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Thread: Amp rating of incoming supply breaker

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    Amp rating of incoming supply breaker

    Hi guys

    New to the forum but I've lurked a lot. Have gotten quite a bit of really good advice from this forum.
    I'll state off the bat I'm a DIYer. I've studied the regulations quite in depth and I'm very serious about safety.

    I live in an apartment in Cape Town. And as is usual for many Cape Town apartments, my incoming supply is 3 phase.

    When I moved in the panel was a mixture of really old breakers and new breakers.
    Some problems I had right off the bat:

    Kitchen plugs weren't on a phase with an earth leakage (ouchie).
    Only some of my plugs were on an earth leakage, everything else was not.
    It became apparent quickly why this was the case, one set of plugs and the lights had a earth/neutral fault, so obviously as soon as I installed a EL everything would trip immediately. Took a really long time to find where it was shorting.

    I've rewired the apartment as:
    Main breaker (3p + N) -> EL (3p + N) -> Everything else (wired from the 3 phase EL)

    Finally my question:
    My incoming supply has a 35amp breaker.
    Now I've noticed every other apartment has a 40a incoming supply breaker or 50a.

    How common is a 35a incoming supply breaker?

    I want to remove my geyser (takes a LOT of space) and replace it with an instant water heater, but it is rated at 21kW, which with 35amp breaker leaves virtually no wiggle room.

    My thinking is the main supply breaker was replaced by the previous installer with whatever he had.

    I obviously don't just want to upgrade the breaker to 40a, I simply don't feel it is safe.
    So what I'm wondering is, how would one go about determining if the breaker can be upgraded, who should do it, etc?

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    Diamond Member AndyD's Avatar
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    Comments in red;

    Quote Originally Posted by Gnome View Post
    Hi guys

    New to the forum but I've lurked a lot. Have gotten quite a bit of really good advice from this forum. Welcome to the forum.
    I'll state off the bat I'm a DIYer. I've studied the regulations quite in depth and I'm very serious about safety.

    Don't take this the wrong way because reading your post it sounds like you certainly have some skills. As a DIY'er you need to do electrical work in conjunction with a qualified and registered electrician. There's no reason you can't do some of the work tasks under his/her instruction / supervision but ultimately he/she's got to inspect, test and certify the work. Failure to work this way is likely to result in invalidation of insurance in the case something goes awry and you could be legally liable if there's injury or damage in the future.

    I live in an apartment in Cape Town. And as is usual for many Cape Town apartments, my incoming supply is 3 phase.

    When I moved in the panel was a mixture of really old breakers and new breakers.
    Some problems I had right off the bat:

    Kitchen plugs weren't on a phase with an earth leakage (ouchie).
    If the plugs were installed to previous regulations that were valid at the time and RCD protection wasn't a requirement at that point then it's possible they were still compliant today if alterations or additions or change of ownership haven't been made since.
    Only some of my plugs were on an earth leakage, everything else was not.
    It became apparent quickly why this was the case, one set of plugs and the lights had a earth/neutral fault, so obviously as soon as I installed a EL everything would trip immediately. Took a really long time to find where it was shorting.

    Before an RCD is installed there's certain tests that need to be made on the existing circuits. You need a particular test instrument called a Mega Tester or Insulation Tester to make these tests.

    I've rewired the apartment as:
    Main breaker (3p + N) -> EL (3p + N) -> Everything else (wired from the 3 phase EL)
    Do you have any final circuits that are three-phase? Having a single upfront RCD (earth leakage breaker) could be highly inconvenient if there's a fault, it's often preferrable to install multiple single phase RCD's.

    Finally my question:
    My incoming supply has a 35amp breaker.
    Now I've noticed every other apartment has a 40a incoming supply breaker or 50a.

    How common is a 35a incoming supply breaker?
    Wrong question, the answer no relevance

    I want to remove my geyser (takes a LOT of space) and replace it with an instant water heater, but it is rated at 21kW, which with 35amp breaker leaves virtually no wiggle room.
    That's a very powerful instant water heater for domestic purposes. Is it to supply the entire house? It's too large obviously for your existing supply even when applying optimistic diversity to the design.

    My thinking is the main supply breaker was replaced by the previous installer with whatever he had.
    This is a dangerous assumption

    I obviously don't just want to upgrade the breaker to 40a, I simply don't feel it is safe. It's definately not safe to do so!!!
    So what I'm wondering is, how would one go about determining if the breaker can be upgraded, who should do it, etc?
    DON'T JUST UPRATE THE EXISTING BREAKER WHATEVER YOU DO. The Amp rating of the breaker has far reaching implications. Firstly the main breaker is there to prevent overloading the supply circuit wiring coming into your apartment. It's maximum size is relevant to the wiring size, wiring length and how the wiring is physically installed. Secondly the breaker is required to provide disconnection of the supply within a time limit when there's a short circuit fault. If you increase the Amp rating of the breaker you will likely increase the disconnection time beyond the allowable limit. There are certain impedance tests that must be done on the live supply before the maximum allowable breaker size can be determined. Thirdly you need to ensure discrimination with the upstream circuit breaker which will be locked away in the supply room somewhere.

    I'm not going to get into explaining all these issues because without a full range of test equipment and knowledge of how to use it it will all be moot. Some of the tests are live tests and definately not performable by unqualified personnel.

    Bottom line is that you're talking about work here that's far outside of the realms of any DIY'er so you need to consult a qualified sparky. Failure to do so will almost certainly result in a real risk of the installation being rendered unsafe and non-compliant.
    Last edited by AndyD; 02-Jun-17 at 11:04 PM. Reason: Spelling and didn't like the shade of red so edited bb-code colour tags
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    Wow, slightly hostile response...

    I wasn't simply going to up rate the breaker. My question is more in regard to what the chances are that the previous install downrated the breaker because I believe it will impact if I will be able to convince the council that I should be allowed to up rate the breaker.

    Although I'm not a qualified electrician, I do have degree in electronic engineering and perhaps it didn't come across in my post above I'm fairly conservative when it comes to considering risks.
    I DIY a lot of electronics in my spare time (Eg. wind my own transformers, built my own inverter as a thought experiment, etc.). I can't convince you on a forum that my technical knowledge of electronics eclipses most electricians so I guess we'll just call me a n00b.

    I didn't have a Insulation Tester at the time I fitted the RCD, so yes, I had to use a multimeter to find the short circuits between neutral and earth. Both were due to damaged insulation that was microscopic. Recently I acquired a really cheap one that tests at 500/1000v, probably not the best but the resistance at 1000v is >10m between all circuits. The cheap tester is obviously not at the level of the professional kind. It just uses a simple flyback transformer and the wave form on my oscilloscope doesn't look anything like the 50Hz sine you get from a wall plug but its voltage is within the correct range which should sufficiently cause insulation break down. (just my 2c on that). Obviously I couldn't do an RCD test that verifies that the trip current is less than 30mA. Sadly I'm hoping that the RCD (CBI 3phase type) meets its specs. I have no reason to doubt it thus far. It did after all trip initially due to the earth neutral fault.

    The wiring DIY was simply because the work that I've experienced so far has been shoddy when done by contractors. No offense to you guys. My entire flat had new modern plugs fitted and the kitchen was redone with one of the kitchen circuit having twin and earth whereas the rest is general purpose. The lady claimed it had been redone less than 5 years ago. I don't see how not having RCD on those circuits was legal.

    I considered using 3 x single phase RCDs but my stove is 3 phase and I don't want any circuits off of an RCD. That said my leakage current is non-existent. 0 nuisance trips so far.

    The instant water heater is a whole house type yes. 21kW is seems pretty standard rating for whole house types from what I can see from my own research.

    21kW = ~92amps, ~31amps per phase, or am I missing something here? I have a 35amp breaker currently...

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    I should say if I was just going to up rate the breaker, I would have done it and not asked here. The wiring coming in is sufficient mm^2 to handle 50amp breaker.
    But I specifically DO NOT want to up rate it without explicitly getting in touch with someone that can verify the whole circuit going back to the council (and in my flat if they so please).

    My question here was more because I'm wondering if they will even entertain my asking to up rate the breaker.

    Is the right path here to simply get an electrician or do I contact the council first?

    Also which electrician

    If I have to get someone who gets here and tells me he doesn't know, it'll be very upsetting.

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    Lol, I wasn't attempting to be hostile, I was aiming for firm but maybe I missed.

    I'll elaborate on the 'wrong question' comment in my previous reply. What I meant was it's irrelevant whether the breaker was downrated by a previous contractor because the proceedure for establishing whether it can be uprated is the same regardless. It's not just about the size (CSA) of the wiring there's many other factors dictate the maximum allowable size of the breaker.

    Uprating the main breaker in your DB wouldn't involve the supply authority (apart from maybe isolating the supply during the work) unless alterations to your apartments supply cable and/or upgrading of the upstream breaker on the council side are necessary. To establish whether this is the case you'll have to get a sparky in. A CoC must be issued.

    Quote Originally Posted by Gnome View Post
    The cheap tester is obviously not at the level of the professional kind. It just uses a simple flyback transformer and the wave form on my oscilloscope doesn't look anything like the 50Hz sine you get from a wall plug but its voltage is within the correct range which should sufficiently cause insulation break down. (just my 2c on that).
    Most normal insulation (megger) testers use a 250/500/1000v DC test voltage, you rarely find an insulation tester that uses an AC test voltage until you get into the realms of Hi Pot testers which aren't usually used for testing domestic installations.
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    Quote Originally Posted by AndyD View Post
    Lol, I wasn't attempting to be hostile, I was aiming for firm but maybe I missed.
    Yeah I was going to edit my post (but I can't). I understand the reason

    Overall, thank you for the input, it is appreciated

    Quote Originally Posted by AndyD View Post
    I'll elaborate on the 'wrong question' comment in my previous reply. What I meant was it's irrelevant whether the breaker was downrated by a previous contractor because the proceedure for establishing whether it can be uprated is the same regardless. It's not just about the size (CSA) of the wiring there's many other factors dictate the maximum allowable size of the breaker.

    Uprating the main breaker in your DB wouldn't involve the supply authority (apart from maybe isolating the supply during the work) unless alterations to your apartments supply cable and/or upgrading of the upstream breaker on the council side are necessary. To establish whether this is the case you'll have to get a sparky in. A CoC must be issued.
    I did not know this. Thanks. Can I get any sparky or are only some of them capable of this?

    Is this basically a case of yellow pages?

    Quote Originally Posted by AndyD View Post
    Most normal insulation (megger) testers use a 250/500/1000v DC test voltage, you rarely find an insulation tester that uses an AC test voltage until you get into the realms of Hi Pot testers which aren't usually used for testing domestic installations.
    That makes sense I guess.

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    Diamond Member AndyD's Avatar
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    Maybe one of the forum regular sparkies would be interested in helping you out. Which side or what area of Cape Town are you?
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    Anyone want to help me out?

    I'm in Sea Point. Obviously I'll pay (just so you don't think I'm trying to pull a fast one). I can even throw in some used CBI breakers (and some unused) if that'll sweeten the deal

    My basic ask is, I want to determine if I can up rate my incoming breaker to 40a at least.
    If it can't be then at least I know.

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    if you have 10mm supply wiring ( you mentioned the wiring was rated for 50 amps ) then you should easily be able to upgrade to 40 amp main breaker. as long as voltage drop isnt a problem.
    try get the council out to see what size breaker they have . if they have a 50 or 45 then yours was probably originally a 40. speak to the inspector at the depot .its near you . i think its in rose street.
    as andy said , get a good electrician that can size the cable ,do the tests and make a recommendation.

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