Cables Connected in Parallel

Not a serious issue, But If smaller wire out of two can reach to breaker with out trouble, Then why not to cut longer one or make both equal, Equal means equal for practical purpose.Not the equal of an over killer.

What do you mean by very small? are you trying to control them in mm?

If you're talking 50/60Hz, I'd say it depends on total length of conductors and total difference in length.

Unless the difference is on the order of 10%, or more, I don't think it would matter.

Usually you are going to work at a current much lesser than their maximum capacity.

The Length is of course directly proportional to the impedance (with a bit of approximation) and Hence the currents sharing would be inversely proportional. So it would boil down to a sharing inequality a bit just less than the length inequality.

It really may not matter under normal circumstances. Afterall there are other things that may give rise to a bit of inequality too.

you shouldnt have any trouble but need to be aware that if you have doubled up the cable to meet current carrying capacity of the device and have a break\failure in one wire your other one will overheat causing you any number of problems and any circuit protection may not work due to undersized conductors.

It is more-or-less standard domestic electrical practice to run a ring circuit from a 32A breaker to a number of socket outlets using 2.5mm² twin-and-earth cable for a floor area not exceeding 100m², and this is covered in British Standard 7671. So there wouldn't appear to be an issue as it is hardly trail-blazing stuff.

"So there wouldn't appear to be an issue as it is hardly trail-blazing stuff".

It's been in use since 1942! And still going strong (until the EU mucks things up again).

As a current divider, each cable will carry it's share of the load proportionate to their individual resistance. R total = R1XR2/(R1+R2). If the length (resistance) difference is say 10% the sharing will be 10% more carried by the shorter (lower resistance) wire, assuming of course both wires are of the same gauge (conduction diameter).

The overall impedance will not change due to unequal sharing of current. But yes, the difference in length will result in an unequal amount of current through each cable since resistance is based on the length. However, this difference should be minimal if the difference in lengths are "very small".

This question will be understood better by doing home work by you. I will provide you the data and equation to do home work.

Cable size is mentioned 16mm² which indicates you are using European cables and not American.

Let the distance between 220V source (Panel board) and load (Device-heater) be 50 meter. Load electrical data is 220V, 30Amps, 50Hz, 1-phase (Phase to Neutral) and unity power factor (pure resistive load or heater). Heater load is used here to minimize complex calculations due to unity power factor.

Owing to some reasons you got two 16mm² cables of each lengths as 55meter(cable-1) and 60(cable-2). The two cables are connected in parallel to electrical source (Panel board) and Load (heater).

Electrical data of the cables from the cable manufacturer are as follows:

Cable-1(55meter) impedance: Z1 = 0.0792+j0.00728 Ohms

Cable-2(60meter) impedance: Z2 = 0.0864+j0.0079 Ohms

Cable -Ideal (50meter) impedance ZI = 0.072+j0.00662Ohms

To make further simplification, we use IEE code which advices to drop reactive Ohms up 16mm² cable due to Resistance(R) value >10 *Reactance(X) value

Therefore cable impedance (Z) becomes as resistance(R) and as follows:

Z1=R1= 0.0792Ohms; Z2=R2= 0.0864Ohms; ZI=RI=0.072Ohms

Therefore the formulas are as given below and substitute above resistance values :

Effective impedance of the two unequal length parallel cables in Ohms= R1*R2/(R1+R2) ----1

In ideal case( two equal & exact length 50meters) effective impedance = RI/2 .......…2

By comparing Equation-1 and equation-2, you will get your answer. Similarly you can do other calculation such as current sharing, power loss in each cable and so and so forth.

I want to give you a practical advice that don't connect small size cables into parallel to load, if next higher size cables having double cross section is available. Always try to use single cable to load. In your case, it is better to use a cable size of 25mm² or 35mm² instead of 2*16mm² and check which cable gives a. % voltage drop as acceptable to local code and b. cable current rating meets the load current.

Mushtaq Hussain-INDIA