Basic three phase questions.

[ΩΩΩ]

I have some basic three phase question, if someone could help with please.
I have left power factor a 1 for simplicity.

If I have a 63A three phase and neutral BS 60390 socket - I presume that means it can carry 63A per phase so 189A.?
But because of the displacement of the windings, you would never draw 186A at the same time as some phases will be in the negative cycle.
But theoretically it can handle 189A?

If I had load of 10kW on a single phase supply that would be 10,000/230 = 43.5A. So a 45 / 50 A MCB would be needed
If I had a load of 10kW on a 3 phase supply that would be 10,000 /( 1.73 x 400) = 14.45A so a 16A 3 phase MCB would be acceptable.

I have seen single pole (online) MCB’s is rated at 125A, they may go higher?
One of the reasons for an appliance to be a three phase load rather than a single phase is that a lower rated protective device is needed, and cable size can be smaller?

So if a load is 63A 3 phase, that means 63A per phase. A protective device above 63A would be needed.
So 63 x 1.73 x PF (1) x 400 = 43.5kW
So the Total load across the 3 phases is 43.5kW
And if I had a load of 43.5kw 43500/ (1.73 x 400 x PF1)=62A A 3 three phase 63A MCB would be needed

Thanks

 

[JohnW2]

If I have a 63A three phase and neutral BS 60390 socket - I presume that means it can carry 63A per phase so 189A.?

Yes - but, as below, that's 63A RMS per phase - which waveform has a peak of about 89A - i.e. the current varies from -89A to +89A,with an RMS of 63A

But because of the displacement of the windings, you would never draw 186A at the same time as some phases will be in the negative cycle.

[ trivial typo there ("186A") ] I understand what you're saying , but I think that's a somewhat confusing statement, since, with the waveforms of the three phases being 120° apart, the 'peak' (+89A) of current in one phase will never happen at the same time as the 'tough' (-89A) in the other.

But theoretically it can handle 189A?

189A RMS, yes. However, because, as you say, the three phases are out-of-phase, it would not ever happen that it had to carry 3 x peak current (i.e. 3 x 89A).

If I had load of 10kW on a single phase supply that would be 10,000/230 = 43.5A. So a 45 / 50 A MCB would be needed
If I had a load of 10kW on a 3 phase supply that would be 10,000 /( 1.73 x 400) = 14.45A so a 16A 3 phase MCB would be acceptable.

I agree with your answers, although personally would probably have undertaken the 3-phase calculation differently.

One of the reasons for an appliance to be a three phase load rather than a single phase is that a lower rated protective device is needed, and cable size can be smaller?

I would say that is a fair statement.

So if a load is 63A 3 phase, that means 63A per phase. A protective device above 63A would be needed. So 63 x 1.73 x PF (1) x 400 = 43.5kW.
So the Total load across the 3 phases is 43.5kW

Agreed with your calculation. Another way of looking at it is as three separate single-phase loads.
Each Phase-Neutral voltage is 400V/1.73 (=~230V) and hence each single phase power is 63 x 230 x PF(1) (=14.9 kW)
Since there are three phases, total power = 3 x 14.9 kW (=43.47 kW)

However, I'm not sure why you say that "a protective device above 63A would be needed" (for a 63A load) ??

And if I had a load of 43.5kw 43500/ (1.73 x 400 x PF1)=62A A 3 three phase 63A MCB would be needed

I'm not sure what you are saying here - you seem to just be talking about very small inaccuracies in the calculations. The square root of 3 is actually 1.73205 [plus a bit!] If you had used that in your first calculation, you would have got 43.64768 [plus a bit!] kW, and if you had plugged that into the above calculation, you would have got 43647.68 / (1.73205 x 400) = 63.00002887A [and a bit!] (and therefore perhaps, if your name were Jobsworth, concluded that a 63A MCB was not OK?).

In fact, if you used the square root of 3 to an infinite number of decimal places you would, by definition, get an answer of exactly 63.0A - and, given what you say above, I don't know what you would say about the required MCB rating!

Kind Regards, John

 

[ericmark]

Each phase could carry 63 amp to neutral, so calculating at 230 volt rather than 400 volt makes the total wattage easier to work out, but at 400 volt that 63 amp must go between two phases, so total for all three phases is around 43 kW.

But that is resistive @JohnW2 has gone further into it, but in the main the problem or beauty of three phase is motors can start up without any special set up to start them turning, but also unless inverters are use can draw massive start currents.

So we traditionally used methods to reduce the start load, star/delta, auto transformers, and resistor starts, today we have electronic devices soft starts and inverters, but with single phase the reverse was the case, we had to use a second winding to get the motor turning, and the main problem was getting the motor to speed before the load was applied. Again using inverters we now can run three phase motors from a single phase supply.

 

[JohnW2]

Each phase could carry 63 amp to neutral, so calculating at 230 volt rather than 400 volt makes the total wattage easier to work out, but at 400 volt that 63 amp must go between two phases, so total for all three phases is around 43 kW.

Indeed, that's roughly what the OP and I both calculated. As for the calculation methodology, as I wrote to him, I would probably have done it differently (per your suggestion), even though we all get the same answers.

Kind Regards, John

 

[333rocky333]

I have seen single pole (online) MCB’s is rated at 125A, they may go higher?

Not sure you have, i believe 63a is the highest rated mcb's you can get. There are mccb's in larger ratings but they will not fit in a standard 3 phase board.

 

[JohnW2]

Not sure you have, i believe 63a is the highest rated mcb's you can get. There are mccb's in larger ratings but they will not fit in a standard 3 phase board.

I can't say I've ever previously had need to look, but here's a couple for starters .....

Kind Regards, John

 

[plugwash]

Those Eaton and Hager MCBs are ~1.5x the width of a normal MCB so they can't be used in normal outgoing ways of regular boards. The EATON one's description says it's for use in special boards, the hager one doesn't seem to say how it's intended to be used.

I remember TLC used to sell including 100a MCBs and special cable assemblies that could be bolted to the busbars so you could fit the higher-rated MCBs in an extension box to one of the brands of board they carried (think it was contactum) but they don't seem to sell them anymore.

 

[JohnW2]

Those Eaton and Hager MCBs are ~1.5x the width of a normal MCB so they can't be used in normal outgoing ways of regular boards.

Fair enough. As I said I was just 'noting their existence' (since I've never had reason to look for them before), so I didn't bother looking into details.

The EATON one's description says it's for use in special boards,

Yes, I noticed that it mentioned "high load boards". I suppose I assumed that just meant a board with something meatier than a 100A incomer, not that the width rendered it unusable for 'regular' busbars etc.

Kind Regards, John

 

[ericmark]

70 amp with the Loadmaster board which was larger than normal miniature contact breakers, but once you go to non miniature then 300 amp moulded breakers are common, and to be frank when using the heavier cables required one really needs a larger breaker with larger terminals.

 

[Adam_151]

Those Eaton and Hager MCBs are ~1.5x the width of a normal MCB so they can't be used in normal outgoing ways of regular boards. The EATON one's description says it's for use in special boards, the hager one doesn't seem to say how it's intended to be used.

I remember TLC used to sell including 100a MCBs and special cable assemblies that could be bolted to the busbars so you could fit the higher-rated MCBs in an extension box to one of the brands of board they carried (think it was contactum) but they don't seem to sell them anymore.

Fair enough. As I said I was just 'noting their existence' (since I've never had reason to look for them before), so I didn't bother looking into details.

Yes, I noticed that it mentioned "high load boards". I suppose I assumed that just meant a board with something meatier than a 100A incomer, not that the width rendered it unusable for 'regular' busbars etc.

Kind Regards, John

Both MEM (eaton) and hager make what they call 'hybrid boards' which have a normal pan assembly at the top, then just above the main switch they generally have 2TPN ways for the wider format breakers. The boards are nomally designed to take the 250A frame size incommers.

Or you can use them in a suitbale enclosure, supplied from a busbar chamber, for example

 

[JohnW2]

Both MEM (eaton) and hager make what they call 'hybrid boards' which have a normal pan assembly at the top, then just above the main switch they generally have 2TPN ways for the wider format breakers. The boards are nomally designed to take the 250A frame size incommers.

Thanks. Needless to say, it's a world that I've never had need to know anything about.

Kind Regards, John

 

[SUNRAY]

One of the managers I worked for ordered some 80A MCB, knowing they are wider he even gave instructions on what existing circuits had to be moved to where withing the DB to 2 to be fitted in 2 spaces, he really struggled to understand they are incompatible with the bussbars... until he turned up on site to show us how to do it.

 

[JohnW2]

One of the managers I worked for ordered some 80A MCB, knowing they are wider he even gave instructions on what existing circuits had to be moved to where withing the DB to 2 to be fitted in 2 spaces, he really struggled to understand they are incompatible with the bussbars... until he turned up on site to show us how to do it.

If the front-to-back positioning of the MCBs terminals were compatible with the busbar, then I suppose one might be able to do it with one of them occupying three 'spaces', by chopping of the busbar lugs on either side (if necessary). However, one would then have the problem of needing non-standard width blanking plates for the 'gaps' (although, depending on design', I suppose one might possibly be able to cut down a 'standard' one).

.. any of which I imagine would well qualify for a description of 'bodging'

Kind Regards, John

 

[Adam_151]

Its not even possible, the MCB would expect the lugs to be at centres of 1/2 MCB width, if you did somehow manage it, I'd be giving a C2 to it on an EICR.

Only way a similar outcome can be acheieved, with some makes (eg merlin gerin), is they produce unfused 100A tap offs for their boards, you mount the 80A or 100A large format MCB in a separate enclosue bushed to the side, and feed in tails from the tap offs

 

[SUNRAY]

If the front-to-back positioning of the MCBs terminals were compatible with the busbar, then I suppose one might be able to do it with one of them occupying three 'spaces', by chopping of the busbar lugs on either side (if necessary). However, one would then have the problem of needing non-standard width blanking plates for the 'gaps' (although, depending on design', I suppose one might possibly be able to cut down a 'standard' one).

.. any of which I imagine would well qualify for a description of 'bodging'

Kind Regards, John

In the board In question they would fit in everyother space without modification, but:
1) we didn't have the luxury of 5 spare positions, only 3.
2) We didn't know if the busbars are rated at 80A.
Our solution was a 4 way enclosure adjacent to the DB with tails dirrecly off its main switch.