Single Cable calculation for hot tub?

[Deleted member 4632]

Can anyone enlighten me on why my max Zs for this cable calc is so high? What am I missing?

 

[andyb]

Probably just a glitch in the software, have you tried re running it or running the calc with a different ocpd?

 

[buzzlightyear]

what is the kw on the tub ?

 

[davesparks]

Can anyone enlighten me on why my max Zs for this cable calc is so high? What am I missing?

Just work the calculation backwards and all should become clear

V=IR
V=0.03x7285
V=218.55

Uo is 230V Cmin is 0.95,
230x0.95=218.5
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Probably just a glitch in the software, have you tried re running it or running the calc with a different ocpd?

There's no glitch, there's a 30mA RCD so technically max Zs is 230xCmin/0.03=7285.5

 

[Deleted member 4632]

what is the kw on the tub ?

Absolutely no Wattage details anywhere in manual just keeps asking for 32Amps.
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Just work the calculation backwards and all should become clear

V=IR
V=0.03x7285
V=218.55

Uo is 230V Cmin is 0.95,
230x0.95=218.5
[automerge]1590152229[/automerge]


There's no glitch, there's a 30mA RCD so technically max Zs is 230xCmin/0.03=7285.5

Ah!!! Thank you.

 

[buzzlightyear]

Absolutely no Wattage details anywhere in manual just keeps asking for 32Amps

what the computer asking for 32amps .

 

[Deleted member 4632]

what the computer asking for 32amps .

Hot tub company.

 

[pc1966]

As already explained, it is technically correct but not that useful in my opinion:

• If it is relying on the RCD trip limit it should be using the limits from 411.5.3 (so the recognisable 1667 ohms for 30mA due to the 50V voltage assumed)
• But even when the RCD can be used as the ADS means, I would prefer if they also list the over-current trip requirement to meet it (for example Table 41.3 where is says it also applies to RCBO) so here I guess the 1.09 ohm Zs for 40A B-curve

That way you can see if the fault would be cleared on the MCB action even if the RCD action were to fail.

 

[andyb]

There's no glitch, there's a 30mA RCD so technically max Zs is 230xCmin/0.03=7285.5

Is that what you put on your certificates Dave? Or do you put the max Zs for the ocpd?

Serious question.
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As already explained, it is technically correct but not that useful in my opinion:

• If it is relying on the RCD trip limit it should be using the limits from 411.5.3 (so the recognisable 1667 ohms for 30mA due to the 50V voltage assumed)
• But even when the RCD can be used as the ADS means, I would prefer if they also list the over-current trip requirement to meet it (for example Table 41.3 where is says it also applies to RCBO) so here I guess the 1.09 ohm Zs for 40A B-curve

That way you can see if the fault would be cleared on the MCB action even if the RCD action were to fail.

I agree totally with this, but he's not using an RCBO he's using a 60898 type B.
There is an RCD in there somewhere, I assume it's a dual RCD board or similar.

 

[Deleted member 4632]

Is that what you put on your certificates Dave? Or do you put the max Zs for the ocpd?

Serious question.
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I agree totally with this, but he's not using an RCBO he's using a 60898 type B.
There is an RCD in there somewhere, I assume it's a dual RCD board or similar.

Yep, split load C/U.

 

[andyb]

Yep, split load C/U.

The question was for Davesparks. But since you answered it, if you put this down on your certs, why did you need to ask the question in the first place?
Surely you must of known why your software gave such a high max Zs as you use it all the time.

 

[Deleted member 4632]

The question was for Davesparks. But since you answered it, if you put this down on your certs, why did you need to ask the question in the first place?
Surely you must of known why your software gave such a high max Zs as you use it all the time.

of course, surely I must have known that. Why cant I ask a question on this flipping forum without someone jumping all over it. Can it not just be answered with friendly advice rather than trying to catch someone out? ?‍

 

[davesparks]

Is that what you put on your certificates Dave? Or do you put the max Zs for the ocpd?

I put the max Zs for the OCPD.

Well actually I'm lazy these days and let the software fill that bit in.

 

[andyb]

of course, surely I must have known that. Why cant I ask a question on this flipping forum without someone jumping all over it. Can it not just be answered with friendly advice rather than trying to catch someone out? ?‍

I'm not trying to catch you or anyone out, it's a question to which I did not know the answer myself. It's never dawned on me to calc the Zs using the rcd and not the ocpd, that's why I asked Dave if he uses this method.
It also prompts the question, if using this method when testing, do you then use 80%?
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I put the max Zs for the OCPD.

Well actually I'm lazy these days and let the software fill that bit in.

That's why I asked, my software populates it for me.

 

[pc1966]

I'm not trying to catch you or anyone out, it's a question to which I did not know the answer myself. It's never dawned on me to calc the Zs using the rcd and not the ocpd, that's why I asked Dave if he uses this method.

The RCD value of Zs is pretty much pointless in virtually every case!

Even for a TT system where Zs would be above any reasonable cable resistance value, it is still mostly going to be the 200 ohm value for adequate stability of the earth rod, and not the 500/1667 ohm for typical 100mA/30mA incomer/RCBO as used.

I suspect only the likes of a large farm is going to have a 300mA incomer on TT where you are down to (in this example) Zs < 167 ohm that has a test based on the RCD value.