Circuit Wire For Long Run

[Cookie]

Ok, say you've got a lighting run with the last fixture being 1,280 meters away. 1,800 watts per phase 3 phase, 277/480Y volts though in this example we can use 230/400Y for the ease of discussion.

My question is- what size wire do I run for each phase and earth? What disconnection time do I use? Do I use a Type B, C or D breaker? What should my max R1+R2 come out to be?

 

[pc1966]

Your 1.8kW/phase is about 8A, so 10A or more OCPD should be fine, subject to inrush limits.

If you control inrush then the 10A B-curve MCB is OK and it needs cable below 3.42 mOhm/m so all examples except 16mm 4C are fine.

If you used a 16A B-MCB for more inrush tolerance then it is 2.15 mOhm/m, so either 25mm 4-core and a supplementary 16mm earth, or 25mm 5-core is needed, etc.

 

[pc1966]

I'm sure there is proper software to do this, just never got round to finding/learning a package!

 

[Cookie]

For what aspect, volt drop or meeting MCB disconnection R1+R2?

Disconnection times.

 

[pc1966]

Should also add that I have not included and Ze or Zs at DB in the above table, assuming it is quite small for this discussion.

Equally you might just be lucky and find the lamp-posts are having 10-20 ohms when in the ground resulting in a small reduction in the R2 value.

I also did not do the parallel CPC & armour calculation as usually the regs assume the worst-case of the better conductor being used. Unless explicitly paralleled and then it is often assumed they are all similar conductors.

So there are possible factors making it better or worse!

 

[Cookie]

I'm sure there is proper software to do this, just never got round to finding/learning a package!

You kidding? I love doing this all by hand. Software doesn't teach you theory.

 

[Cookie]

OCPD​	Zs (from OSG)​	Zs No heating (Ohm)​	R1+R2 (mOhm/m)​
10A BS88-2 fuse​	3.7​	4.63​	3.61​
10A B-MCB​	3.5​	4.38​	3.42​
16A B-MCB​	2.2​	2.75​	2.15​
16A BS88-2 fuse​	1.9​	2.38​	1.86​
10A C-MCB​	1.75​	2.19​	1.71​
SWA choice​	R1​	R2​	R1+R2 (mOhm/m)​
16mm 4C​	1.15​	3.1​	4.25​
16mm 5C​	1.15​	1.15​	2.30​
25mm 4C​	0.727​	2.3​	3.03​
25mm 4C + 16mm​	0.727​	1.15​	1.88​
25mm 5C​	0.727​	0.727​	1.45​

By any chance, would you know the max Zs for this 15 amp MCB?

https://library.industrialsolutions.abb.com/publibrary/checkout/DES-079?TNR=Time%20Current%20Curves%7CDES-079%7CPDF&filename=DES079.pdf

 

[pc1966]

By any chance, would you know the max Zs for this 15 amp MCB?

https://library.industrialsolutions.abb.com/publibrary/checkout/DES-079?TNR=Time%20Current%20Curves%7CDES-079%7CPDF&filename=DES079.pdf

Looking at the data sheet, and applying the UK regulation's approach but with USA value, it would be:

• Max current for 0.4s trip time = 30 * In (slightly less, but that is marked on horizontal scale) so that = 30 * 15A = 450A
• Assuming you are looking at the USA supply of 277V nominal line-neutral and that is +/-10% then the lowest supply is 277 * 0.9 = 249V
• So max Zs = 249V / 450A = 0.55 ohms
• As for the table I did earlier, I am not applying the typical 0.8 factor for hot cable resistance compared to cold design/test values.

That is about 4 times lower than the European style of B-curve MCB would require, and that puts you cable size up massively! Assuming copper and matching CPC, that is no more than 0.27 ohms per conductor, and over 1280m that is 0.215 mOhm/m so you are looking at 3/0 AWG!

Realistically if you are faced with that sort of a breaker curve I would be looking at putting in a high quality RCD of 300mA or even up to 5A trip point as it would deliver far faster disconnection in the event of an earth fault, not having inrush trip risk, and allowing a sane cable size based on 5% VD of something like #5 AWG copper.

 

[pc1966]

As mentioned before, two RCD in series could be used for "no single point of failure" and even at the $1k MCCB style pricing it is going to work out cheaper than that extra copper (and the time & effort to install and terminate it).

 

[Cookie]

Looking at the data sheet, and applying the UK regulation's approach but with USA value, it would be:

• Max current for 0.4s trip time = 30 * In (slightly less, but that is marked on horizontal scale) so that = 30 * 15A = 450A
• Assuming you are looking at the USA supply of 277V nominal line-phase and that is +/-10% then the lowest supply is 277 * 0.9 = 249V
• So max Zs = 249V / 450A = 0.55 ohms
• As for the table I did earlier, I am not applying the typical 0.8 factor for hot cable resistance compared to cold design/test values.

That is about 4 times lower than the European style of B-curve MCB would require, and that puts you cable size up massively! Assuming copper and matching CPC, that is no more than 0.27 ohms per conductor, and over 1280m that is 0.215 mOhm/m so you are looking at 3/0 AWG!

Realistically if you are faced with that sort of a breaker curve I would be looking at putting in a high quality RCD of 300mA or even up to 5A trip point as it would deliver far faster disconnection in the event of an earth fault, not having inrush trip risk, and allowing a sane cable size based on 5% VD of something like #5 AWG copper.

Pure awesome! Though I would use -5% instead of -10% but it works either way.

Yup, I noticed that two. US breakers keep giving me a much lower Zs which kept throwing me off. Still confuses me to be honest. Basically most US breakers are a type E of F, if such a category existed after type D lol.


3/0 is a massive amount of copper. I'm basically calculating voltage rings around the post in hopes of being able to get away with a longer disconnecting time based on lower hand to foot voltage than 138 volts:

For your calcs, are you using 30*C or 75*C for the AWG wire system?

 

[pc1966]

Pure awesome! Though I would use -5% instead of -10% but it works either way.

The UK voltage tolerance is +10% / -6% but the calculations are actually on 0.95 so -5% of nominal. However Aus/NZ seems to use the nominal cases so is more "optimistic".

What is actually applied would depend on local code/regulations but I have no idea of the details of USA policy.

Yup, I noticed that two. US breakers keep giving me a much lower Zs which kept throwing me off. Still confuses me to be honest. Basically most US breakers are a type E of F, if such a category existed after type D lol.

That seems to be the case, though to be fair the EU range of MCCB also come with that style (basically fixed magnetic trip point for the family, but different thermal curves) as well as ones more like MCB with the "instant" magnetic point being at a fixed ratio to 'In'.

As well as the fancy but expensive ones with adjustable electronic trip up to the very high energy-limiting magnetic "last resort" trip.

3/0 is a massive amount of copper. I'm basically calculating voltage rings around the post in hopes of being able to get away with a longer disconnecting time based on lower hand to foot voltage than 138 volts:

With local rods you have no real hope of safe disconnection or protection, unless very deep and wet. Having a CPC is far safer but adds cost, though if you can use SWA armour and still disconnect it is not really making a difference to project cable cost (assuming armoured is used).

But from above it looks like an RCD is needed in any case to deal with that sort of length, even though it is not an RCD for direct-touch shock itself.

For your calcs, are you using 30*C or 75*C for the AWG wire system?

I assume 30C (i.e. cold, as measured), just got the AWG size & resistance from Wikipedia!

 

[Cookie]

Agreed.


Can you post the link to the 30*C AC resistance? The NEC is only giving me 75*C.

 

[pc1966]

Agreed.


Can you post the link to the 30*C AC resistance? The NEC is only giving me 75*C.

I used this:

American wire gauge - Wikipedia

en.wikipedia.org

Looking at details it says 68F = 20C which is room temperature around my part of the world.

Also probably they are DC, starts to become a source of error for large conductors as skin effect kicks in.

 

[telectrix]

personally i'd be quoting at least less than half the price. how?...... fit gas lamps. no volt drop or disconnection issues then.

 

[pc1966]

Or solar powered ones

 

[Soi disant]

Also probably they are DC, starts to become a source of error for large conductors as skin effect kicks in.

At 50/60Hz??