Replace an MCB

Forums › Home Workshop DIY › Replace an MCB

1 | 2 | 3

474 posts

Ultimate Geek

#1157991 19-Oct-2014 18:24

as an electrician i can say that some of you have misread what i said...
the breaker is specc'd to protect the wiring. period.
the size of cable used in a run is calculated by the allowable volt drop in the longest cable run so theoretically you could have a long lighting circuit fed by 2.5mm but only having a 10A breaker. . In 2011 they doubled the allowable volt drop so now you can actually have longer runs for a size of cable.
Yes socket outlets are only 10A rated, but thats just how it is and how its always been, the socket is the weak link. a single socket outlet fed by 2.5mm TPS on a 20A breaker is perfectly legal.
99% of houses are specced for 10A lighting circuits and 20A power outlets. 16A circuits are so rare now i dont think ive seen one in years i used 16A as an example as 16A is very common in those old push in breakers. thats because some of those old power circuits are only 1.5mm cable, whereas now all power ccts are 2.5mm.
5 amp lighting circuits are very rare. even 1mm tps is rated for 10A you dont derate a breaker to allow for volt drop...thats the wrong way to do it, you are supposed to up the cable size
having cable in insulation makes no difference, as i said before the volt drop allowed went from 2.5% to 5% allowing greater tolerance. a single cable run is not going to generate enough heat by itself even at max VD.

gregmcc

2147 posts

Uber Geek

ID Verified

Trusted

Lifetime subscriber

#1158033 19-Oct-2014 19:37

floydie: as an electrician i can say that some of you have misread what i said...
the breaker is specc'd to protect the wiring. period.
the size of cable used in a run is calculated by the allowable volt drop in the longest cable run so theoretically you could have a long lighting circuit fed by 2.5mm but only having a 10A breaker. . In 2011 they doubled the allowable volt drop so now you can actually have longer runs for a size of cable.

um....no, it's always been 2.5% for the mains and 2.5% for the subcircuits

Yes socket outlets are only 10A rated, but thats just how it is and how its always been, the socket is the weak link. a single socket outlet fed by 2.5mm TPS on a 20A breaker is perfectly legal.
99% of houses are specced for 10A lighting circuits and 20A power outlets. 16A circuits are so rare now i dont think ive seen one in years i used 16A as an example as 16A is very common in those old push in breakers. thats because some of those old power circuits are only 1.5mm cable, whereas now all power ccts are 2.5mm.
5 amp lighting circuits are very rare. even 1mm tps is rated for 10A you dont derate a breaker to allow for volt drop...thats the wrong way to do it, you are supposed to up the cable size
having cable in insulation makes no difference, as i said before the volt drop allowed went from 2.5% to 5% allowing greater tolerance. a single cable run is not going to generate enough heat by itself even at max VD.

Insulation does make a difference, there is a de-rating factor that needs to be applied if the cables are run covered in insulation.

floydie

474 posts

Ultimate Geek

#1158720 20-Oct-2014 18:32

the insulation factor is so small, it wont make any appreciable difference in most house runs.
IMO derating cables by using smaller breakers is not best practice...
And if you look in the new regs...the VD has been increased from the old 2.5% back in the regs update in (2011?) although i always use 2.5 anyway.

gregmcc

2147 posts

Uber Geek

ID Verified

Trusted

Lifetime subscriber

#1158735 20-Oct-2014 18:54

floydie: the insulation factor is so small, it wont make any appreciable difference in most house runs.
IMO derating cables by using smaller breakers is not best practice...
And if you look in the new regs...the VD has been increased from the old 2.5% back in the regs update in (2011?) although i always use 2.5 anyway.

I would suggest you have a look at AS/NZS3008.1.2:1998, this details the derating factor for thermal insulation.

The only item that has changed regarding the % volt drop calculation is allowing an extra 1% if there is a transformer on site

floydie

474 posts

Ultimate Geek

#1158751 20-Oct-2014 19:15

no thats not right.
I worked for a contracting firm at the time and all our calcs changed to 5% pretty much as soon as the law was changed. it also changed in gencalc. the inspector confirmed this change as it just didnt seem right at the time. i couldnt understand why they tighten up the regs but loosen the allowable VD.

gregmcc

2147 posts

Uber Geek

ID Verified

Trusted

Lifetime subscriber

#1158755 20-Oct-2014 19:23

floydie: no thats not right.
I worked for a contracting firm at the time and all our calcs changed to 5% pretty much as soon as the law was changed. it also changed in gencalc. the inspector confirmed this change as it just didnt seem right at the time. i couldnt understand why they tighten up the regs but loosen the allowable VD.

For a total installation it is 5%, that's 2.5% for the mains and 2.5% for the subcircuits, as previously stated in an earlier post

floydie

474 posts

Ultimate Geek

#1158783 20-Oct-2014 19:49

AS/NZS300:2007 clause 3.6.2

it doesnt say anything about the Installation splitting percentages in such a way. IT'S A PERCENTAGE...thats the point. you calc it on the MAXIMUM DEMAND or the MCB rating of the circuit you dont add up volt drops you add up maximum demand. every sub circuit is now calc'd at 5% because the upstream cable needs to be able to carry the maximum demand of all the ccts at .....5%
Whether its 5% of 10A or 5% of the mains maximum demand...its still 5%.

Sharesies

Trade NZ and US shares and funds with Sharesies (affiliate link).

gregmcc

2147 posts

Uber Geek

ID Verified

Trusted

Lifetime subscriber

#1158806 20-Oct-2014 20:15

floydie: AS/NZS300:2007 clause 3.6.2

it doesnt say anything about the Installation splitting percentages in such a way. IT'S A PERCENTAGE...thats the point. you calc it on the MAXIMUM DEMAND or the MCB rating of the circuit you dont add up volt drops you add up maximum demand. every sub circuit is now calc'd at 5% because the upstream cable needs to be able to carry the maximum demand of all the ccts at .....5%
Whether its 5% of 10A or 5% of the mains maximum demand...its still 5%.

2 posts back you were talking about VD (volt drop) now you are talking about Amps and maximum demand.

The clause below quite clearly states "Voltage" not current, and not 5% of a sub circuit, but 5% from the point of supply and any point in the installation.

3.6.2 Value

The cross-sectional area of every current-carrying conductor shall be

such that the voltage drop between the point of supply for the low

voltage electrical installation and any point in that electrical

installation does not exceed 5% of the nominal voltage at the point of

supply.

3.6.2 Value

The cross-sectional area of every current-carrying conductor shall be

such that the voltage drop between the point of supply for the low

voltage electrical installation and any point in that electrical

installation does not exceed 5% of the nominal voltage at the point of

supply.

floydie

474 posts

Ultimate Geek

#1158810 20-Oct-2014 20:20

which is calc'd from max demand values..the sub cct will always be a smaller contributer than in the mains.
(edit) which is why you have to use whats available and base it off maximum demand calcs.thats how we did it and it was fine by the engineer and the inspector.

anyway your points simply reinforce my point...20A breaker for power circuits and 10A for lighting even factoring insulation in most modern homes. no point overthinking things.
(this is why i never go on the elec forums..too much overthinking).

gregmcc

2147 posts

Uber Geek

ID Verified

Trusted

Lifetime subscriber

#1158816 20-Oct-2014 20:31

floydie: which is calc'd from max demand values..the sub cct will always be a smaller contributer than in the mains.

anyway your points simply reinforce my point...20A breaker for power circuits and 10A for lighting even factoring insulation in most modern homes. no point overthinking things.

Take a look at gencalc, what's the % that it defaults to when started up? 2.5%

Smaller cable means BIGGER volt drop and a sub circuit will be a big contributor, obviously you have been under thinking volt drop calculations, applying a 5% maximum volt drop on sub circuits is not right.........unless you have been installing some super conductor mains in peoples home.

Niel

3267 posts

Uber Geek

Trusted

#1158848 20-Oct-2014 21:43

Let's do some rough calcs. 2.5mm square cable is about 6.5 Ohm per km. At 240V and 10A, a 5% voltage drop requires 1.2 Ohm resistance. That is 185m of cable return loop, or 93m from the distribution board. Suppose it is 2.5%, that is still 46m. Few homes are than long. Most of your resistance will be in connections, and that is what the voltage drop test is for, to identify poor connections that could heat up. This cable can handle 200A for 10 seconds, 800A for 1s, or 4500A for 32ms. It needs to handle such high currents or else it will be destroyed as soon as your old heat pump started up, or from the smallest lightning surge induced into the cables.

A 20A breaker is designed to trip at 20A. A 10A socket is designed to carry 10A, not to fail at 10A. There is nothing wrong with a 20A breaker for a 10A socket. But follow whatever the regulations say.

Agree with Floydie, people on electrical forums often over-think things.

You can never have enough Volvos!

nickb800

2721 posts

Uber Geek

Trusted

#1435104 26-Nov-2015 12:20

Does anyone know anything about using inserts to protect users from live terminals when replacing breakers?

There's mention of it here http://www.electricalforum.co.nz/index.php?action=more_details&id=1367272357 but the conclusion seems to be that the inserts are no longer available.

I replaced my plug in rewirable fuses with NZI MCBs, and noticed there can be a small (say 1mm gap) between the body of the MCB and ceramic base. What's best practice for this?

mdooher

Hmm, what to write...
1425 posts

Uber Geek

ID Verified

Trusted

Lifetime subscriber

#1435171 26-Nov-2015 13:04

nickb800: Does anyone know anything about using inserts to protect users from live terminals when replacing breakers?

There's mention of it here http://www.electricalforum.co.nz/index.php?action=more_details&id=1367272357 but the conclusion seems to be that the inserts are no longer available.

I replaced my plug in rewirable fuses with NZI MCBs, and noticed there can be a small (say 1mm gap) between the body of the MCB and ceramic base. What's best practice for this?

No the inserts are no longer available. The reason is the inserts were designed to be placed in the ceramic base of the old rewirable fuse and then the breaker plugged in over it. This is no longer allowed. If you wish to replace a ceramic fuse with a plug in breaker you must get a sparky to replace the base as well.

If you have a ceramic base with a new plug-in breaker attached you will notice they don't mate properly leaving exposed contacts for fingers to touch. Obviously this is not permitted.

Matthew

richms

28192 posts

Uber Geek

Trusted

Lifetime subscriber

#1435178 26-Nov-2015 13:10

nickb800: Does anyone know anything about using inserts to protect users from live terminals when replacing breakers?

There's mention of it here http://www.electricalforum.co.nz/index.php?action=more_details&id=1367272357

Wow, that site is... an eyeful!

Richard rich.ms

nickb800

2721 posts

Uber Geek

Trusted

#1435212 26-Nov-2015 14:00

mdooher:
nickb800: Does anyone know anything about using inserts to protect users from live terminals when replacing breakers?

There's mention of it here http://www.electricalforum.co.nz/index.php?action=more_details&id=1367272357 but the conclusion seems to be that the inserts are no longer available.

I replaced my plug in rewirable fuses with NZI MCBs, and noticed there can be a small (say 1mm gap) between the body of the MCB and ceramic base. What's best practice for this?

No the inserts are no longer available. The reason is the inserts were designed to be placed in the ceramic base of the old rewirable fuse and then the breaker plugged in over it. This is no longer allowed. If you wish to replace a ceramic fuse with a plug in breaker you must get a sparky to replace the base as well.

If you have a ceramic base with a new plug-in breaker attached you will notice they don't mate properly leaving exposed contacts for fingers to touch. Obviously this is not permitted.

Aha. Can a sparky replace all of the bases without triggering the need for RCDs?

1 | 2 | 3