40A MCB will trip, but 63A won't: Is this logic even logical?

[Wrichik Basu]

Let's get the basics out of the way first. We receive 220-230V RMS mains @ 50Hz, and have a maximum sanctioned load of 60A. The current load, though, is 32A. And we have a fuse of 32A just after the meter.

We have planned some expansions and need a new line to be drawn from the main meter for a new power outlet. Which means upgrading the fuse to an MCB. The issue starts here: I wanted to upgrade to a 63A MCB directly. At the moment, we won't have a full load of 63A, but why not keep something upgraded for the future if it is feasible? The electrician, however, is adamant that a 63A MCB will be detrimental as it won't trip, and is pushing for a 40A MCB instead.

My theory is, say, we have a full load of 63A. Which means we have the ability to run three air conditioners simultaneously, or two air conditioners and one microwave oven at full power. But that does not mean that we will have 63A being drawn throughout the year. In winter, there will be no air conditioners, so usage will anyway drop to around 40A. So if we have a short circuit in winter, the 63A MCB won't trip? That doesn't sound logical to me.

Should I push back and ask for a 63A MCB instead, or agree to the electrician and keep the 40A?

One thing to note, though, is that here in my country, electricians do not have certificates or licences. You can get into the trade by starting as an assistant, and then move out and have a new shop and get clients. Nobody follows codes (well, for low tension work, there are no codes other than colour codes, and these guys don't even follow that).

 

[DaveC426913]

One thing to note, though, is that here in my country, electricians do not have certificates or licences. You can get into the trade by starting as an assistant, and then move out and have a new shop and get clients. Nobody follows codes (well, for low tension work, there are no codes other than colour codes, and these guys don't even follow that).

I'd say your best solution is to move to any other country.

 

[Averagesupernova]

Your electrician is clueless. The whole idea is that the MCB does not trip, UNLESS IT NEEDS TO.

 

[DaveC426913]

Your electrician is clueless. The whole idea is that the MCB does not trip, UNLESS IT NEEDS TO.

Right but isn't the breaker's job to protect the rest of the circuit?

i.e. if the rest of the circuit risks overheating at, say, 50A, then the 63A won't trip when it needs to.

(This is not my wheelhouse, so I am talking through my hat.)

 

[Averagesupernova]

Right but isn't the breaker's job to protect the rest of the circuit?

i.e. if the rest of the circuit risks overheating at, say, 50A, then the 63A won't trip when it needs to.

(This is not my wheelhouse, so I am talking through my hat.)

Ok. I see I misread the first post. Doesn't change much though. If the electrician means that the conductors upstream are not adequate for a 63 amp breaker he should simply say so. From what I've read in the past concerning things where @Wrichik Basu lives it's hard to say if the conductors upstream are even adequate now.

 

[Wrichik Basu]

if the rest of the circuit risks overheating at, say, 50A, then the 63A won't trip when it needs to.

Do MCBs work like that? Afaik, MCBs trip when the current drawn increases by three or four times the rated value. The exact "number of times" is determined by the curve of the MCB. Here, we have a C-curve MCB.

 

[Wrichik Basu]

Ok. I see I misread the first post. Doesn't change much though. If the electrician means that the conductors upstream are not adequate for a 63 amp breaker he should simply say so. From what I've read in the past concerning things where @Wrichik Basu lives it's hard to say if the conductors upstream are even adequate now.

We are changing the wires from the fuse board inside the house to the MCB downstairs. The new set of wires will be 6mm² for the entire house (including the new outlet), and a separate 4mm² for the air conditioner. The air conditioner line will have an additional 20A MCB inside the house; electrician said it's an extra safety and I felt that's fine.

 

[Averagesupernova]

6mm wires is the equivalent of 10 AWG according to the chart that popped up with a Google search. AWG is what I am familiar with. 10 AWG would not be protected at more than 30 amp in the US.

 

[Rive]

The electrician, however, is adamant that a 63A MCB will be detrimental as it won't trip, and is pushing for a 40A MCB instead.

63A is awful lot. Entirely in the industrial range (at the bottom of it, but still...) Insane, if you ask me.
It's indeed possible that smaller (through thinner, more common wires) shorts or rather: overloads just won't trip it (before the wires burns down, that is).

What you need is a segmented setup, with a bunch of smaller breakers for separate (type and place and wire) loads.
That's what I would do. (I mean, what I would make the electrician do )

 

[DaveE]

In the US, overcurrent protection in a facility is designed to prevent fires in the downstream distribution wires. That is all. Not the upstream wiring and not the connected equipment*. As such, the only thing you should need to know is the size (ampacity) of the downstream wiring, the applicable building codes and/or the circuit breaker trip curves. Undersized breakers are feasible, but can result in nuisance tripping. They also have no real benefit since a well designed system should support the higher loads.

If you live in a country that doesn't have well defined and implemented building codes, then I don't see how you can reliably do anything except do your best to mimic countries that do.

Anyway, hire a real electrician, even if it's more expensive. They should be able to clearly explain why they choose what they do, even if it's just "the rules say I have to". Tread carefully, fires can kill people.

*Of course you might have special purpose circuits that are designed to protect unique connected loads. It never makes sense to try to protect the upstream wiring. Also GFCI (RCD) protection exists, but is out of the scope of this thread.

 

[Averagesupernova]

In the US, overcurrent protection in a facility is designed to prevent fires in the downstream distribution wires.

In branch circuit wiring yes this is the case. However, it is not entirely true that in all cases the fuse or breaker is not meant to protect upstream wire. Obviously it cannot protect against a short upstream but it can protect the wire upstream against a short that is downstream. In other words, it is assumed that the short will seldom occur upstream. Look up tap rules in the NEC. They would not exist as they do if it didn't make sense.
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Edit: As far as hiring a 'real' electrician, this is likely as good as it gets where the OP resides.

 

[Averagesupernova]

I also would like to add that dedicated circuits such as for a permanently connected central air conditioning unit manufacturers installation instructions will state a circuit breaker size larger than the normal size used to protect that specific size wire. For instance a 40 amp breaker on a number 10 AWG wire. The reason for this is the breaker is relied upon to interrupt current during a short circuit event. This type of event is generally of a duration short enough as to not damage the wire. The device in the air conditioner unit is relied upon to protect against overload. This way motor start current is less likely to trip a breaker in the main panel. Of course, there is never anything wrong with over sizing the wire feeding the unit as long as it is not too large to work with in the space allotted or to make terminations. So, my comment about protecting a number 10 wire with nothing larger than 30 amps was a bit incorrect. @russ_watters I am sure you are familiar with what I've said in this post.

 

[russ_watters]

@russ_watters I am sure you are familiar with what I've said in this post.

Somewhat, but I'm an ME, not an EE so I just pass the unit data to the electrical department. I'm aware units/motors have their own overload protection but I'm not up on the sizing.

 

[Wrichik Basu]

6mm wires is the equivalent of 10 AWG according to the chart that popped up with a Google search. AWG is what I am familiar with. 10 AWG would not be protected at more than 30 amp in the US.

See, this is why I get frustrated. There's not a proper chart that says how much capacity a wire of a certain cross-section is. Yesterday, I was told that 1.5mm² can carry 6-10A and 2.5mm² supports >16-25A. And today I was told that 4mm² can carry up to 40A.

63A is awful lot. Entirely in the industrial range (at the bottom of it, but still...) Insane, if you ask me.
It's indeed possible that smaller (through thinner, more common wires) shorts or rather: overloads just won't trip it (before the wires burns down, that is)

Ok, that makes sense.

What you need is a segmented setup, with a bunch of smaller breakers for separate (type and place and wire) loads.
That's what I would do. (I mean, what I would make do by the electrician )

Yeah, we do have something like that. From the meter, there's one large fuse of 32A, then individual 16A fuses for each room/section. The air conditioner is currently directly connected to the 32A fuse; that will be separated out into a 20A MCB as we will be including the new outlet in its place. The 32A fuse will be replaced by a 40A DP MCB. We also have plans to replace each of those 16A fuses with 16A SP MCBs, but that will take time.

 

[Wrichik Basu]

Can anyone please link me to a reliable chart where I can look up the max capacities of wires? Can be w.r.t. SWG or AWG; I can convert mm² to SWG to AWG.

 

[DaveE]

...dedicated circuits such as for a permanently connected central air conditioning unit manufacturers installation instructions will state a circuit breaker size larger than the normal size used to protect that specific size wire.

Yes, we did exactly this with our large ion lasers (3Φ, 10 - 50KW). Our installation requirements were for a dedicated branch with a circuit breaker, local disconnect, plus our supplied, and pretty short, hard service cable to the power supply (plus a whole bunch of other custom expensive things). But the facility wiring wasn't really effected. I wouldn't consider that like a part of facility wiring since it had load specific requirements. Other configurations required somebody's explicit approval. Outside of the big countries, deviations weren't unusual, which sometimes was a real PITA for our field service people.

 

[DaveE]

Can anyone please link me to a reliable chart where I can look up the max capacities of wires? Can be w.r.t. SWG or AWG; I can convert mm² to SWG to AWG.

There are many, depending on the country. They are also much more complex than most people realize. You have to read the fine print and choose the right table or derating, etc.

Here's one from the US: NEC NFPA 70E Table 310.16

 

[DaveE]

https://www.nfpa.org/customer-support/how-can-i-access-nfpa-codes-and-standards-for-free

 

[Averagesupernova]

Realize that the ampacity of the wire is dependent upon the termination. If the termination is rated for 90 degrees, then you can use that column in the chart providing there isn't another rule that prohibits it. This is very seldom. Also, the table does not concern itself with voltage drop. So just because you have a setup with 90 degrees termination on each end and the wire itself is rated for that does not mean you will not have unacceptable voltage drop.

 

[DaveE]

See, this is why I get frustrated. There's not a proper chart that says how much capacity a wire of a certain cross-section is. Yesterday, I was told that 1.5mm2 can carry 6-10A and 2.5mm2 supports >16-25A. And today I was told that 4mm2 can carry up to 40A.

It is frustrating. Ampacity basically depends on the temperature rating of the insulation. This means that in addition to the type of insulation it depends on the thermal situation in the application. Things like the ambient temperature, confinement, and (most importantly) other heat sources nearby, which is usually another wire running next to it. What's around the wire is often as important as the size of the wire. So the table is useful, but it's really a whole section of the code books that you have to read to make sure you're using the table data correctly. The tables are the normal, simple, cases and are a basis for other deratings. It's a lot of work to do it right, which is why the building industry defaults to standard solutions that someone else figured out long ago. Fortunately they are a bit conservative, so you can guess a little bit, if you understand the issues.

So, for example, if you are getting UL, CE, CSA safety approvals for a product (not building codes), they will ask about wire sizes, but that's just so they know where to measure temperatures of the insulation in what they think is the worst case during product testing.