Replacing Oven, Wiring Question

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corvus

5 posts

Wannabe Geek

#317416 13-Oct-2024 17:33

Hello,

My house has an 11.4kW freestanding oven/stove in it, which I plan to replace with a 12.1kW one. These both fall into the "Assessed Current Load" of 32amp according to the electrical regulations.

Being a house built in the late 60s, it has a 30amp re-wireable fuse, which I planned to have replaced with the a 32amp MCB just for simplicity of a breaker would be easier to reset than a fuse wire. However, it turns out that isn't so simple as the 30amp fuse holder is larger, and electricians will not drill new holes into what is probably an asbestos panel (fair call). So, I could just leave the 30amp fuse, and the 30amp switch for the oven.

But then there is the wiring, which is 4mm². Generally 4mm² is considered fine for 32amp loads, although there seems to be a few electricians websites that don't agree with that, even though technically the calculations will tell you that it's fine for up to 39amps (https://www.jcalc.net/cable-sizing-calculator-as3008).

However, I've had a number of local electricians all insist that it has to be upgraded to 6mm² cable, and I can't work out why, other than that is the recommendation for new wiring runs. I even had some try to tell me the whole house wiring needs to be replaced (sight unseen) just because it was an older house. Of course that it nonsense, and yes all the cabling is at least TPS and appropriately rated.

Is there anything in the code that says that because I am replacing my oven with an almost identical oven, I need to replace the wiring with 6mm² wire? I would just reconnect the new oven with the existing cable out of the wall, but it is too short, and adding the convenience of a plug and socket seemed like an easy job. Now I'm not so sure.

Further to this, I was going through the as/nzs 3008 calculator https://www.jcalc.net/cable-sizing-calculator-as3008, and I think I have worked out why 6mm² is stated. 4mm is fine for 32amp insulated spaces, but not for insulated spaces. The cut off is 31amp, so I should be perfectly fine to keep a 30amp fuse, even if my walls and ceiling were properly insulated (which they aren't - the ceiling is partially, the walls are not).

Cheers,
M

dimsim

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#3297000 13-Oct-2024 18:04

I'd be interested to see what others have to say about this as I have other tradesman insisting on using code compliant products when effecting a repair. For electrical work I can understand this as they need to issue an electrical certificate which I assume is based on current standards. My understanding is that homeowners are allowed to replace "like for like" electrical fittings within the home as long as you don't make any changes to wiring or circuit (e.g add something which could potentially increase current draw)

I have a new oven that I need to fit and while it is pretty much an identical model (same oven wattage and electrical spec) it does have a slightly different electrical plug on it. I'm planning on simply fitting the existing oven plug to my new oven, so I'm curious if that is something that I'm allowed to do.

Recently I've had glaziers wanting to replace standard high wall window glass in my 50's built house, with safety glass quoting building code. While I can understand that for a new door or a repair to a door or something else that someone could possibly fall through, I don't think that's it's reasonable to have to replace a single pane of standard glass in a high up window with safety glass just because that is what the current building code, which new homes are held to says.

Johnk

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#3297007 13-Oct-2024 18:47

Generally 4.0mm cables installed a NZ building are not "fine" for 32a, the 4.0mm cable sold in NZ can generally run 32a for approx 17m if not in direct sunlight, and not touching anything else.

Using the calc from a NZ cable supplier (First flex), taking into account NZ building temps and insulations, 4.0mm is only rated to carry 19a if installed "Thermal insulation, completely surrounded, unenclosed" and 31a if it is installed "Thermal insulation, Partially surrounded, unenclosed"

Even 6.0mm can only carry 25A if installed "Thermal insulation, completely surrounded, unenclosed".

I would look at having the sparky install beside the switchboard a small surface mount MCB enclosure if he is unhappy to drill new holes in the existing DB.

Note, most electricians will recommend a switchboard upgrade if you have fuse holder and or plug in style MCBs. These are incredibly old, offer very little in the way of protection, and will not save your life if something went wrong. Modern MCBs, (not just a new plug in style MCB) have far greater protection for over load, short circuit, fault current protection.

And note to home owners, may pay to double check NZECP 51, I am under the impression homeowners can do some of their own work, but are not legally allowed to connect anything without electrical inspection "Before connection to a power supply, the work is tested and certified by a registered electrical inspector in accordance with regulations made under that section"

Eva888

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#3297011 13-Oct-2024 19:03

dimsim:

I'd be interested to see what others have to say about this as I have other tradesman insisting on using code compliant products when effecting a repair. For electrical work I can understand this as they need to issue an electrical certificate which I assume is based on current standards. My understanding is that homeowners are allowed to replace "like for like" electrical fittings within the home as long as you don't make any changes to wiring or circuit (e.g add something which could potentially increase current draw)

I have a new oven that I need to fit and while it is pretty much an identical model (same oven wattage and electrical spec) it does have a slightly different electrical plug on it. I'm planning on simply fitting the existing oven plug to my new oven, so I'm curious if that is something that I'm allowed to do.

Recently I've had glaziers wanting to replace standard high wall window glass in my 50's built house, with safety glass quoting building code. While I can understand that for a new door or a repair to a door or something else that someone could possibly fall through, I don't think that's it's reasonable to have to replace a single pane of standard glass in a high up window with safety glass just because that is what the current building code, which new homes are held to says.

I’ve had the same words from glaziers when quoted for a bathroom window. Reasoning in case you slip on water and fire yourself through the glass. Even if there is a vanity in front of said window.

I’ve been told there’s a way around this. You can install ordinary glass but you must put a film over the inside.

Frankly though I wouldn’t want high windows with ordinary glass in them. We have them already and I worry in a storm that they will come down on us if hit by flying debris. Yes it’s a bit more expensive to go laminated but peace of mind makes it worth it.

corvus

5 posts

Wannabe Geek

#3297012 13-Oct-2024 19:04

Johnk:

Generally 4.0mm cables installed a NZ building are not "fine" for 32a, the 4.0mm cable sold in NZ can generally run 32a for approx 17m if not in direct sunlight, and not touching anything else.

Using the calc from a NZ cable supplier (First flex), taking into account NZ building temps and insulations, 4.0mm is only rated to carry 19a if installed "Thermal insulation, completely surrounded, unenclosed" and 31a if it is installed "Thermal insulation, Partially surrounded, unenclosed"

Even 6.0mm can only carry 25A if installed "Thermal insulation, completely surrounded, unenclosed".

I would look at having the sparky install beside the switchboard a small surface mount MCB enclosure if he is unhappy to drill new holes in the existing DB.

Note, most electricians will recommend a switchboard upgrade if you have fuse holder and or plug in style MCBs. These are incredibly old, offer very little in the way of protection, and will not save your life if something went wrong. Modern MCBs, (not just a new plug in style MCB) have far greater protection for over load, short circuit, fault current protection.

And note to home owners, may pay to double check NZECP 51, I am under the impression homeowners can do some of their own work, but are not legally allowed to connect anything without electrical inspection "Before connection to a power supply, the work is tested and certified by a registered electrical inspector in accordance with regulations made under that section"

Thanks John, yeah, I found the AS/NZ 3008 calcs and the insulation de-rating information just after posting this, despite hours of looking over stuff previously. It's a real pain that current cut of was 31amps - couldn't have been 32amps and made life easy for everyone. LOL

I believe what you quoted out of NZECP 51 is specific to any new wiring you add, and needs to be checked before connection, and you must not connect cables to your main board (or even feed the cable into the main board). Things like replacing a like for like switch and power point, and connecting/disconnecting a fixed appliance don't require an inspector.

gregmcc

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#3297023 13-Oct-2024 20:11

The issue here would be carrying the liability, it's borderline at best, you may have run it thru the cable calculator but no mention of the length of the run of cable which has a direct bearing on the current carrying capacity.

Who ever does the new oven connection needs to sign an electrical safety certificate, with it been a borderline case it makes sense that the electrician is ensuring the safety and not risking your life, and your house or putting their electrical license on the line.

With an older house the costs start to mount up making things compliant and safe to todays standards.

t92300

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Master Geek

#3297033 13-Oct-2024 20:26

When I did the calculation it gave me 19A as the maximum 4mm could handle. Im guessing you put in touching surface for the installation conditions. See my quote from the standards below

From ASNZS 3000:2007 (What we work to in NZ)

3.4.1 General

Every conductor shall have a current-carrying capacity, in accordance with the AS?NZS 3008.1 series, not less than the current to be carried by the conductor.

In determining the required current-carrying capacity, provision shall be made for reasonably foreseeable changes to external influences, such as the installation of thermal insulation in ceiling spaces and walls.

In New Zealand only, wiring systems in domestic installations shall be installed on the assumption that thermal insulation in ceilings, walls and under floors, if not currently installed, will be installed in the future.

Table C5 from ASNZS 3000:2007

This table has ratings that would generally be used in domestic situations, most cabling would be fused for in thermal insulation completely surrounded

What a cable can be fused at depends on a number of things, including but not limited to length, volt drop, earth resistance.

corvus

5 posts

Wannabe Geek

#3297034 13-Oct-2024 20:45

gregmcc:
The issue here would be carrying the liability, it's borderline at best, you may have run it thru the cable calculator but no mention of the length of the run of cable which has a direct bearing on the current carrying capacity.

Who ever does the new oven connection needs to sign an electrical safety certificate, with it been a borderline case it makes sense that the electrician is ensuring the safety and not risking your life, and your house or putting their electrical license on the line.

With an older house the costs start to mount up making things compliant and safe to todays standards.

Valid point, the wire length is less than 10 metres, which as far as copper in open air goes would mean 2.5mm² wire would carry 32amps, but clearly not to code. 4mm² is to code for open space. The only argument would be if the cable running the ceiling space is in open air (crossing beams above insulation). The subjective consideration would be if the insulation can move and partially enclose it.

Yeah, I perhaps should have just put in a new indoor board in 10 years ago, when it would have been somewhat cheaper

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corvus

5 posts

Wannabe Geek

#3297036 13-Oct-2024 20:56

t92300:
When I did the calculation it gave me 19A as the maximum 4mm could handle. Im guessing you put in touching surface for the installation conditions. See my quote from the standards below

From ASNZS 3000:2007 (What we work to in NZ)

3.4.1 General

Every conductor shall have a current-carrying capacity, in accordance with the AS?NZS 3008.1 series, not less than the current to be carried by the conductor.

In determining the required current-carrying capacity, provision shall be made for reasonably foreseeable changes to external influences, such as the installation of thermal insulation in ceiling spaces and walls.

In New Zealand only, wiring systems in domestic installations shall be installed on the assumption that thermal insulation in ceilings, walls and under floors, if not currently installed, will be installed in the future.

Table C5 from ASNZS 3000:2007

This table has ratings that would generally be used in domestic situations, most cabling would be fused for in thermal insulation completely surrounded

What a cable can be fused at depends on a number of things, including but not limited to length, volt drop, earth resistance.

I agree that is the standard for installing new cable, but that doesn't technically cover existing cables in existing houses. Otherwise there would be need to upgrade every house every time the code changes.

corvus

5 posts

Wannabe Geek

#3297041 13-Oct-2024 21:18

t92300:

When I did the calculation it gave me 19A as the maximum 4mm could handle. Im guessing you put in touching surface for the installation conditions. See my quote from the standards below

From ASNZS 3000:2007 (What we work to in NZ)

3.4.1 General

Every conductor shall have a current-carrying capacity, in accordance with the AS?NZS 3008.1 series, not less than the current to be carried by the conductor.

In determining the required current-carrying capacity, provision shall be made for reasonably foreseeable changes to external influences, such as the installation of thermal insulation in ceiling spaces and walls.

In New Zealand only, wiring systems in domestic installations shall be installed on the assumption that thermal insulation in ceilings, walls and under floors, if not currently installed, will be installed in the future.

Table C5 from ASNZS 3000:2007

This table has ratings that would generally be used in domestic situations, most cabling would be fused for in thermal insulation completely surrounded

What a cable can be fused at depends on a number of things, including but not limited to length, volt drop, earth resistance.

Oh, you calculate for completely surrounded in insulation, yeah, definitely not the case. Also based on most figures I have seen elsewhere, they are usually based on partially surrounded figures. Fully surrounded would been 10mm² for 32amps, and 16mm² for 40amp (both common range circuit ratings). I guess it depends how someone runs the cables, but most people don't cut slots into their insulation to pack it around the cables, the cables just squash between the insulation and one side of the wall (which is defined as partially surrounded).