3-phase cooker circuit installation

[hightower]

Morning all,

Been asked by a mate to drop a cooker circuit in to a commercial unit. Not having dealt with 3-phase cookers before, thought it best to post my thoughts up here as a sanity check before I proceed.

Cooker is L1 = 26A, L2 = 26A, L3 = 14A. So In = 32A Type B 3-p MCB. Need to do cable calcs properly but I'm thinking it's likely to be 4mm in PVC conduit (not asking for help with this, I'll do it some time this morning).

Anyway, this is the part I need advice on. I'm thinking wired from board in to one of these near the cooker:

32 Amp TP&N Rotary Switch Insulated Weatherproof - IP65 - https://www.tlc-direct.co.uk/Products/CMDX032.html

Then in to one of these:

415v 32 Amp 5 Pin MK Commando Socket - Red - IP44 - https://www.tlc-direct.co.uk/Products/MKK9241RED.html

Then fit a cable to the cooker with one of these on:

415v 32 Amp 5 Pin MK Commando Plug - Red - IP44 - https://www.tlc-direct.co.uk/Products/MKK9045RED.html

Is that the standard way to do these types of cookers or is there another preferred method? The cooker is brand new, however, it has a cable coming out the back (looks like 2.5mm) that has been cut off. I'm thinking it should have had a plug on it and it's been cut off, so I'll just refix a length of 5-core cable.

That sound ok?

 

[telectrix]

sounds fine, but you don't really need the rotary isolator. the commando plug is a means of local isolation if it's accessible once the cooker is in position.

 

[hightower]

sounds fine, but you don't really need the rotary isolator. the commando plug is a means of local isolation if it's accessible once the cooker is in position.

I know the MCB is too, but just thought local rotary isolator would be a better install given the level of competence the user will have - a big red switch saying COOKER, can't get much more obvious than that.

Oooo, this is a socket though, need to check regs but think that then calls for RCD protection. If that's the case, can I do away with 3-p socket/plug and wire straight in to rotary isolator, or would you think it's best to keep the cooker on the end of an unpluggable.

EDIT: 411.3.3 (i) socket outlets not exceeding 20A, thought it was 32A off top of head but obviously getting mixed up with (ii), so no RCD protection required. Would it be alright going straight to isolator still though, gonna save my mate £20-30 in materials if I can.

 

[telectrix]

i'd prefer the commando plug/socket without isolator . as you say, RCD protection is not required.

 

[hightower]

Also, from what I can see in the OSG, there's no reduction allowed at the point of utilisation for business cooking appliances, that only applies to household cooking appliances, so for cable calculations I need to take this at full whack (26A per phase)

 

[telectrix]

also check 433.3.1. (ii) regarding fixed load. ( reg. from green book, might be different in whatever colour book is current ).

 

[hightower]

Hmmm, 4mm can only carry 28A in a 3-p circuit, looks like 6mm may be required.

 

[Rpa07]

You’re not alone Tel, my favourite colour is Green but, I guess, next it’ll be blue - just like me!
Good thread @hightower following with interest for my 1st 3phase cooker install!

 

[hightower]

Right, cable calcs for confirmation - it's going in PVC conduit with no other circuits, so ref method B, Cg = 0, Ca = 0, Ci = 0, and Cf = 0 (just going off OSG appendix for calc), so It = In / 0 = 32A.

Table F4(i) says 4mm is 28A, 6mm is 36A.

6mm VD checks out at around 3v so that's good.

5 x 6mm cables will fit in 20mm conduit according to the OSG so that's good.

Does this all look okay, or have I made any glaring errors. I'm shocked as at a guess I'd have said 4mm would have been okay, but then I haven't really done 3-p stuff and the CCCs are a little different.

If that's all good and 6mm is the answer, why the hell has the cooker got 2.5mm cable cut off the back of it? I'll buy a length of 6mm H07 too to reterminate the cooker with.

 

[The Ghost]

I did a 3 phase cooker install and used a rotary isolator. Although the load worked out at technically 17.3a per phase, the manufacturers recommended 4mm² as opposed to the minimum 1.5mm² I could have used. I don't know if it may be wise to check that or not. I was thinking as 1.5mm² was just about adequate, I would go to 2.5mm², but then decided to use 4mm² as per manufacturers spec. They offered a service of installing the cooker to the customer and said they never use anything less.

 

[hightower]

The cooker instructions say nothing about cable sizes or anything remotely useful for installation.

 

[davesparks]

Out of interest what has made you choose PVC conduit?
Personally I wouldn’t have thought it was the best choice for the environment and would favour steel.
With the large variations in temperature in a commercial kitchen I can imagine the expansion and contraction of the conduit being a problem, plus steel gives far better mechanical protection.

 

[hightower]

Out of interest what has made you choose PVC conduit?
Personally I wouldn’t have thought it was the best choice for the environment and would favour steel.
With the large variations in temperature in a commercial kitchen I can imagine the expansion and contraction of the conduit being a problem, plus steel gives far better mechanical protection.

PVC mainly for cost. It's not a commercial kitchen (as in a deli with loads of cooking equipment on the go), it's a cooker in a commercial unit. It's a vast open space so I can't see there being much variation in temperature.

 

[telectrix]

would SWA look too out of place?

 

[davesparks]

PVC mainly for cost. It's not a commercial kitchen (as in a deli with loads of cooking equipment on the go), it's a cooker in a commercial unit. It's a vast open space so I can't see there being much variation in temperature.

Well if the important factor in the design is cheapness then why not just chuck in a bit of flex in sticky back trunking?

 

[hightower]

would SWA look too out of place?

Yeah, white walls, black SWA, wouldn't be the prettiest.

Well if the important factor in the design is cheapness then why not just chuck in a bit of flex in sticky back trunking?

Wind it in a bit mate, you asked me a question with regards to a certain issue you highlighted. I responded saying that heat fluctuation isn't going to be an issue. Tell you what mate, I'll just start lashing in 25mm armour everywhere shall I, since anybody that wants to save a quid or two must be cutting corners.

 

[gazdkw82]

I've worked in many commercial kitchen environments. Rotary isolators are always used for cookers/steamers within this environment.

 

[suffolkspark]

Id stick a 25mm or 32mm gland out the bottom of the rotary isolator if you need to keep the price down a socket arrangement being forward thinking may be nice but If its brand new cooker the likelyhood of any imminent reason to unplug itnis probably small

 

[SparkyChick]

@hightower ,

When I did my first 3 phase job, I had a chat with my wholesaler regarding sockets and isolators. They had some nice looking combined units that included mechanical interlock so they couldn't be turned on unless the socket was properly connected.

If memory serves, they worked out slightly cheaper than a separate isolator and socket, and if you felt the need to go all the way and provide RCD protection, they had add on sections to accomodate a 3 phase RCD.

Just a thought

 

[telectrix]

hightower has gone quiet. customer has just informed him that it's a gas cooker.