Reference method A & B current rating

Gardner · Apr 19, 2015

Using IEC reference method A and B for loose wires in conduit a I have derived the following max OCPD and in turn maximum current loading for general use circuits:

1.5mm2: 10 amp MCB reference method A, 16 amp MCB reference method B

2.5mm2: 20 amp MCB (Both A & B)

4mm2: 25 amp MCB reference method A; 32 amp MCB Reference B

6mm2: 32 amp MCB reference method A, 40 amp MCB reference method B

10mm2: 40 amp MCB reference method A, 50 amp MCB reference method B

16mm2: 63 amp MCB reference method A, 70 amp MCB reference method B

Do my numbers sound correct? I may have rounded a few numbers in method B, so A would probably be the best approach. Just double checking.

davesparks · Apr 19, 2015

You've gone about this backwards, you need to select your OCPD which is suitable for the load and then you calculate the cable size required from that.

Gardner · Apr 19, 2015

Does this mean the OCPD determines the current or the load? I may need an example.

davesparks · Apr 19, 2015

Gardner said:
Does this mean the OCPD determines the current or the load? I may need an example.

You work out the design current, which is the current required by the load.
Then you select an OCPD which is equal to or higher than this current, this is the nominal current.
You then apply the cable rating factors to the nominal current which gives you the minimum permissible tabulated current carrying capacity of the cable.
You then select a cable size from the appropriate table which has a current carrying capacity equal to or greater than the required minimum.
You then use the tabulated voltage drop to confirm whether this will comply.

telectrix · Apr 19, 2015

Ib<In<Iz is your starter.

shanky887614 · Apr 19, 2015

deleted

Plonker 3 · Apr 19, 2015

shanky887614 said:
i would recommend the onsite guide or the electricians guide to the wiring regs

either one will help you in your calcs (you will use the onsite guide in your am2 eventually so it is useful to get familiar with it)

Do they have to do the AM2 in the USA?

Gardner · Apr 19, 2015

davesparks said:
You work out the design current, which is the current required by the load.
Then you select an OCPD which is equal to or higher than this current, this is the nominal current.
You then apply the cable rating factors to the nominal current which gives you the minimum permissible tabulated current carrying capacity of the cable.
You then select a cable size from the appropriate table which has a current carrying capacity equal to or greater than the required minimum.
You then use the tabulated voltage drop to confirm whether this will comply.

Ok so:

A 5kw imersion heater, 5000/230=22amps. This would equate to a 25 amp MCB being the next standard size up. Thus my nominal current is not the load current but rather my MCB rating, so I would examine my installation method and base that off of 25 amps. Assuming 3 wires (L+N+E) in conduit in a thermall insualted wall with the conduit touching the inner surface I would use IEC refernce A giving me 4mm2 copper wire.

Assuming for example a 15 meter run and using V= {2I (Rc cos + Xc sin) L}/1000 I get a voltage drop of about 4 volts of 1.6%, which as long as it does not exceed 5% this is ok?

Sound about right?

And my earth fault loop impedance calculation assumes a conductor temprature of 70*Celcuis?

shanky887614 · Apr 19, 2015

deleted

Gardner · Apr 19, 2015

telectrix said:
Ib<In<Iz is your starter.

Load, breaker wire? Im confused lol.

shanky887614 · Apr 20, 2015

Gardner said:
Load, breaker wire? Im confused lol.

[ElectriciansForums.net] Reference method A & B current rating

happysteve · Apr 20, 2015

Gardner said:
Ok so:

A 5kw imersion heater, 5000/230=22amps. This would equate to a 25 amp MCB being the next standard size up. Thus my nominal current is not the load current but rather my MCB rating, so I would examine my installation method and base that off of 25 amps. Assuming 3 wires (L+N+E) in conduit in a thermall insualted wall with the conduit touching the inner surface I would use IEC refernce A giving me 4mm2 copper wire.

Assuming for example a 15 meter run and using V= {2I (Rc cos + Xc sin) L}/1000 I get a voltage drop of about 4 volts of 1.6%, which as long as it does not exceed 5% this is ok?

Sound about right?

And my earth fault loop impedance calculation assumes a conductor temprature of 70*Celcuis?

Sounds generally good to me, some minor notes (which don't affect the overall outcome):

(1) 25A MCBs are not common, you might struggle to find one, generally your choice is 20A or 32A

(2) The design is for a fixed load, and so overload protection is not required. The load on the circuit will not increase unless the heater is replaced by one having a higher rating, in which case you'll need to check the adequacy of the circuit. The OCPD in this case is for protection against fault currents only. Hence it is your design current, Ib (=22A), not the rated current of the protective device, In (=25A, or more realistically, 32A) that you use, so:

It >= Ib/(Ca Cg Cs Cd Ci Cc)

You would still end up with the same CSA (=4mm2)

(3) In determining voltage drop, it is always the design current (rather than the rated current of the protective device) that you use. From table 4D1A of BS 7671, VD = 11mV/A/m (ref A&B, 2 cable, single phase ac) = 11 x 22 x 15 = 3.63V, which as you say is fine.

(4) You mention "3 wires (L + N + E)", for the sake of clarity, the "E" wire is not "counted" when looking up the number of wires in the table. The current-carrying cables, the live ones, are the ones that count.

Gardner · Apr 20, 2015

Ok thanks!

Over load proection is not required for all fixed in place applainces some are exampt? So In theory it is ok to have a higher MCB relative to the current carrying capacity of the wire provided the load does not exceed the capcity of the wire and the breaker is only upsized because the next required size not avaliable? However anything to a socket must have over load protection so the current carry capcity must be equal to or greater than the MCB?

Good point on not counting the E. It never should be. My understaning, and correct me if I am wrong, is that in a 3P+N circuit the neutral is not counted as a current carry conductor for de-rating unless the load in none linear in wave form?

happysteve · Apr 20, 2015

Regarding your first paragraph, basically, yes... although it's not that some are "exempt", it's just that if it's a fixed load, you know what the load is. If there are socket-outlets, then you don't know what the load is (depends on what's plugged in).

Regarding your second point, you've got that spot on. Third order harmonics (and higher), even in a balanced 3P system, mean you'll get a neutral current and this needs to be taken into account in the rating factor.

Gardner · Apr 20, 2015

But this openly says that someone may replace a shower with a higher load? Or its determined that only qualified personal will be replacing the shower?