Zs and circuit length

[dokkan1080]

One final thing to consider for unusually long cables are:

• If you are failing to meet the Zs for MCB disconnections times, or
• If you fail to meet 5% voltage drop at the MCB rating (even if it is OK at the fixed load's current that you intended)

They are both a warning signs that you need to consider the design in far more detail.

Basically you might use an RCD to mitigate the high Zs situation, but you also have to consider the risk of a L-N fault also failing to disconnect quickly (as that is not considered a fault by the RCD). So you need to make sure that the over-current aspect (i.e. MCB) will prevent a fire if the cable is overloaded for a long time due to a end-of-line short not resulting in enough current to trip the MCB quickly.

So if its long best bet is to just use a smaller mcb go from 6amp to a 4amp or a c type to a b type or use a bigger cable

 

[pc1966]

So if its long best bet is to just use a smaller mcb go from 6amp to a 4amp or a c type to a b type or use a bigger cable

That is the simplest / easiest option to avoid much more analysis. If the cable is already in then looking at a smaller MCB (if available for the distribution board) would be the simplest approach.

If designing from scratch then looking at "6A B-curve" as readily available and usually in the van as a spare would point to using a bigger cable, as here going from 1.5mm to 2.5mm is not a big cost factor.

 

[dokkan1080]

That is the simplest / easiest option to avoid much more analysis. If the cable is already in then looking at a smaller MCB (if available for the distribution board) would be the simplest approach.

If designing from scratch then looking at "6A B-curve" as readily available and usually in the van as a spare would point to using a bigger cable, as here going from 1.5mm to 2.5mm is not a big cost factor.

But what would count as an overly long circuit?
100m? 150m?

 

[pc1966]

But what would count as an overly long circuit?
100m? 150m?

One where Zs or VD on the chosen cable CSA, length, and MCB are not being met!

Actual length is immaterial.

Simpler answer is one that is longer than the values giving in the OSG for your situation.
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While an electrician should know how to do all of the calculations, etc, as needed, in practice the whole point of "standard circuits" such as the OSG tables is to avoid the effort and make for a quicker, safer, and cost-effective design process.

You look at the load(s) current requirement, look up a readily available cable/OCPD combination that should be OK for that, and see if your expected length is within the table's values of length & method. If there is nothing else unusual (hot zone, big bundle of cables to need more derating, high inrush currents, manufacturer specific requirements, etc) you don't even need a calculator as it is ready to be fitted!

 

[dokkan1080]

?

So of you was tncs type b 6 amp you are pretty much good up to 90 meter.

But if you was to drop to a 4 amp type b mcb you can go longer but as a rule you know 90 meters will comply at 6 amps

So its best to design a circuit to the breaker size and not your loads...?

 

[pc1966]

If it is not in the OSG table you need to go through the full design process really. But for that you also need to know what choices you have for cable and MCBs, etc, that are actually available to you when trying out combinations to see if they meet all of the goals.

MCB/RCBO in particular have to be approved by the DB/CU manufacturer to be sure they are safe (e.g. line up with the busbar, etc) so you need to check the DB make/model and then see what MCB/RCBO/etc are made for it first.
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Many sparks have one or two brands they always use as they know them, have had no problems in the past, can get them are a reasonable price, and usually can get any spares quickly as needed. So they would have a good idea of what they can get for a new design from memory.

If it is an existing design you generally have to work with whatever DB is installed (unless major upgrades are in order) and then you have to do your research.

 

[davesparks]

So if its long best bet is to just use a smaller mcb go from 6amp to a 4amp or a c type to a b type or use a bigger cable

No, whatever the length of the circuit you calculate what size cable is required for compliance.
You select the correct size and type of MCB for the circuit and calculate the cable size after that.