32A breaker with a C20, at least then the cable from the outlet to the compressor would be protected too

 

[madkayaker]

the motor control circuit should have protection incorpurated that will protect the motor from over current. I would leave it as is.

 

[brightspark]

the motor control circuit should have protection incorpurated that will protect the motor from over current. I would leave it as is.

the thermal trips protect against overload but they dont protect against an earh fault so policy to protect the wiring on comp to the correct mcb rating

 

[madkayaker]

if the circuit has passed inspection for 32A then its safe regardless of how small the load is

 

[brightspark]

if the circuit has passed inspection for 32A then its safe regardless of how small the load is

hmmm am i missing the point here i regularly down rate the mcb on cooker supply circuits from 30 amp that only require a 20 amp supply to ovens . the manafacturers guarantee is void if fuse is over rated irespective of what the load is

 

[tigler]

Surely the MCB in the consumer unit is to protect the circuit on that MCB, individual appliance protection should be provided by a suitably rated switched fuse that supplies that appliance, like the fuse in a plug on small appliances.

 

[madkayaker]

the MCB is there to protect the cabling. the appliance (what ever it is) should have its own inbuilt protection

 

[brightspark]

Surely the MCB in the consumer unit is to protect the circuit on that MCB, individual appliance protection should be provided by a suitably rated switched fuse that supplies that appliance, like the fuse in a plug on small appliances.

anything less than 16 amp can be fused down in a 13 amp plug down to 3 amp. if the cable feeding a compressor is wired for a 30 amp supply and it only needs to be 20 amp it can be down rated with a switchfuse on the end of line or the mcb at source changed to a smaller mcb and an isolator fitted at the comp end

 

[tigler]

the MCB is there to protect the cabling. the appliance (what ever it is) should have its own inbuilt protection

Agreed, but to protect something like a compressor wired directly without a fused plug a switched fuse outlet is required with an appropriately rated MCB.

 

[brightspark]

Agreed, but to protect something like a compressor wired directly without a fused plug a switched fuse outlet is required with an appropriately rated MCB.

a switchfused outlet isnt required just an isolator switch with apropriate mcb back at main source if it is a radial supply . it does not require being fused at both ends just isolation within a meter of apliance this is all thats needed for maintenance purposes

 

[madkayaker]

Agreed, but to protect something like a compressor wired directly without a fused plug a switched fuse outlet is required with an appropriately rated MCB.

the compresser will have overload protection all ready on the motor as its bigger than 750W. or it should do unless it some dodgy grey import job

 

[tigler]

What form does that normally take as I've often seen single phase compressors with 3hp motors with no obvious protection ?

 

[madkayaker]

normally in the pressure switch there is a thermal overload or a thermal overload on the motor. with fuses or MCB its mostly imposable to get the protection set correctly for the motor

 

[tigler]

Thermal or overload isn't included in standard pressure switches and many motors do not have thermal protection. Thats why a switch fuse is a good idea, or, for full motor protection a starter switch with an appropriately rated overload cutout included.

 

[brightspark]

Thermal or overload isn't included in standard pressure switches and many motors do not have thermal protection. Thats why a switch fuse is a good idea, or, for full motor protection a starter switch with an appropriately rated overload cutout included.

as i stated a switchfuse isnt needed just an isolator as the compressor is fused at the supply .[radial circuit ]a switchfuse is an isolator and fuse carrier combined . the only time one is needed is when the supply line is fused at say 30 amps and needs reducing on the end of line to a smaller rating . if the fuses at source arent down rated . the only protection the motor needs is a direct on line starter fitted with thermal overloads and wired through pressure switch i hope this clears any confusion

 

[madkayaker]

i checked all (20) 230V compressers today (time for there zuric inspection anyway) and they all have a thermial overload in the motor. the 4 3ph compressers are star delter with overload in the control box.


the only way to offer adequte protection for motors is with a termial overload either in the motor (for small ones) or wired as per the diagran above.

 

[tigler]

Interesting, when you say the motors have thermal protection, what form does it take ?