# Interest group forums > Electrical Contracting Industry Forum >  Cable sizing for a stove.

## @SamboLindoh

Hi All,

Kindly see the 3 attached document, what are your thought?

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## AndyD

They tell you what cable is suitable although my personal preference would be H07RNF which is the uprated neoprene version. I don't see any reason not to follow their recommended size.

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## Leecatt

> They tell you what cable is suitable although my personal preference would be H07RNF which is the uprated neoprene version. I don't see any reason not to follow their recommended size.


As long as the cable is protected correctly with a 20 amp circuit breaker and not the recommended 25amp

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## @SamboLindoh

> They tell you what cable is suitable although my personal preference would be H07RNF which is the uprated neoprene version. I don't see any reason not to follow their recommended size.


Hi AndyD, thank you for the reply. Did you view the PDF document attached together with these 2 pictures? I would like to hear your response and view on my question and analysis respectively.

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## @SamboLindoh

> As long as the cable is protected correctly with a 20 amp circuit breaker and not the recommended 25amp


Hi Leecatt, 

Thank you for your input on the subject matter. I assume your response is based on the burners alone, the burners when they are all operated to maximum at the same time they can draw a maximum current of 25 A. In your recommendation you recommend a 2,5mm2 Cable and a 20A C/B wouldn't this breaker trip when all the burners are turned on to maximum?

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## Leecatt

> Hi Leecatt, 
> 
> Thank you for your input on the subject matter. I assume your response is based on the burners alone, the burners when they are all operated to maximum at the same time they can draw a maximum current of 25 A. In your recommendation you recommend a 2,5mm2 Cable and a 20A C/B wouldn't this breaker trip when all the burners are turned on to maximum?


I am not taking the load into account at all.
My concern is simply for the amperage in the cable. Protecting a 2.5mm cable with a 25 amp circuit breaker is incorrect. The maximum rating in the cable is 23 amps, therefore the recommendation to use a 20 amp circuit breaker to protect it. 

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## AndyD

These types of trailing cables are fine stranded conductors plus the CPC is the same CSA as the line conductors and the insulation has a higher temperature rating than standard PVC  so they carry a relatively higher current rating than PVC installation cables with solid conductors like flat twin. 2.5mm 3-core H07 for example is usually rated at 25Amps. Data sheet attached below, see page 4.

I'd personally follow the manufacturers recommendations regarding cable size and overcurrent protection. 

h07rn-f-bs-en-50525-2-21-flexible-rubber-cable.pdf

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## AndyD

The load of the stove is limited to 3.4 kw (about 15A depending on the actual voltage) at any given time. There will be a mechanism inside the stove to prevent all the elements running simultaneously.

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## @SamboLindoh

> I am not taking the load into account at all.
> My concern is simply for the amperage in the cable. Protecting a 2.5mm cable with a 25 amp circuit breaker is incorrect. The maximum rating in the cable is 23 amps, therefore the recommendation to use a 20 amp circuit breaker to protect it. 
> 
> Sent from my SM-G570F using Tapatalk


Hi Leecatt,

In my opinion I wouldn't say using a 25A circuit breaker for a 2.5mm2 cable is incorrect.  The maximum protection rating for a 2.5mm2 is indeed 25A. The maximum amount of current that a cable can handle depends on various factors such as the method of installation (Pg 89), the length of the cable, the type of cable (i.e single core,multicore, PVC, armoured, unarmoured, copper,aluminum, etc) see. table 6.2(a), table 6.3(a) and table 6.4(a)....SANS 10142-1

Please see the attached pdf document and go through the analysis of the scenario there, I would appreciate your comment on the analysis there.

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## Leecatt

You are correct the max rating depends upon the installation method. 
According to your diagram the cable is a 2.5mm twin core and earth and is to be installed into conduit on a wall. 
This means I have to refer to table 6.2(a), installation method 2.
The maximum current is limited to 24 amps. 
In my world this requires a 20 amp circuit breaker for protection not a a 25 amp. 




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## Leecatt

Using this same procedure the max amperage for a 6mm cable is 41 amps. 

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## Leecatt

I've been looking at the wrong table. Using table 6.3(a) the max current for the 2.5mm cable is 23 amps using installation method 2. Same solution though, needs to be protected by a 20 amp cb.


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## Leecatt

The maximum for 6mm is 3 8 amps unless you surface mount the cable 

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## @SamboLindoh

> You are correct the max rating depends upon the installation method. 
> According to your diagram the cable is a 2.5mm twin core and earth and is to be installed into conduit on a wall. 
> This means I have to refer to table 6.2(a), installation method 2.
> The maximum current is limited to 24 amps. 
> In my world this requires a 20 amp circuit breaker for protection not a a 25 amp. 
> 
> 
> 
> 
> Sent from my SM-G570F using Tapatalk


In this case you are 100% correct in selecting the 20A breaker. I thought you were saying it is incorrect (under any conditions) to use a breaker that is more than 20A on a 2.5mm2 cable, because in other methods  a 25A C/B can then be used to protect a 2.5mm2 cable.

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## @SamboLindoh

Let me clarify my problem here:

I've got burners and an oven.
Burners rated current=25A
Oven rated current= 15A
Total rated current=40A

I wanna use 1 cable (Multicore PVC Insulated cable, unarmoured, one two-core cable, single phase) to supply power to both the oven and the burners.

Lenght of cable from DB to isolator is 6meters and from the isolator to the burners and oven is less than a meter 

Installation method to be used is Method 2.

When I select a cable for this installation I find that a suitable cable for this combined load is a 10mm2 protected by a 45A C/B. Now, i've never seen a 10 mm2 used for a stove before.  Am I right with this cable size for a stove? Am I right in using the combined maximum rated current (40A) in sizing this cable? Where am I missing it?

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## Leecatt

> Let me clarify my problem here:
> 
> I've got burners and an oven.
> Burners rated current=25A
> Oven rated current= 15A
> Total rated current=40A
> 
> I wanna use 1 cable (Multicore PVC Insulated cable, unarmoured, one two-core cable, single phase) to supply power to both the oven and the burners.
> 
> ...


This is a good exercise as it has made me revisit those darned confusing tables. 
Now I see them in a second light and it's all fallen into place.

If you use single core cables inside of conduit instead of a multicore cable, as you said you were going to install conduit, then your rating changes. 
The rating for a 6mm single core cable, installation method two, becomes 41 amps. Table 6.2(a)

Normally the conduit and Flexi conduit will run from the distribution board into the oven and the rest of it is joined together with internal cables which usually have those heat-resistant cables and plugs. 

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## AndyD

Maybe I missed something here, from the opening post I thought this was a discussion around the trailing cable recommendation in the manufacturers instructions, I didn't realize you're asking about installing the entire circuit from the DB.

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## @SamboLindoh

Hi Leecatt,

Thank you for your insight  :Thumbup:

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## @SamboLindoh

Hi AndyD,

No problem, thanks for your participation.

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## Canuck

Just an observation and a subsequent question.  In Canada we use the wattage information for load calculations when it is on the nameplate data.  Also one of the things happens a lot here is manufacturers of the tops and oven are different and the two are not designed to be connected to the same circuit as there is no approved overload devices between the units to prevent the top from drawing too much current from a breaker that would be to large for the top itself.  The question is are you allowed to load a breaker and conductor to 100% in a residential application?  Here the breaker is not allowed to loaded to more the 80% in residential but the conductor can be loaded to 100%.  In commercial the breaker and conductor must be sized to 125%  Do you do anything like that?

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## AndyD

> Just an observation and a subsequent question.  In Canada we use the wattage information for load calculations when it is on the nameplate data.  Also one of the things happens a lot here is manufacturers of the tops and oven are different and the two are not designed to be connected to the same circuit as there is no approved overload devices between the units to prevent the top from drawing too much current from a breaker that would be to large for the top itself.


 The circuit breaker is sized to protect the cabling of the electrical installation from overload, not to protect the actual appliances that are plugged in or connected. If an appliance requires its own overload protection it must be built into the appliance or designed into the appliance. For example many stoves have an internal wiring arrangement that prevents all the hob elements and the oven running simultaneously. 




> The question is are you allowed to load a breaker and conductor to 100% in a residential application?  Here the breaker is not allowed to loaded to more the 80% in residential but the conductor can be loaded to 100%.  In commercial the breaker and conductor must be sized to 125%  Do you do anything like that?


 Yes, you can load a circuit to 100% of its capacity. There's sufficient safety allowances in the reference tables in the regs that dictate a circuits maximum load capacity so no additional blanket derating is required.

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Dave A (16-Nov-18)

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## Canuck

Thanks for the reply Andy.  Regarding the stove and top comment I probably was not clear enough.  What seems to happen here on occasion is an older wall oven and separate cook top have been installed previously using a 40A feed.  The oven has all of its own protective fusing in its own junction box.  In these older arrangements there was also a sub feed from the oven box to the cooktop box of smaller conductors based on its draw.  The cooktop also has its own fusing in a junction box.  The conductors in between the units are not rated for the 40A breaker that is feeding them as well as the stove.  These cooktop conductors are usually within 3 meters of each and are classed as tap conductors.  When the units are replaced the homeowner buys two separate units that are now of a much higher rating (often 40A and 30A, sometimes more) that require their own individual feeds from the panel.  The owner gets upset because "the old ones were fine like that so just hook them up the same".  In this case to do it correctly they must have their own individual feeds as the existing conductors from the panel only have a 40A rating whereas the new loading would be well over that. 

Regarding circuit loading, are you saying then that in a commercial application a known load of lets say 30A would only require 30A conductors and a 30A overcurrent device?  (Here both would have to be sized at 125% of rated current of the load)  In a residential application however there is are no demand factors and the circuit can be sized to 100% of rated current.

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## AndyD

> ...........Regarding circuit loading, are you saying then that in a commercial application a known load of lets say 30A would only require 30A conductors and a 30A overcurrent device?  (Here both would have to be sized at 125% of rated current of the load)  In a residential application however there is are no demand factors and the circuit can be sized to 100% of rated current.


 There's no electrical safety reason you couldn't supply a 30A load using a cable that's rated at 30 A and an MCB rated 30A, (Ib ≤ In ≤ Iz). 

From memory there's a reg that states a circuit should be designed in such a way to avoid unnecessary nuisance tripping so it could be argued that if Ib = In = Iz you may be in breach of this reg if tripping occurs on days where ambient temperatures are high (making the thermal tripping curve of the MCB more sensitive) for example. Also a highly inductive or capacitive load or a load with a high inrush current or very poor power factor might be a consideration that would encourage me to upsize the circuit.

A manufacturer will state the appliance current in the perfect storm worst case scenario ie with the highest permissible supply voltage, under absolute maximum load and even then they err on the side of caution. In real life I've never taken a current reading on an appliance and found it to be more than 95% of the manufacturer stated current. If the appliance was a fairly linear load with no notable inrush or start current and was manufacturer rated at max 30A I wouldn't have an issue installing it on its own 30A circuit as long as the ambient temperatures weren't going to be above average. That would hold for both domestic and commercial applications.

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## ians

> Yes, you can load a circuit to 100% of its capacity.


You can load a circuit to way more than 100 % of its capacity...if correctly designed for the application...an old aircon unit is a good example...using a D or C1 curve breaker you can load the circuit more than 10 times its capacity for a short period of time...new aircon no load require this type of overload protection...in fact we have had to replace these type of breakers due to nuisance buzzing for some reason (i havent done any research into  this issue yet) ...this is why it is important for the person supplying the breaker to understand the application...we have also had to use type 2 breakers for electro magnets for holding down tools in CNC machines...due to the type of inrush current.

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## Canuck

Thanks for the discussion.  Obviously there are some differences in our codes which of course is what determines what we can and cannot do in each country.  I think the reasoning for the demand factors applied here are that in a residential type occupancy the actual loading on average is less and the duration of loading is also less then what may be in a commercial application.  At any rate it is interesting to view the comments and questions on the forum.  Hats off to you though for sure in the area of safety.  Your requirements of leakage protection has been in place for a long time and over here they are only starting to enforce such things like arc fault protection on all receptacle circuits of 15 and 20A configurations.  The leakage type (which we call ground fault circuit interrupters) has been a requirement around washbasins and outside for quite some time.  With the new arc fault requirement as well we often install beakers that are AFCI and GFCI combined for use outdoors and the AFCI inside.  This requirement only applies to receptacles not lighting.

ians: Like in your examples, we also have situations where the breaker size is larger then the current handing of the conductors.  Usually in motor or compressor situations where the counter EMF does not rise fast enough to prevent overcurrent from tripping the breaker.  Our rules allow the rating of the overcurrent device to be increased up to 400% in order to allow the motor to start.  This of course does not mean that you would install a breaker at 400% of FLA but rather that it the breaker can be increased up to 400% to allow the motor to start.  In most case the over load devices can be set at 115% or 125% of full load amps depending on the service factor of the motor.  Is "aircon" short for airconditioner?  What is a type 2 breaker?

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AndyD (21-Nov-18)

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## ians

> Thanks for the discussion.  Obviously there are some differences in our codes which of course is what determines what we can and cannot do in each country.  I think the reasoning for the demand factors applied here are that in a residential type occupancy the actual loading on average is less and the duration of loading is also less then what may be in a commercial application.  At any rate it is interesting to view the comments and questions on the forum.  Hats off to you though for sure in the area of safety.  Your requirements of leakage protection has been in place for a long time and over here they are only starting to enforce such things like arc fault protection on all receptacle circuits of 15 and 20A configurations.  The leakage type (which we call ground fault circuit interrupters) has been a requirement around washbasins and outside for quite some time.  With the new arc fault requirement as well we often install beakers that are AFCI and GFCI combined for use outdoors and the AFCI inside.  This requirement only applies to receptacles not lighting.
> 
> ians: Like in your examples, we also have situations where the breaker size is larger then the current handing of the conductors.  Usually in motor or compressor situations where the counter EMF does not rise fast enough to prevent overcurrent from tripping the breaker.  Our rules allow the rating of the overcurrent device to be increased up to 400% in order to allow the motor to start.  This of course does not mean that you would install a breaker at 400% of FLA but rather that it the breaker can be increased up to 400% to allow the motor to start.  In most case the over load devices can be set at 115% or 125% of full load amps depending on the service factor of the motor.  Is "aircon" short for airconditioner?  What is a type 2 breaker?


I didnt say the breaker is rated higher than the current carrying capacity of the cable...you can use a breaker which is rated for the correct cable size...with a slower trip curve...ie curve 1 or curve 2...depending on the application...for example curve 1 motor start up (referred to as in rush current) for "old" type air conditioners pump motors etc...curve 2 for a big transformer...like a magnetic table or cnc machines which have 400-230 3 phase transformers... required to reduce the voltage so they can be connected to 400 volt supplies in SA.

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