Change of a Cable/Conductor Size at a Junction Box in a Long Circuit

It is alright with me if code compliance is not mandatory. Otherwise, you have to go by the code, no escape from that.

Well, if one of the branches is as low as 2.5mm² then why not go 2.5mm² all the way?

One must always consider the prospective short circuit current [PSC] in selecting cable size and the overcurrent protection device. There is no difference in the PSC either side of the junction box, so either:

• There needs to be additional overcurrent protection within the junction box for the smaller circuits or
• The upstream protection needs to be selected for the smaller cable size and not the larger or
• the smaller branch circuits are undersized and must be uprated to the main feeder size

all of which is covered by the protocols in British Standard 7671.

Otherwise one could have 25mm² feeding the house and no protection for the 1mm² lighting circuits within it; in this scenario, the fuse is the cable.

What does your copy of AS3000 say?

I don't have a copy here in the USA.

A professional electrician in Australia wouldn't need to ask this question in a global internet forum. The telephone and Yellow Pages provide the answer to this one.

Replace the PIT/JB with a CCU. Saves a lot of trouble with AS3000

Try AS/NZS 3000 section 3.4 (current carrying capacity) and section 3.6 (voltage drop).

As long as you comply with the requirements of the applicable sections of the standard (and use some common sense) you will be fine.

As per BS 7671 whenever a change in cross sectional area of conductor takes place there should be PD(fuse/cb).

No. Only need another protective device if the existing one is too large for the smallest cable.

I agree.

Read post#12.

...in which case the larger of the two cables is oversized.

On a long run the volt drop can cause the cable size to increase. In some instances the cable size will be such you cannot terminate it safely into the device. You can install a smaller diameter cable to terminate into the device but you must ensure it is of sufficient size to carry the normal circuit current safely and be protected by the fuse or breaker. The constraints are a safe installtion and that the cable is protected by the protective device.

In your situation as long as the protective device protects the smallest branch I can't see an issue. If it doesn't then you you will forced to the more usual distribution board. You will have earth loop to consider as well.

Thanks everybody for your help,

I think I agree with what Silverfox said here.

Cheers

I know what NEC (National electric Code) says and how to apply it in the USA, but I don't know what Australian stdandards are. This question is like asking you if I have to go by the speed limit in Shannon Hills. Try 31 MPH when that cop is in a bad mood and see! -- JHF

In Aus under no circumstances are you permitted to reduce the size of a cable in a circuit unless there is a protective device to protect the smaller cable. In the instance where the cable is of a larger size because of voltage drop a short tail that is rated to carry the full current supplied by the original circuit, with out needing to allow for voltage drop may be fitted to the end of the larger cable at the end of the circuit only & for termination purposes only, but must be as short as practical.

In the case of the 2 cables splitting & going to different locations from a pit with no protective device the same size cable must be continued to the end of the circuit in both directions from the pit.

I am sure if you got a copy of our rule book AS_NZS 3000-2007 you would be able to find this information & if you are an electrician in Aus you are required to have a copy of this book.

You have come on CR4 & asked this question so it appears to me you are not qualified to do this work so get some 1 that is qualified to do the work. It is illegal for unqualified persons to do electrical work in Aus & if caught the electricity supply authority can refuse to allow you to have power connected in your name for life & you face prosecution for doing the work.

This question you have asked is simple & basic it is taught to apprentices in their first year & I would be concerned about the quality of the rest of the job because of this. So get an electrician to complete the job before you kill some 1 or burn down a building.

Just so you know I am a qualified electrician of 30 yrs in Aus & know the rules. I have not given you any references to the rules because I do not have the time or inclination to search through the rule book to find the specific rules that apply.

No offense to our foreign colleagues but your rules do not apply in Aus & are not accepted here

I thought my answer would earn a GA as my facts were accurate, correct & were backed up by Azzamaniac. I thank him for identifying the clause that backs up my statement when I was unable to research the exact clause.

Hi,

The answer to this question is that AS3000:2007 Clause 2.5.1.2 Submains and Final subcircuits states:

An OCP or protection device SHALL be placed at the origin of every circuit and at each point where a reduction occurs in the CCC of the conductors.

I have been working as an Australian Federal Inspector servicing WA, SA and NT for over 2 years and i have to agree, any unlicensed works carried out by unqualified persons WILL be prosecuted by the State Regulators. Use a licensed Contractor and qualified sparky.

I'm sorry Silverfox, but after rereading your post you would be recieving a lovely defect notice and possible disconnection of service by not following the AS3000.

So am I getting this defect notice or not? I'm not up to speed with Oz regs - I'm UK. Bottom line is are the circuits electrically protected against thermal overload and will they trip in the event of a fault? If these boxes are ticked I can't see any regulations sending a defect notice.

Have you looked at the exceptions to that clause? 2.5.3.4 (b) (i) for the overload protection and 2.5.4.3.2 for the short circuit protection.

Therefore, as long as the circuit breaker at the switchboard provides overload and short circuit protection for the feeder cable into the pit, the larger branch and smaller branch going out of the pit, it's ok. :)

More information is needed to asses if these rules apply to this installation

What is the purpose of the circuit? is it a final circuit or a sub mains ?

What is the current rating of the protective device supplying the circuit?

What is the load/ maxim demand on each leg of the circuit?

What is the length of the circuit to the pit & the individual length of the 2 legs splitting after the pit?

do the 2 legs from the pit supply sub boards or are terminated as a final sub circuit for equipment?