Requirements to power a shed

[JohnW2]

Maybe there's a field with a drop-down list to select installation method, next to the field with the drop-down list to select cable type.

You know you could be right

He was right - apologies for missing that!

Kind Regards, John.

 

[bernardgreen]

I tend to think that calculator can give mis leading results. As it says

NO guarantee as to the accuracy of these values is given.
And they should be checked against some other source.

Two "oddities " are where cables from a CU are bunched as they leave the CU before separating in different directions.

The voltage drop calculation seems to treat the entire cable run as being bunched and applies that voltage drop per amp per metre value to the whole cable run. One metre of bunching on a 15 metre long circuit will only have one metre of bunched voltage drop and 14 metres of un-unbunched voltage drop. Provided that the maximum safe carrying capacity of the cable when bunched is not exceeded a smaller cable than offered in the results will quite likely be within the voltage drop limitation.

With bunched cables in terms of maximum safe current carrying capacity cannot diversity be better applied. If one circuit is carrying less than 30 % of its rated current then it seems this cable can be ignored in the bunch calculation. What is the situation if the sum of all the maximums of all the cables in the bunch is more than the maximum possible limit set by the incoming main supply fuse.

 

[JohnW2]

The load is determined by the breaker on the supply. If you limit it to a 10amp 30ma Rcbo You are controling it to a designated 1500w sub circuit. for a specific use.

Your arithmetical error aside, the size of the 'breaker' is (as required by the regs) dictated by the design current of the circuit. If you had a 'sub circuit, for a specific use' to which you had a hard-wired load drawing 10A or less, then your design current would be 10A or less and the 10A 'breaker' would be fine. However, the moment you have one or more 13A sockets at the end of the cable, it would IMO be a very dubious design practice to say that the design current for the circuit (and hence 'breaker' rating) was only 10A.

Kind Regards, John.

 

[JohnW2]

The voltage drop calculation seems to treat the entire cable run as being bunched and applies that voltage drop per amp per metre value to the whole cable run. One metre of bunching on a 15 metre long circuit will only have one metre of bunched voltage drop and 14 metres of un-unbunched voltage drop. Provided that the maximum safe carrying capacity of the cable when bunched is not exceeded a smaller cable than offered in the results will quite likely be within the voltage drop limitation.

Indeed. I strongly suspect that most people would ignore that one metre of 'bunching' for the purpose of a VD calculation. Having said that, I think that many of the 'calculators' around (seemingly not Alarm's one) simply consider the worst-case scenario of the cable operating at it's maximum permitted operating temperature, and hence over-estimate VD (which is essentially what the tabulated VD figures in the regs do).

With bunched cables in terms of maximum safe current carrying capacity cannot diversity be better applied. If one circuit is carrying less than 30 % of its rated current then it seems this cable can be ignored in the bunch calculation.

I assume that the assumption is that all the bunched cables equilibrate to the same temperature. As you say, diversity is certainly an issue, but a formal calculation would be very tedious, and would require a lot of assumptions. If one did get down to calcs from basic principles, I suspect one would find that the 'bunching factors'(and some of the other 'factors') in the regs are very conservative ('worst case scenarios' etc.).

Kind Regards, John.

 

[Alarm]

Lads, what did I post earlier?

Look very carefully at this at the top

Calculator for General Guidance Only

 

[ban-all-sheds]

The load is determined by the breaker on the supply.

Err, no - the load is what the load is, and In must be greater than it.

It is the load which determines the breaker size, not the other way around.

If you put a 32amp breaker on, as a radial circuit, you will end up with big cable sizes, to combat voltage drop.

Voltage drop is determined by the actual current, not by the breaker rating.

Do you know anything about circuit design?

Ask them to quote the IEE reg that supports their theory. That should keep them quiet.

What theory?