2 X Ovens on 4mm T&E

[Kuffs]

Hi All,

I have 1 X Fan oven 3.3Kw and an oven/microwave combo 3Kw. I had already installed a 4mm cable, as originally, there was only going to be one oven. After doing the calcs and looking at diversity, it seems to me that I'm safe to go with both ovens on this cable.

Total Load: 6.3Kw
Cable Rating: 37A
Reference Method: C (Cable is clipped direct to the ground floor joists above a concrete pad with plenty of air circulation. It is then clipped to a plywood wall as it enters the CU)
Load after diversity applied: 15.27A
Cable run to CU is 5M so voltage drop is negligible.

I welcome any constructive comments. Thank You.

 

[Dustydazzler]

all good to me , I regularly put 4mm on a 32a mcb for kitchen goods

 

[DefyG]

As you say you've done the calcs and happy with them so good to go!

 

[Kuffs]

Thanks Guys, Although I'm Part P, I am not practicing, other than my own house refurb, so it's just a little reassurance I needed. Thank you for your responses.

 

[Dustydazzler]

I’m still practicing, been practicing my whole life and still no good at it ...


bomtish

 

[sxdave]

all good to me , I regularly put 4mm on a 32a mcb for kitchen goods

Hello, I have a question about your use of 4mm on a 32 amp MCB. My reading of table 4D5 in wiring regs is that the only time 4mm T&E is OK for more than 32 Amps is with ref method C so all other fixing methods with 4mm T&E would be with an MCB rated at less than 32 amps - is this correct? Of course I am open to correction in my thinking on this.

 

[Strima]

Hello, I have a question about your use of 4mm on a 32 amp MCB. My reading of table 4D5 in wiring regs is that the only time 4mm T&E is OK for more than 32 Amps is with ref method C so all other fixing methods with 4mm T&E would be with an MCB rated at less than 32 amps - is this correct? Of course I am open to correction in my thinking on this.

You're correct. The vast majority of domestic installations are Ref Method C so no problem.

If you can guarantee a fixed load is below the CCC of the cable then you could install it on a 32A OCPD. For example you have a heater with a fixed load of 26A and the installation method is Ref A (conduit in an insulated wall) then you could use a 32A OCPD as there's no risk of overload on the circuit. This wouldn't apply to loads such as cookers/sockets as they're not a fixed load. This is also why we can use 2.5mm for a spur for a socket-outlet from a 32A RFC as the load technically will not exceed 20A so no risk of overloading the cable.

 

[telectrix]

i would class a cooker as a fixed load as it can't draw more than it's rated current. even though the actual load varies according to use.

 

[pc1966]

i would class a cooker as a fixed load as it can't draw more than it's rated current. even though the actual load varies according to use.

It is a fixed load, but that is the max rating. Once you get a big drop in operating current due to diversity you can't really treat it as fixed at the 10A + 30% sort of rating.

 

[telectrix]

It is a fixed load, but that is the max rating. Once you get a big drop in operating current due to diversity you can't really treat it as fixed at the 10A + 30% sort of rating.

agree, but my point is that it cannot overload the circuit in normal operation (assuming that said circuit is designed accordingly). over current can only be due to a fault, in which case, only fault protection is required. tin hat firmly in place.

 

[sxdave]

You're correct. The vast majority of domestic installations are Ref Method C so no problem.

If you can guarantee a fixed load is below the CCC of the cable then you could install it on a 32A OCPD. For example you have a heater with a fixed load of 26A and the installation method is Ref A (conduit in an insulated wall) then you could use a 32A OCPD as there's no risk of overload on the circuit. This wouldn't apply to loads such as cookers/sockets as they're not a fixed load. This is also why we can use 2.5mm for a spur for a socket-outlet from a 32A RFC as the load technically will not exceed 20A so no risk of overloading the cable.

Thanks for your reply. I am uncertain about something though. I am assuming you are still considering 4mm cable for your example. So my concern is with regard to 433.1.1 in the regs. I see the practical nature of your example but think it might not meet the letter of that reg with regard to In (32 amps) < Iz (26 amps for method A).

 

[Strima]

Thanks for your reply. I am uncertain about something though. I am assuming you are still considering 4mm cable for your example. So my concern is with regard to 433.1.1 in the regs. I see the practical nature of your example but think it might not meet the letter of that reg with regard to In (32 amps) < Iz (26 amps for method A).

Take a look at 433.3.1 (ii).

The problem with the regs is you may find the answer you're looking for but you may not be looking for the answer that negates the answer you find.

 

[sxdave]

Take a look at 433.3.1 (ii).

The problem with the regs is you may find the answer you're looking for but you may not be looking for the answer that negates the answer you find.

Thanks, as you say 433.3.1 (ii) makes this example acceptable.