# Voltage drop

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#### aaelectric

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Hi
i keep getting confused when trying to work out voltage drop please can some one point me in the right direction.
63amp boiler supply 25 meter run in plastic trunking fixed to wall. Will 16mm twin and earth be sufficent and can someone show the calculation cheers

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K

How far have you got with the calculations?

#### telectrix

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agreed. give us your thoughts and we can the tell if you're doing it right. not much use having it on a plate.

S

#### Silly Sausage

Hint...
look at Table 4D5. You know the current, you know the length of cable and csa of the cable...

A

#### aaelectric

Mv/a/mxLxIb%1000

2.8x25x601000=4.2 percent 5% max
is that ok.

#### Richard Burns

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Well you have used 60A as the design current and stated that it is a 63A boiler, but the 63A may just be the protective device and if the boiler is actually only taking 60A you should use that as the design current.

So based on your calculation using 60A as the actual usage from the boiler then you have correctly undertaken the calculation.
As you have stated the maximum volt drop for a power circuit is 5% of the supply voltage, assuming you have a supply of 230V (nominal) then the maximum volt drop permissible in the absence of any manufacturers instructions is (5%of 230V) 11.5V.
You calculated volt drop is 4.2V (1.826% of 230V) and so is well within the limits.

Good work.

A

#### aaelectric

Now how can i work out min earth by adiabiatic equation please i have never been able to get my head around it tns system

#### Richard Burns

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S=(√I²t)/k
S is the csa of the earthing conductor/cpc that you need to find
I is the prospective fault current at that point in in the circuit (normally PSC for the installation)
t is the disconnection time for the protective device you are using at the fault current I, if this is a new installation and the device is a BSEN60898 MCB and the fault current is higher than the "instantaneous" trip time shown in appendix three of the BGB (5x the MCB rating for type B MCBs) then this value is 0.1s (though the actual trip time may be quicker)
k is the correction factor for the material of the conductor, since in domestic situations the conductor is invariably copper then you only need to consider if the conductor is enclose din a cable or not. If it is in a cable then the value of k is 115. If it is not in cable then the value of K is 143.
The csa of the earthing conductor in a TNCS system should be selected from table 54.8 as it is also a bonding conductor, the csa of a cpc that is not incorporated in cable and not mechanically protected must be at least 4mm².

The value S that is obtained is increased to the next available size of conductor, so if you get a value of 3.4 then the conductor would need to be 4mm² minimum.

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