The basic circuit of a (1/4 wave resonant) Tesla Coil is shown above. It is not too complicated, unlike its bigger brother the Tesla Magnifier.

Below is a diagram of the real-world wiring of this circuit, with the addition of a strike rail (see 'Primary Coil' section for details). The HV supply comes from the NST and the tank cap is a MMC in this instance.


Wire Types and Sizes

The type and size of wire used in a Tesla Coil system should be chosen to match the specific needs of each part of the circuit. The system can be split into four sections:


The Control section uses correctly rated mains cable.
Typical cables are 'E' & 'D' (see below).

The Charge section is high voltage and low current.
Suitable cables are 'A' & 'B'.

The Primary section is high voltage and high current.
Suitable cable is 'F'.

The Secondary section. The RF ground connection is low voltage and high current.
Suitable cable is 'F'.

A bare single strand (1.0 sq mm)
B EHT cable 25KVDC
C twin and earth (2.5 sq mm)
D twin and earth (1.0 sq mm)
E 3 core flex 16A (1.5 sq mm)
F car battery lead (overall diameter 8mm)



Because of the high voltages found in a Charge & Primary sections it seems natural to look for cable with high voltage ratings, such as neon sign cable, medical X-ray equipment cable & car engine spark plug leads. The first two would be fine for connecting the NST to the spark gap but the spark plug leads are useless (too high a resistance). However, you don't need to spend lots of money on fancy HV cable because all sorts of ordinary wire would work too. All you need to do is think about insulation.


For a circuit to work you need wires to conduct the electricity between the components. A bare piece of wire will do this perfectly but you now have the problem of wires accidentally touching and shorting out. Depending on the voltage being used you may also have the added risk of electrocution! Insulation is used to solve both these problems but for high voltages, like in a Tesla Coil, the cost of suitably insulated cable is high.

The solution is to use ordinary low voltage cable, or bare wire, and use air as your insulatior (just like the big boys do with the National Power Grid!). All you need to do is to secure the wires in such a way that they cannot short against anything conductive and are at a safe distance from operator's hands.


All conductors, except for 'superconductors', resist the flow of current through them. The value of this resistance is determined by the material used for the conductor and by its size.

As the cross-sectional area 'a' gets smaller, or the length of the wire 'l' gets longer, the resistance 'R' increases.The higher the resistance the more electrical energy is used just in fighting against it. This causes power to be lost in the form of heat.

Example: The heating element in an electric kettle uses wire with a high resistance to generate heat. The wire connecting the kettle to the wall socket has a very low resistance, to avoid heating.

High current:
High current
connections in the 'Primary' section require thick cables to keep resistance values to a minimum. Cables 'C', 'D' and 'E' can be used for high current Tesla Coil use but should be treated as single conductors (see below). Cable 'F' is good for high currents too but not as easy to find in the shops.

Conductors joined together at both ends for high current use

Low Current:
The low current 'Charge' section can use thin cable without any measurable loss of power. Cables 'A' and 'B' are fine. By removing the outermost layer of insulation from cables 'C', 'D' and 'E' the individual wires are perfect too (see below).

Conductors separated for low current use

Skin Effect

Current flowing in a wire is normally assummed to use all of the available metal to travel but this is not the case for alternating current.

Skin effect is the term used to describe current flower primarily in the outer layer, or skin, of a conductor. The skin depth gives the thickness of conductor in which most of the total current flows.

(Cross-sections of a 3mm diameter copper wire showing current density for three different frequencies. The yellow areas indicate current density.)

The above cross-section pictures show that as frequency increases the centre of the conductor becomes less and less used for current flow. In practice this means that although your big fat cable may have a low resistance to DC current flow, at RF frequencies the resistance increases considerably. Note how the metal at the centre of the wire is not used and is therefore wasted expense.
Hollow copper tubing is a good choice for high current, RF conductors.
For my RF ground cable I cut up a roll of copper sheet to make a 2 inch wide by 6 thou strip. If this amount of copper had been a round conductor, there would have been wasted metal at its centre.

Here are some graphs showing skin depth in copper for a range of frequencies.