Discuss how to work out what breaker to use and why (level 3 hotel extension assignment) in the Electrical Forum area at ElectriciansForums.net

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I have 10 7.5A lights in a room on a breaker 10x7.5a=75a so 75a ÷ 230v is something crazy like 0.3A so what am I going to use a 1A breaker 😂 another example I have 700w AC unit 700w ÷ 230v= something like 3.2A yet again ovs not gonna use a 4amp breaker for a AC unit??? What am I doing wrong guys need to know thanks
 
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pc1966

Arms
Esteemed
Well for a start I presume you have 10 * 7.5W lamps!

You also have to allow for two other aspects:

First is the switch-on surge of anything that is not a simple resistor. For that you further need to consider if all lights are switched as one load (i.e. peak current is simultaneous), or if they are individually switched so the peaks occur independently.

Second aspect is you should only fit breakers that are approved for the type of board you have. Yes, it is a bit of a annoyance at times, but sadly the busbar arrangements, etc, are not standardised and so may not properly match and so manufacturers will only guarantee arrangements they have tested. So if your chosen board only has the option for, say, a 6A MCB as the smallest then that is your only choice for < 6A rating, even if another manufacturer offers 1A or 2A models, etc.
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But for any answer to a question you need to show what you work out as needed, and then your logic/reasons for what you select to provide that functionality.
 

JAWS

-
Trainee
I have 10 7.5A lights in a room on a breaker 10x7.5a=75a so 75a ÷ 230v is something crazy like 0.3A so what am I going to use a 1A breaker 😂 another example I have 700w AC unit 700w ÷ 230v= something like 3.2A yet again ovs not gonna use a 4amp breaker for a AC unit??? What am I doing wrong guys need to know thanks
Probably best to tell you Thomas that calculations for the Hotel is not required.
 
  • Thread Starter Thread Starter
  • #4
Well for a start I presume you have 10 * 7.5W lamps!

You also have to allow for two other aspects:

First is the switch-on surge of anything that is not a simple resistor. For that you further need to consider if all lights are switched as one load (i.e. peak current is simultaneous), or if they are individually switched so the peaks occur independently.

Second aspect is you should only fit breakers that are approved for the type of board you have. Yes, it is a bit of a annoyance at times, but sadly the busbar arrangements, etc, are not standardised and so may not properly match and so manufacturers will only guarantee arrangements they have tested. So if your chosen board only has the option for, say, a 6A MCB as the smallest then that is your only choice for < 6A rating, even if another manufacturer offers 1A or 2A models, etc.
Post automatically merged:

But for any answer to a question you need to show what you work out as needed, and then your logic/reasons for what you select to provide that functionality.
You have confused me 😂 I'm not very academic sorry
Post automatically merged:

Probably best to tell you Thomas that calculations for the Hotel is not required.
Isit not ? But question 3 u must say what breakers you need
 
you have allowed for the running currents only,
What about the start up surges ?
Have you allowed for them,
with ten lights there will be a surge.
 

Vortigern

Arms
Esteemed
Might be helpful to look at Ohms law. So in your example I/V(U) (is there such an equation?) in the first part as against P/V=I is correct. Get an Ohms law wheel and look at the transpositions.
 
That's actually watts law.
I don't know it's correct but I calculated all my Lights at 100w like diversity in OSG for worse case scenario.
 

pc1966

Arms
Esteemed
If you don't know what lights are in use then assuming the 100W as per on-site guide, etc, is consistent and justifiable. Even if not actually right, but you are unlikely to be overloaded as above 100W is very rare these days!

But the problem of doing it correctly is often the information you need is not given, or is not in any accessible format. For example one company does a similar calculation for their LED driver and gets two rather different results:


There example is a 75W LED driver and they use a 16A B-type MCB as the example breaker (so already very big by domestic light standards) and calculate:
  • Steady-state you could have 42 on the 16A circuit.
  • Due to switch on surges you can have 8 simultaneous.
So while it is not in any standard, and may vary a lot from supplier to supplier, you could guess that whatever you compute for LED lighting steady-state, if simultaneously switched you need to look at 5-6 times that current for selecting a B-curve MCB

Presumably more generous with a C-curve MCB, but as the surge is very short at 240us = 0.24ms = 0.00024s that may not be guaranteed.
 
Even at 100w they were well within a 6Amp Mcb, Does anyone actually fit smaller than a 6Amp for lights? There not that common are they smaller MCBs
 

pc1966

Arms
Esteemed
Even at 100w they were well within a 6Amp Mcb, Does anyone actually fit smaller than a 6Amp for lights? There not that common are they smaller MCBs
More than 100W of LED lighting, in particular simultaneously-switched, is very unusual for a home. Maybe if you have a 5-a-side pitch outside and want floodlights for dark evenings...

Most consumer boards only have matching MCB down to 6A, a couple of brands offer 1A or 2A, but often you need to look at the industrial type of three-phase boards, etc, to get a wide range of small currents. Hager, for example, go down to a 0.5A C-curve MCB.

But as with a lot of electrician's work, the majority of installations fall inside the "typical" cases and for that you can look up the on-site guide table 7.1 and if your setup falls within the typical cases of 6A lights / 20A radial / 32A ring then no fancy maths needed at all.

And the typical sizes are pretty generous. For example, even with 1mm T&E on a 6A B-curve MCB you have 68m length. Quite a size for a single circuit to cover!

Going by the above LED example and working backwards, allowing x6 for the surge factor that is 1A steady-state, and then you are looking at 230W of lights (around 1.5kW in tungsten lamp brightness terms) simultaneously-switched. You can probably increase by a factor of 2-3 for a typical home or small office setup as most rooms won't have more than a half dozen on the one switch.

As most LED lamps are under 15W, that is over 15 outlets per switch, subject to maybe 30-45 total. Again, a lot for the area a typical single circuit would cover.

But the reason you have to do these questions in gaining your qualifications is not for day-to-day work, but to demonstrate you have a grasp of the electrical principles for the odd case where you have an unusual set up that falls outside of the norm.
 

hasel5

-
Arms
Surely the breaker is there to protect the cable from overload and that is the more important issue.
 

pc1966

Arms
Esteemed
Well there needs to be some protection of the cable and it could be one or more of the following:
  • Short circuit currents
  • Overload
  • Leakage currents
OK rest of my last answer was for another thread!

Back to this one, yes it needs to protect the cable. But equally it has to be reliable in service, which means not tripping on surges that ought not to be seen by the people using it.

The problem with electronics, like the LED drivers, is the switch on surge can be very big (they are talking about 67A at 230V AC per driver) but it is very short-lived (0.24ms) as it charges some fairly small capacitors, and that is far less than the typical 3-5ms response time of the "instantaneous" magnetic trip part of the MCB curve.

So in the example LED driver they give the 16A B-curve MCB trip has a sensitivity of 560A for 0.24ms, and not the value of 3-5*In = 48-80A that you would normally see on the time-current plots for a MCB that usually go down to around 10ms at the shortest.
 
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