When do amp ratings matter

[Neptune]

Got some amateurish questions. Please excuse me but want to set this straight in my head.

Do amp ratings only matter when current is being drawn i.e. an appliance plugged in, light fitting, etc.

• To expand, if I have a 50amp MCB with a 1mm T&E cable going into a 5amp junction box, would that matter or would it have any adverse effects?
• If in the above example, I had a socket at the end of the cable and a heater plugged in, I assume that would be problematic as the 1mm cable isn't correctly sized
• Similarly, if I had a ring main where every point was terminated into 5amp junction boxes instead of sockets, I assume that would be fine too
• However, if one of the points had a socket and the socket had a fridge plugged into it, the 5amp junction boxes would be an issue as you now have a higher current being drawn through the ring.

I would really appreciate your wisdom on the above scenarios. Thanks in advance.

 

[pc1966]

Pedantically 'no', as without current flowing none of it matters.

In reality 'yes', as you also have to consider what happens if/when there is a fault, possibly in the fixed cable or accessories themselves and not in any planned load. Then the lack of correct over-current protection for the down-stream cable and accessories, etc, can lead to damage or even a fire.

 

[Neptune]

Pedantically 'no', as without current flowing none of it matters.

That’s what I expected. Thanks

Then the lack of correct over-current protection for the down-stream cable and accessories, etc, can lead to damage or even a fire.

I don’t understand this. Can you give an example please?
How can you have a fire if nothing will be plugged in or draw current from the circuit? Clearly if there will be a current draw, the I understand that junction boxes, cables and breakers need to be sized appropriately.

 

[pc1966]

I don’t understand this. Can you give an example please?
How can you have a fire if nothing will be plugged in or draw current from the circuit?

• Someone nails a cable doing DIY
• A local mouse chews a cable
• The person installing the sockets trapped a cable against a back-box and eventually the pressure punctures the insulation
• There is poor manufacturing control for an accessory or reel of cable

I am sure others on this forum will have examples of how you can get a fault in an installation!

 

[ipf]

A basic factor to consider is that the current carrying capacity of a cable determines the size of protection device to be used. Obviously I.0mm cable is nowhere near 50amp rated. Well over 50amps could be flowing with a 15amp (approx) rated cable which, as a result, could become a bit warm, to say the least.......an extreme fire hazard, of course.
This is not even taking into account the danger of short circuit fault currents possible, as could be involved with some of the examples indicated above.

 

[Dustydazzler]

Under 'fault conditions' a relatively small csa cable will survive a large fault current so long as the over current device disconnects the supply quickly

The bigger issue is a prolonged over current , this is when insulation breaks down

JW did some interesting videos on passing large current through small cables and to see how long it took before the cable got warm

 

[Dustydazzler]

 

[ipf]

As per basic point made…….?

 

[Dustydazzler]

As per basic point made…….?

Yep

Seeing it in a video really gives a beginner in the electrical trade a good visual image of what happens under certain conditions

 

[Petej999]

Got some amateurish questions. Please excuse me but want to set this straight in my head.

Do amp ratings only matter when current is being drawn i.e. an appliance plugged in, light fitting, etc.

• To expand, if I have a 50amp MCB with a 1mm T&E cable going into a 5amp junction box, would that matter or would it have any adverse effects?
• If in the above example, I had a socket at the end of the cable and a heater plugged in, I assume that would be problematic as the 1mm cable isn't correctly sized
• Similarly, if I had a ring main where every point was terminated into 5amp junction boxes instead of sockets, I assume that would be fine too
• However, if one of the points had a socket and the socket had a fridge plugged into it, the 5amp junction boxes would be an issue as you now have a higher current being drawn through the ring.

I would really appreciate your wisdom on the above scenarios. Thanks in advance.

KAre you a Spark? if yes then I WOULD TRY SOMETHING EKSE TO DO AS A LIVING

 

[littlespark]

KAre you a Spark? if yes then I WOULD TRY SOMETHING EKSE TO DO AS A LIVING

He's not.... He's a DIYer eager to learn.

 

[DPG]

KAre you a Spark? if yes then I WOULD TRY SOMETHING EKSE TO DO AS A LIVING

He's DIY - hence the DIY tag.

 

[freddo]

When sizing cables and circuit breakers the much simplified rule is:

The cables maximum current carrying capacity must be greater than the current the connected equipment draws.
The fuse/circuit breaker used needs to have a higher current rating than the connected load, (so the fuse/mcb doesn't operate during normal conditions) but lower current rating than the cable used (so under fault conditions the circuit is switched off before the cable is thermally damaged. Remember, fuses and circuit breakers are mostly designed to protect the cables not the equipment connected to the electrical circuit

There are more factors to consider when selecting cable and circuit protection sizes and ratings such as:
The cables length.
How and where the cable is installed.
How many other cables your cable is bunched with.
The external earth loop impedance value.

Under 'fault conditions' a relatively small csa cable will survive a large fault current so long as the over current device disconnects the supply quickly

Even when everything is correctly sized and rated the conductors can still get surprisingly hot during heavy fault currents! Just not hot enough to cause damage to the cable.

JWs experiment is way more scientific than my attempt of just connecting bell wire across the mains: