Oven and hob

tmccar

I have just done some kitchen remodelling and one item I have changed is an old gas cooker which have replaced with a built-in oven and worktop electric hob.
Do these need to have their own circuit? And do I need to enlist an electrician to connect them?

aido79

tmccar wrote: »

I have just done some kitchen remodelling and one item I have changed is an old gas cooker which have replaced with a built-in oven and worktop electric hob.
Do these need to have their own circuit? And do I need to enlist an electrician to connect them?

Are they separate units? If so you should put them on seperate dedicated circuits with the correctly rated cable and breakers.
And yes you will need to hire a registered electrical contractor.

tmccar

aido79 wrote: »

Are they separate units? If so you should put them on seperate dedicated circuits with the correctly rated cable and breakers.
And yes you will need to hire a registered electrical contractor.

Yes, 2 seperate units

aido79

tmccar wrote: »

Yes, 2 seperate units

Ok. The next step is to get in contact with an electrician or 2 and get some quotes.

Jack the Stripper

My hob and oven are on one switch is this dangerous?

aido79

Jack the Stripper wrote: »

My hob and oven are on one switch is this dangerous?

The average oven will draw around 10 amps max(based on 2400 watts). I'm going to assume the oven and hob are on a 32 amp breaker?
If so then in the event of a fault the oven is unlikely to trip the breaker. Even if it does there is a greater chance of it going on fire first which is why I think they should be on separate circuits.

Jack the Stripper

It's a ticking bomb so?

dodzy

Jack the Stripper wrote: »

My hob and oven are on one switch is this dangerous?

You, me and hundreds of thousands of homes around the country. Don't worry.

Risteard81

aido79 wrote: »

Jack the Stripper wrote: »

My hob and oven are on one switch is this dangerous?

The average oven will draw around 10 amps max(based on 2400 watts). I'm going to assume the oven and hob are on a 32 amp breaker?
If so then in the event of a fault the oven is unlikely to trip the breaker. Even if it does there is a greater chance of it going on fire first which is why I think they should be on separate circuits.

I can't agree with this. If there is a short circuit or an earth fault then during initial verification we should have verified that sufficient current will flow to cause the protective device to operate within the required time, and we should also verify that the conductors will withstand the fault current until disconnection has occured without damage to the cable.

As fixed loads the oven and hob cannot overload. We will have used discrimination (selectivity) to ensure nuisance tripping doesn't occur (e.g. up to 15kW of connected load on a standard cooker circuit). So there simply isn't any danger.

aido79

Risteard81 wrote: »

I can't agree with this. If there is a short circuit or an earth fault then during initial verification we should have verified that sufficient current will flow to cause the protective device to operate within the required time, and we should also verify that the conductors will withstand the fault current until disconnection has occured without damage to the cable.

As fixed loads the oven and hob cannot overload. We will have used discrimination (selectivity) to ensure nuisance tripping doesn't occur (e.g. up to 15kW of connected load on a standard cooker circuit). So there simply isn't any danger.

Feel free to correct me if I'm wrong(I don't do domestic work) but don't most ovens these days come with a 3 pin plug? If so then I don't think it's a good idea to connect them to a 32 amp breaker with no RCD protection. It's irrelevant that the hob is on the same circuit because a lot of the time the oven will be used by itself.
Although I suppose if the 3 pin plug is fused then it will blow before the 32 amp breaker trips.
Also they are only fixed loads when working correctly. If a fault occurs this may change.

Risteard81

aido79 wrote: »

Also they are only fixed loads when working correctly. If a fault occurs this may change.

Not really. It's why it is permissible to omit overload protection for a fixed load which cannot overload. Think of busbar chambers for example. If there is a flex which requires a BS 1362 fuse for fault protection then a fused connection unit should be fitted after the cooker switch, perhaps alongside the cooker connection unit. But generally there shouldn't be any issue ensuring disconnection within the thermal constraints of the cable.

An example of this is an unfused spur off a ring final circuit. In this instance the cable cannot support the 32A rating of the circuit, however a single or double socket outlet will not allow excess current to be drawn so only short circuit and earth fault protection is required. IMPORTANT NOTE: Unfused spurs on a ring final circuit are NOT currently permitted by ET101, however they are permitted by BS7671 for the reasons mentioned.

aido79

Risteard81 wrote: »

Not really. It's why it is permissible to omit overload protection for a fixed load which cannot overload. Think of busbar chambers for example. If there is a flex which requires a BS 1362 fuse for fault protection then a fused connection unit should be fitted after the cooker switch, perhaps alongside the cooker connection unit. But generally there shouldn't be any issue ensuring disconnection within the thermal constraints of the cable.

An example of this is an unfused spur off a ring final circuit. In this instance the cable cannot support the 32A rating of the circuit, however a single or double socket outlet will not allow excess current to be drawn so only short circuit and earth fault protection is required. IMPORTANT NOTE: Unfused spurs on a ring final circuit are NOT currently permitted by ET101, however they are permitted by BS7671 for the reasons mentioned.

Fair enough.

It's just my opinion that an oven would be better off on it's own 15 amp circuit/socket. I know it's not the done thing in Ireland though.

2011

A few points according to ET101:2008 that are worth noting when it comes to overload and short circuit protection of cables / conductors:

131.4
Persons and livestock shall be protected against injury, and property shall be protected against damage, due to excessive temperatures or electromechanical stress caused by overcurrents likley to flow in live conductors.

This protection is achieved by one of the following means:

i) automatic disconnection on the occurrence of an overcurrent before it attains a dangerous value taking into account its duration.

ii) limiting the maximum overcurrent to a safe value and duration.

430.2 General Requirements

Protective devices shall be provided to break any overcurrent, whether short-circuit or overload, flowing in the circuit conductors before such a current could cause a danger due to the thermal or magnetic effects detrimental to insulation, joints, terminations or surroundings of the conductors.

So this tells us that overcurrent protection must be provided for all circuits.
Simply put, an overcurrent in a cable is a current that exceeds the current carrying capacity of the cable.

2011

Another point to note is 533.2.1 of ET101:2008 (page 105).

This essentially tells us that the current for which a circuit is designed must be lower than the rating (or current setting) of the protective device which must in turn be lower than the current carrying capacity of the cable in question. In other words a 2.5 mm sq. cable can not "be protected" by a 32A MCB.

2011

Back to the OP's question:

555.2.1
Where an oven and hob are separate appliances, a common supply and isolator may be used provided:

- each appliance is within 2m of the switching device, and

- the fault-loop impedance value for each is sufficiently low to ensure the correct operation of the overcurrent protective device in accordance with 411.3 and Chapter 61.

So one cooker circuit may be sufficient for both units. Generally a 6mm sq. cable with at least a 32A MCB would be the absolute minimum required.

As already stated a circuit for each appliance would be best and you will need a REC to install new circuits. Many modern hobs such as induction hobs are best on a dedicated circuit as they can be relitively large loads.

Risteard81

2011 wrote: »

A few points according to ET101:2008 that are worth noting when it comes to overload and short circuit protection of cables / conductors:

131.4
Persons and livestock shall be protected against injury, and property shall be protected against damage, due to excessive temperatures or electromechanical stress caused by overcurrents likley to flow in live conductors.

This protection is achieved by one of the following means:

i) automatic disconnection on the occurrence of an overcurrent before it attains a dangerous value taking into account its duration.

ii) limiting the maximum overcurrent to a safe value and duration.

 

430.2 General Requirements

Protective devices shall be provided to break any overcurrent, whether short-circuit or overload, flowing in the circuit conductors before such a current could cause a danger due to the thermal or magnetic effects detrimental to insulation, joints, terminations or surroundings of the conductors.

So this tells us that overcurrent protection must be provided for all circuits.
Simply put, an overcurrent in a cable is a current that exceeds the current carrying capacity of the cable.

It should also be read alongside 433.3 - Omission of devices for protection against overload.

Tuco88

If a independent circuit cant be given.

So what is the possible solutions to take?

1)Supply a Switched Fused spur outlet in 6mm² from the Cooker circuit and hard wire the ovento the spur (Fixed appliance)

2)Spur to non standard t-bar style socket Or Spur to SRCD not having to remove appliance plug.

3)Or place cooker circuit on RCD and spur Socket so on...

Jack the Stripper

Tuco88 wrote: »

If a independent circuit cant be given.

So what is the possible solutions to take?

1)Supply a Switched Fused spur outlet in 6mm² from the Cooker circuit and hard wire the ovento the spur (Fixed appliance)

2)Spur to non standard t-bar style socket Or Spur to SRCD not having to remove appliance plug.

3)Or place cooker circuit on RCD and spur Socket so on...

I'll ring my electrician tomorrow and tell you what he says.

2011

Risteard81 wrote: »

It should also be read alongside 433.3 - Omission of devices for protection against overload.

Interesting, I didn't spot that to be honest.
For the past few years ET101 is not applicable to the projects I am on.
Anyway it is a little "woolly" in my opinion as it states that this can only be applied "...provided the location is not an area of fire risk..".
Some may consider a hob with a chip pan on it a fire risk.

On the other hand when installing Variable Frequency Drives only short circuit protection is provided on the cable from the MCC busbar to the drive (no overload protection). This is often achieved by using HRC fuses.
The VFD itself provides overload protection to the final motor circuit.

Tuco88

Correct, If I remember there is a parameter to set the max current. Usually it's set to the F.L.C for the motor, it's been a while so I am not 100%.