Wiring For A Guest Bungalow

[SamSipEt]

We will be building a small one bedroom bungalow to use as guest accommodation and I want to ensure that the electrics are done properly but would be the first to admit that I have no prior experience in this field. I have read a fair bit on the subject and think that the following should cover the requirements.

2 core 6mm cable from existing consumer unit in yellow plastic pipe buried about 40 cm under ground running for about 40 meters to another consumer unit serving the bungalow. The circuit breaker in the existing consumer unit (and the main one in the new unit) will be 32 amps.

The 5 circuit breakers in the bungalow will be

15 amps for internal lighting

15 amps for external lighting

20 amps for power sockets (6 double outlets)

20 amps for 3 ceiling fans

20 amps for small air-con unit for the bedroom.

N.B. no water heater – solar power!

Lighting cables will be 1.5mm and the rest will be 2.5mm. All cables will be 3 core with an earth rod at the bungalow.

I have no intention in doing the wiring myself but just want to make sure that the electrician will use the right materials for the job so any comments on the above would be gratefully received.

[Pink Mist]

That is very genorous with breaker size,

Personally would use 10amps for light 16 for power,

ceiling fans can go in with the lights, (fractional horsepower motors)

16 for the AC, use a split inverter

all breakers RCD's and you would comply with AS/NZS 3000:2000

[elkangorito]

Some suggestions you may consider;

1. the size of the electrical conduit should be 32mm diameter (to allow for heat dissipation).

2. if you use two 32 amp CB's (one supplying your sub-board from the Main board, one as the Main Switch for the sub-board), an overload may trip both CB's. It is normal practice to have a 'Discrimination Factor' of at least 1.2. That is, the upstream CB must have an overload trip current of at least 1.2 times the downstream CB. WARNING!!! Increasing the size of the upstream CB may exceed the current rating of the cable it supplies & therefore this cable will need to be upgraded.

The workaround for this is to use a "non-auto' CB (just a switch - no trip mechanism) as the Main Switch for the sub-board. In your case, I'd use a 63 amp switch.

3. I'd bury the cable at least 600mm beneath the road surface AND place 150mm of sand on top of the conduit followed by aggregate (Blue Metal or similar) for at least 300mm on top of the sand. The rest can be filled with soil etc.

4. Follow Bronco's advice about CB sizing etc.

[SamSipEt]

Thank you bronco and elkangorito for the advice and recommendations; I'll follow both

[chownah]

Why is having two circuit breakers trip simultaneously worse than having only one trip? Seems to me that it is obvious to see which breakers have tripped and that on the rare occasion that you will need to reset a breaker it seems like doing two of them is a minor inconvenience. Is the inconvenience of having to reset two breakers intead of one the only reason to use a discriminating factor of 1.2?

Also, how is using a switch better than using an over-rating on a circuit breaker? A switch is like a circuit breaker that does not trip....seems like the worst thing you could do.

Chownah

[elkangorito]

Why is having two circuit breakers trip simultaneously worse than having only one trip? Seems to me that it is obvious to see which breakers have tripped and that on the rare occasion that you will need to reset a breaker it seems like doing two of them is a minor inconvenience. Is the inconvenience of having to reset two breakers intead of one the only reason to use a discriminating factor of 1.2?

Also, how is using a switch better than using an over-rating on a circuit breaker? A switch is like a circuit breaker that does not trip....seems like the worst thing you could do.

Chownah

Hi Chownah.

There are 2 issues related to having 2 identical CB's configured as previously mentioned. They are

1) Redundancy & 2) Inconvenience.

If both CB's are in the same building/room, then it becomes the issue of redundancy only. In other words, why pay the money for 2 circuit breakers when you only need to buy 1. Also, 2 identical CB's acting in series, do not necessarily facilitate a further benefit than having 1 CB (see "cascading" below).

On the other hand, if the upstream CB is in a separate building, which can be locked, then it is obviously inconvenient to reset this CB (since the downstream CB will have also tripped).

Generally, it is engineering 'best practice' to discriminate between protective devices primarily to improve protection performance & reduce unnecessary cost. Downstream Main Switches (not CB's) can be used when the upstream supply cable is protected by a CB (as it is in this case) & all end sub-circuits are protected by their individual CB's.

If discrimination is used carefully between the 'same brand' CB's, "cascading" of these CB's can be used to limit fault current (should this be required).

At the end of the day, having 2 identical CB's will cost double the money when there is clearly no need for this. No extra protection is gained by this practice. A switch is generally cheaper than a CB.

[chownah]

Why is having two circuit breakers trip simultaneously worse than having only one trip? Seems to me that it is obvious to see which breakers have tripped and that on the rare occasion that you will need to reset a breaker it seems like doing two of them is a minor inconvenience. Is the inconvenience of having to reset two breakers intead of one the only reason to use a discriminating factor of 1.2?

Also, how is using a switch better than using an over-rating on a circuit breaker? A switch is like a circuit breaker that does not trip....seems like the worst thing you could do.

Chownah

Hi Chownah.

There are 2 issues related to having 2 identical CB's configured as previously mentioned. They are

1) Redundancy & 2) Inconvenience.

If both CB's are in the same building/room, then it becomes the issue of redundancy only. In other words, why pay the money for 2 circuit breakers when you only need to buy 1. Also, 2 identical CB's acting in series, do not necessarily facilitate a further benefit than having 1 CB (see "cascading" below).

On the other hand, if the upstream CB is in a separate building, which can be locked, then it is obviously inconvenient to reset this CB (since the downstream CB will have also tripped).

Generally, it is engineering 'best practice' to discriminate between protective devices primarily to improve protection performance & reduce unnecessary cost. Downstream Main Switches (not CB's) can be used when the upstream supply cable is protected by a CB (as it is in this case) & all end sub-circuits are protected by their individual CB's.

If discrimination is used carefully between the 'same brand' CB's, "cascading" of these CB's can be used to limit fault current (should this be required).

At the end of the day, having 2 identical CB's will cost double the money when there is clearly no need for this. No extra protection is gained by this practice. A switch is generally cheaper than a CB.

Would your method then be to bring some wires out of the supply panel straight off of the bus bars, not from a circuit breaker?.....and then connect these unprotected wires to a switch?.....would these wires have to be big enough to handle the current required to trip the main breaker?....in my house that would be 100 amps so the wires would have to be large cables.....I guess. I thought it was bad form to bring wired out of a supply panel without them being protected by a circuit breaker. Also it seems like more of a hassle than just popping in another circuit breaker (if you have room) and proceeding just like any other circuit. If you do your job correctly you might never trip that breaker...or its duplicate in the other building.

Chownah

[elkangorito]

Would your method then be to bring some wires out of the supply panel straight off of the bus bars, not from a circuit breaker?.....and then connect these unprotected wires to a switch?.....would these wires have to be big enough to handle the current required to trip the main breaker?....in my house that would be 100 amps so the wires would have to be large cables.....I guess. I thought it was bad form to bring wired out of a supply panel without them being protected by a circuit breaker. Also it seems like more of a hassle than just popping in another circuit breaker (if you have room) and proceeding just like any other circuit. If you do your job correctly you might never trip that breaker...or its duplicate in the other building.

Chownah

I think you have misunderstood what I said.

The "Upstream" consumer board is the board that will supply the "downstream" consumer board with power. Therefore the cables from the upstream board must be protected by a correctly sized CB. These cables will then become the supply cables for the downstream board & will be connected to the "line" side of a CB or switch within this board. This switch or CB will be connected to the main bus of the board & supply all the load CB's.

Generally, switches are cheaper than circuit breakers. In the situation outlined by the OP, a switch is all that is required in the downstream board, providing it is rated at least the size of the upstream supply CB.

"Doing your job correctly" will not stop a circuit breaker from tripping, because it is designed to trip under fault conditions. Subsequently, if a circuit breaker doesn't trip, it is because;

1. The CB is faulty.

2. A fault has not occured.

[chownah]

I think you have misunderstood what I said.

The "Upstream" consumer board is the board that will supply the "downstream" consumer board with power. Therefore the cables from the upstream board must be protected by a correctly sized CB. These cables will then become the supply cables for the downstream board & will be connected to the "line" side of a CB or switch within this board. This switch or CB will be connected to the main bus of the board & supply all the load CB's.

Generally, switches are cheaper than circuit breakers. In the situation outlined by the OP, a switch is all that is required in the downstream board, providing it is rated at least the size of the upstream supply CB.

"Doing your job correctly" will not stop a circuit breaker from tripping, because it is designed to trip under fault conditions. Subsequently, if a circuit breaker doesn't trip, it is because;

1. The CB is faulty.

2. A fault has not occured.

I did misunderstand but I think I'm getting closer.

I think you are saying that:

1. A cascading circuit breaker configuration has the disadvantage of costing more than a switch (assuming that installing a circuit breaker is more expensive than installing a switch) at the sub-panel and a circuit breaker at the main panel but it has the advantage of convenience in that a fault will only trip a circuit breaker in the building where the fault is located. With this option you must be careful to not use a circuit breaker that is rated at more than the buried cable can carry.

2. Two identical circuit breakers have the disadvantage of of costing more than a switch (assuming that installing a circuit breaker is more expensive than installing a switch) at the sub-panel and a circuit breaker at the main panel and it also will probably have the inconvenience (in the likely event that both circuit breakers trip when a fault occurs) of a tripped circuit breaker in a remote location (the other building).

3. A switch at the sub-panel as the main disconnect with a circuit breaker at the main panel has the advantage of the cheapest solution (assuming that installing a switch is cheaper than installing a circuit breaker) but it has the disadvantage that any trip will be in a remote location (the other building).

Is this right?

Chownah

[elkangorito]

I think you have misunderstood what I said.

The "Upstream" consumer board is the board that will supply the "downstream" consumer board with power. Therefore the cables from the upstream board must be protected by a correctly sized CB. These cables will then become the supply cables for the downstream board & will be connected to the "line" side of a CB or switch within this board. This switch or CB will be connected to the main bus of the board & supply all the load CB's.

Generally, switches are cheaper than circuit breakers. In the situation outlined by the OP, a switch is all that is required in the downstream board, providing it is rated at least the size of the upstream supply CB.

"Doing your job correctly" will not stop a circuit breaker from tripping, because it is designed to trip under fault conditions. Subsequently, if a circuit breaker doesn't trip, it is because;

1. The CB is faulty.

2. A fault has not occured.

I did misunderstand but I think I'm getting closer.

I think you are saying that:

1. A cascading circuit breaker configuration has the disadvantage of costing more than a switch (assuming that installing a circuit breaker is more expensive than installing a switch) at the sub-panel and a circuit breaker at the main panel but it has the advantage of convenience in that a fault will only trip a circuit breaker in the building where the fault is located. With this option you must be careful to not use a circuit breaker that is rated at more than the buried cable can carry.

2. Two identical circuit breakers have the disadvantage of of costing more than a switch (assuming that installing a circuit breaker is more expensive than installing a switch) at the sub-panel and a circuit breaker at the main panel and it also will probably have the inconvenience (in the likely event that both circuit breakers trip when a fault occurs) of a tripped circuit breaker in a remote location (the other building).

3. A switch at the sub-panel as the main disconnect with a circuit breaker at the main panel has the advantage of the cheapest solution (assuming that installing a switch is cheaper than installing a circuit breaker) but it has the disadvantage that any trip will be in a remote location (the other building).

Is this right?

Chownah

Very bloody close

In part 3. of your reply, you said, "...but it has the disadvantage that any trip will be in a remote location (the other building)." This is not entirely true. The upstream CB (in the Main Board) should only trip on a very large fault if the time/current curves on the downstream CB's are slower than the time/current curve on the upstream CB. This is why discrimination & "using like brands" of CB's is so important. It's all to do with "time versus current" & "fault current" (not so much about overload current).

[chownah]

I think you have misunderstood what I said.

The "Upstream" consumer board is the board that will supply the "downstream" consumer board with power. Therefore the cables from the upstream board must be protected by a correctly sized CB. These cables will then become the supply cables for the downstream board & will be connected to the "line" side of a CB or switch within this board. This switch or CB will be connected to the main bus of the board & supply all the load CB's.

Generally, switches are cheaper than circuit breakers. In the situation outlined by the OP, a switch is all that is required in the downstream board, providing it is rated at least the size of the upstream supply CB.

"Doing your job correctly" will not stop a circuit breaker from tripping, because it is designed to trip under fault conditions. Subsequently, if a circuit breaker doesn't trip, it is because;

1. The CB is faulty.

2. A fault has not occured.

I did misunderstand but I think I'm getting closer.

I think you are saying that:

1. A cascading circuit breaker configuration has the disadvantage of costing more than a switch (assuming that installing a circuit breaker is more expensive than installing a switch) at the sub-panel and a circuit breaker at the main panel but it has the advantage of convenience in that a fault will only trip a circuit breaker in the building where the fault is located. With this option you must be careful to not use a circuit breaker that is rated at more than the buried cable can carry.

2. Two identical circuit breakers have the disadvantage of of costing more than a switch (assuming that installing a circuit breaker is more expensive than installing a switch) at the sub-panel and a circuit breaker at the main panel and it also will probably have the inconvenience (in the likely event that both circuit breakers trip when a fault occurs) of a tripped circuit breaker in a remote location (the other building).

3. A switch at the sub-panel as the main disconnect with a circuit breaker at the main panel has the advantage of the cheapest solution (assuming that installing a switch is cheaper than installing a circuit breaker) but it has the disadvantage that any trip will be in a remote location (the other building).

Is this right?

Chownah

Very bloody close

In part 3. of your reply, you said, "...but it has the disadvantage that any trip will be in a remote location (the other building)." This is not entirely true. The upstream CB (in the Main Board) should only trip on a very large fault if the time/current curves on the downstream CB's are slower than the time/current curve on the upstream CB. This is why discrimination & "using like brands" of CB's is so important. It's all to do with "time versus current" & "fault current" (not so much about overload current).

I think I see. In part 3, the sub-panel has a switch for a disconnect and this sub-panel supplies a few circuits the largest of which is protected by a 20 amp circuit breaker. Since it is smaller than the 32 amp circuit breaker in the main panel (by more than a factor of 1.2 which is the criteria for eliminating the double trip) a fault will almost assuredly trip the local circuit breaker (the 20 amp one in the sub-panel) and not trip to remote one (the 32 amp in the main panel).

Is that it?

You wrote: "the upstream CB must have an overload trip current of at least 1.2 times the downstream CB. " Does this mean that a 20 amp breaker in the sub-panel would require a minimum of a 24 amp breaker in the main panel? Or is the "overload trip current" a different specification?

Chownah

Chownah

[elkangorito]

I think you have misunderstood what I said.

The "Upstream" consumer board is the board that will supply the "downstream" consumer board with power. Therefore the cables from the upstream board must be protected by a correctly sized CB. These cables will then become the supply cables for the downstream board & will be connected to the "line" side of a CB or switch within this board. This switch or CB will be connected to the main bus of the board & supply all the load CB's.

Generally, switches are cheaper than circuit breakers. In the situation outlined by the OP, a switch is all that is required in the downstream board, providing it is rated at least the size of the upstream supply CB.

"Doing your job correctly" will not stop a circuit breaker from tripping, because it is designed to trip under fault conditions. Subsequently, if a circuit breaker doesn't trip, it is because;

1. The CB is faulty.

2. A fault has not occured.

I did misunderstand but I think I'm getting closer.

I think you are saying that:

1. A cascading circuit breaker configuration has the disadvantage of costing more than a switch (assuming that installing a circuit breaker is more expensive than installing a switch) at the sub-panel and a circuit breaker at the main panel but it has the advantage of convenience in that a fault will only trip a circuit breaker in the building where the fault is located. With this option you must be careful to not use a circuit breaker that is rated at more than the buried cable can carry.

2. Two identical circuit breakers have the disadvantage of of costing more than a switch (assuming that installing a circuit breaker is more expensive than installing a switch) at the sub-panel and a circuit breaker at the main panel and it also will probably have the inconvenience (in the likely event that both circuit breakers trip when a fault occurs) of a tripped circuit breaker in a remote location (the other building).

3. A switch at the sub-panel as the main disconnect with a circuit breaker at the main panel has the advantage of the cheapest solution (assuming that installing a switch is cheaper than installing a circuit breaker) but it has the disadvantage that any trip will be in a remote location (the other building).

Is this right?

Chownah

Very bloody close

In part 3. of your reply, you said, "...but it has the disadvantage that any trip will be in a remote location (the other building)." This is not entirely true. The upstream CB (in the Main Board) should only trip on a very large fault if the time/current curves on the downstream CB's are slower than the time/current curve on the upstream CB. This is why discrimination & "using like brands" of CB's is so important. It's all to do with "time versus current" & "fault current" (not so much about overload current).

I think I see. In part 3, the sub-panel has a switch for a disconnect and this sub-panel supplies a few circuits the largest of which is protected by a 20 amp circuit breaker. Since it is smaller than the 32 amp circuit breaker in the main panel (by more than a factor of 1.2 which is the criteria for eliminating the double trip) a fault will almost assuredly trip the local circuit breaker (the 20 amp one in the sub-panel) and not trip to remote one (the 32 amp in the main panel).

Is that it? Yep...that's it.

You wrote: "the upstream CB must have an overload trip current of at least 1.2 times the downstream CB. " Does this mean that a 20 amp breaker in the sub-panel would require a minimum of a 24 amp breaker in the main panel? Well, yes but you will not be able to buy a 24 amp CB. You may be able to buy a 25 amp CB & you certainly will be able to buy a 32 amp CB. Or is the "overload trip current" a different specification? To properly address the discrimination of 2 CB's, the time/current curves must be used. Generally, to avoid nuisance "overload" trips, use the minimum discrimination factor of 1.2. To avoid nuisance "fault current" trips, the discrimination may need to be determined by consulting the time/current curves for the CB's. Sometimes, nuisance "fault current" tripping of a series CB cannot be avoided.

Chownah

[ThailandTony]

I sure know where to post my electrical concept when I have it....you guys give excellent advice selflessly....expext a pm from me when my electrical concept is finalised.

Chok Dee