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Posted By Topic: GenCalc and other cable selection

Jul 23 2018 21:32

Hi all , I\'m supplying 3 phase power to a new build and using gencalc to figure out what sort of cable I\'m going to need. But I\'m not sure about 2 of the parameters it asks for. The first is what is your supply voltage. Is it 230 , or 400v supply? . Well I\'m not using any 3 phase machinery or appliance in the home. So could my supply voltage actually be 230 as I\'m simply using it for 3 x 230v supply\'s.

The second question is; is your Setup balanced or unbalanced. Is this referring to the network supply being balanced ie 3 phases 120deg shifted or is your load spread over all 3 phases.


Jul 23 2018 22:09

A single phase calculation is the same as a 3ph unbalanced anyway.

The difference may come in the voltage drop calculation due to the three conductors in proximity being a higher temperature, but it\'s usually only minor.

But as you\'re using a 3ph supply, without any 3ph loads, you should select 3ph, unbalanced, then the type of cable (multicore or whatever), and the installation conditions.

Keep in mind, that if your getting close to the rating for a cable size, you may need to calculate different installation conditions for the different parts of the run.

Or just go for the next size up.

And don\'t forget you should calculate the voltage drop to the main switchboard first, so you know what voltage drop you have left for these submains.

Jul 23 2018 22:10

when you select 3 phase as your supply the supply voltage will change from 230 to 400v accordingly.
also the load will be unbalanced within the installation.

Jul 23 2018 22:21

Sorry. forget about half of what I said. For some reason I thought you were talking about submains.

But you\'ll still need to work on a voltage drop figure for these mains.

Somewhere around 2-2.5% is usually sufficient.

Jul 23 2018 22:30

Thankyou , my volt drop came in at 2.17% from the pillar to the main swbd location, so all ok there. Using a 16mm 3c neutral screen xlpe on a 51 meter run from the pillar, i think it\'s going to allow me 40.8 amps back down the screen , which is fine as I\'m only getting 3 40amp fuses anyway.


Jul 23 2018 22:39

About the same as I get on the PC version of Gencalc.

Jul 23 2018 22:40

check with your local lines co. some have a requirement for minimum cable sizes.
you suggested a 51m run, this could be a back section by that distance, which could result in a 25mm cable being required as a minimum based on some lines co\'s requirements.

Jul 23 2018 22:46

Oh really , I didnt know that thanks, will check.

Jul 24 2018 08:25

There\'s no valid basis for any network to \"require\" a minimum size of cable.
They are entitled to set the supply capacity steps, which may well mean that their fault & short circuit protection - that ESR 32 requires them to provide - may be rated higher than your mains cable\'s CCC. But that does NOT empower them to insist on a larger cable, it just means you have to install your own device for overload protection.

Jul 03 2019 20:48

I don’t use the “GenCALC” app, I use 3008 and calculator.
I have been doing mains calcs recently but another electrician told me I had to use 230V v.d (eg 5.75V instead of 10V in a 400V for 3 phase unbalanced mains. I disagree.
The cable ends up 3 times larger mm2 going by his suggestion...

Jul 03 2019 21:44

The idea is that if you can realistically have one phase fully loaded and the other two with no current, it's basically the same as single-phase as far as volt drop goes.

But that scenario is fairly unrealistic in most cases, with the exception of e.g. single phase submains (e.g. one apartment per phase, with two people not home and one cooking tea and having a shower...).

Especially unrealistic if you can't hit maximum demand without having three-phase loads running, or base loads are on the less-loaded phases.

Remember volt drop only has to be sized to maximum demand, not the protection device.

Jul 04 2019 09:33

There's no official value of current for volt drop. The rules are written in a way that the rule is not broken unless / until the volt drop exceeds the limit.

It's also specific to the point at which the volt drop is measured. If that's a single-phase load, then use 230 V. If a 3-phase balanced load item, then the nominal voltage is 400. If 3-phase + N, then you have to consider both cases.

Using single-phase values for 3-phase submains is NOT required; and excessively cautious (but you can't be found to be "wrong"). Equally, assuming all loads will be balanced will put you on the wrong side. You have to consider the worst-likely-case. Which makes max demand a good starting point, but not an absolute bum-cover.

Of course, doing a MD calc takes time & effort, simplest is to use "limitation", in which case it's the rating of the protection. And that's what I use for mains & submains. On a very large site, that could be over-cautious. But I never found an installation yet that hasn't had a lot of things added over time. Using limitation for MD, and 3-phase balanced for mains & submains; you're unlikely to be on the wrong side.


Jul 04 2019 21:35

“”Equally, assuming all loads will be balanced will put you on the wrong side. You have to consider the worst-likely-case.””

So the worst likely case is if 2 phases ‘drop out’ and you’re left with one phase in a 3 phase mains cable?

Jul 04 2019 21:40

What I’m getting at is in an unbalanced 3 phase mains, we have to use single phase vd calculations eg 5.75V instead of 10V if using 2.5%.
And we dont use v.c x 1.155 as per 3008.

Jul 04 2019 23:28

The "worst case" is the same as calculating as VD for single phase - which is what Gencalc and other applications do for unbalanced 3ph.

Yes you still use the 3ph Vc from the tables, x 1.155 - that's how you calculate the voltage drop on the conductors for single phase or unbalanced 3ph.

It's when expressing the values as percentages, that people get screwed up.

For a single phase example. The 3ph Vc of the cable is 4mV/A/m @ 30A and the run is 25m.
So (4 x 30 x 50 x 1.155)/1000 = 6.93V
6.93/230 = 3%

But the same cable on a balanced 3ph system, would be:
(4 x 30 x 50)/1000 = 6.0V
6/400 = 1.5%

Jul 04 2019 23:33

The advantage of calculating VD with an application like Gencalc (which is the best I've found BTW), is that by entering all the correct conditions and load, it will calculate the actual conductor temperature and use the resistance of the conductors at that temperature - which saves you having to work that out to select the operating temperature from the tables.


Jul 05 2019 09:07

Agree that using third-party software such as Gencalc can give very good results. But it isn't perfect, as it only considers results one cable run at a time, and mostly we have multiple runs to consider, each with different parameters (mains, submains, final subcircuit).

Also agree that working in % instead of V (or mV)can lead to problems of understanding.

Which are not helped by the fact that Gencalc & simuilar hide some of the steps (eg cable temp); so end up with a different figure from what you might calculate by hand. They are a tool you either learn to use, and trust, or decide to leave alone.
They won't help you understand what's going on .

The ONLY way to get an understanding of & a feel for volt drop is longhand calculation of each run. It's not difficult, it doesn't take long, and you can file the page as a record of evidence.

When pushing to the limit, you can work out whether it's better / cheaper to use larger mains and standard subcircuit cables; or smaller mains and larger subcircuits.
Once you've developed the understanding, is time to pass the actual calculations to to a machine.


Jul 05 2019 10:55

AlecK of course I agree that it's necessary to know the calculations first, before simply relying on the application. And the application has limitations.

But just a point that both the PC version and the (Android) phone app do tell you the cable temperature at the specified conditions. On the Android app, there's an arrow at the bottom of the screen which pops up all the useful information CCC, reactance, resistance @ temp, temp, ohms/km @ x temp and more.

Jul 05 2019 12:18

true, but I was tying to convey that unless the user already understands the significance of that data; they won't realise that's why the final result is different to a calc done without reference to conductor temp, or even using3008 where temp is in bands. In fact most users probably don't even notice that the temp is given.