DC wiring (part 1)

Selecting the right electrical cable

In a system, it is important to use the correct thickness of the electrical wire. To select the correct electric cable, it is necessary to know the currents in a system.

To avoid using very thick cables, the first thing to consider is increasing the system voltage. A system with a large inverter will generate significant DC currents. If the DC system voltage is increased, the DC current will decrease, and the power cables may be finer. The most suitable upper limits of inverter power according to the system voltage are:

  • 12V: up to 3000 VA.   • 24 V: up to 5000 VA  .   • 24 V: up to 5000 VA.        • 48 V: from 5000 VA.

 

If you want to increase the system voltage, but DC loads, or DC power supplies that only support 12V, you may consider using a DC / DC converter instead of selecting a low voltage for the entire system.

As we have explained before, it is very important to use the correct wire thickness. You can find the appropriate wire thickness in the product manual. Using too thin a cable has a direct negative effect on system performance.

Generally, the thickness of the electrical cable conductor is indicated in mm². This refers to the area of ​​the cable conductor section. But other units are also used, such as the American wire gauge (AWG or American wire gauge).

To find out the conductor diameter of a multifilament cable, look at the cable’s insulation. There will be some mark on the wire that indicates the thickness of the conductor.

Be aware that some cables have very thick insulation and may appear thicker than they actually are. You can find out the diameter by looking at the inscriptions on the cable itself or its specifications. A physical check can also be done. Strip some of the insulation from the wire and estimate the diameter of the copper wire. In a rigid cable, the surface area can be calculated by measuring the cable conductor’s diameter, but in a flexible cable with several strands, this method is not very accurate. Please note that we do not recommend the use of flexible cables.

Area = π x radius²

Area = π xr (diameter / 2) ²

A = π x (d / 2) ²

If you can’t find a thick enough wire, double it. Use two cables per connection instead of just one very thick one. If you do this, check that the sum of the two cables’ sections is equal to the recommended section. For example, two 35mm² cables are equivalent to one 70mm² cables. The larger Victron inverter/chargers have two positive and two negative battery connections for precisely this purpose.

When selecting cables to avoid making these mistakes:

  • Do not use thick-stranded cables. • Do not use cables that are not flexible. • Do not use AC cables. • In marine environments or wet conditions, use “marine cables.” They are cables with tinned copper wires.

Calculating the thickness of the cables can be difficult. To choose the correct wire thickness,

you can use:

  • The product manual.
  • The general rule
  • The document on recommended cables for batteries.

 

Product manuals

All manuals list the battery cable sizes (and type of fuses) appropriate for that product.

Recommended Battery Cables Document

This document contains a table showing the maximum current for various standard cables where the voltage drop is 0.259 volts.

General rule

For a quick and general calculation for cables up to 5 meters, you can use this formula:

Current / 3 = wire size in mm²

Connection bars

They are like cables, but they are rigid metal bars. They are made of copper or tinned copper. They are used in large systems with high currents. They provide a common positive and negative point between batteries and various inverters. They are also used in smaller systems, especially when there are many DC equipment. In this case, they are a suitable location to connect the different DC cables.

To calculate the thickness of the busbar; just use the recommended wire section area and apply it to the cross-sectional area of ​​the busbar. 

Section area = width x depth

It is almost always necessary to protect the busbars, especially if they are outside. This is done to prevent someone from touching the busbar and prevent a short circuit if a metal object accidentally falls between the positive and negative busbar and short the two busbars. An easy way to do this is to place a sheet of methacrylate on top of or in front of the busbar.

It is easy to make your own busbar; you only need a copper bar in which holes are drilled to be able to connect the electrical cables. For marine applications, use tinned copper busbars. Connecting bars can be purchased from metal product distributors wholesale warehouses.

Wired connections

There are several ways to connect dc cables to batteries.

Bolts, nuts, and bolts

Usually available in different sizes: M5, M6, M8, or M10. Bolts for electricity are usually made of tinned brass. So when tightening, always use the proper torque. Over-tightening could break the nut or bolt. You can check the product manual for the correct torque.

Round cable lugs are used to connect the cable to a bolt. The cable terminal must be adjusted to the thickness of the cable. A special crimping tool is needed to secure the terminal to the wire. If the wire terminal has no insulation, you will need to put it in.

When connecting the round end of the cable to the bolt, fit a washer and a spring washer, and then the nut. Make sure the round terminal lies flat on the bottom surface. Do not put anything between the terminal and the mounting surface, such as washers or fuses. This would reduce the current carrying capacity of the connection.

Use insulated tools to tighten the nut. An accidental short circuit of a battery can be very dangerous, and currents can melt your uninsulated wrench, or sparks can cause the battery to explode.

Screw connectors

Screw connectors are available in various shapes and sizes, suitable for thick and thin cables.

Strip a sufficient length of insulation from the wire before inserting the bare end into the socket in the connector. Prevent cable insulation from entering the connector. This can produce too high a resistance, and the connector will become hot and could melt. Do not show bare wire (bare wire) on the outside of the connector. This is dangerous as it can cause electrocution or a short circuit.

The screws for electrical connectors are normally made of tinned brass. When tightening, always use the proper torque. Over-tightening could break the screw. See the product manual.

Quick connectors (pressure)

Strip a sufficient length of the cable insulation.

Push down the orange part with a flat screwdriver.

Insert the bare wire.

Prevent cable insulation from entering the connector. This can produce too high a resistance, and the connector will become hot and could melt.

Do not let bare wire (bare wire) show on the outside of the connector. This is dangerous as it can cause electrocution or a short circuit.

Release the orange part.

The cable is now secured in place. Gently pull on the cable to check that it is securely attached.          

Toecaps

Theyare cylindrical pieces that are placed on the stripped end of a cable.

A special tool is required for crimping.

Used to straighten the wires in a bare cable and to prevent them from splitting when the cable is inserted into a screw or pressure connector.

Its use is recommended if you want to get a wiring job with a good finish.

Faston terminals

The terminals have to be mounted on the cable.

A special tool is required for crimping.

Within the range of these terminals, there are those with insulation

at the ends and others with special characteristics such as mixed.

MC connectors

These connectors are exclusively used to connect solar panels to MPPT. The most common is MC4, but there are also MC, MC2, and MC3, although they are no longer used. The letters’ MC’ stands for MultiContact, which is the name of one of the original manufacturers. Digits 1 to 4 indicate the cross-section of the contact washer in mm2.

Some data:

They are waterproof (IP67) and can be used outdoors.

There are male and female connectors.

They accept 20 A and 600V (the latest versions up to 1500 V).

A special tool is required for crimping.

Can be purchased as pre-assembled cables.

MC4 Y pieces (or Y cables) are used to connect solar panels inparallel.            

For more information, see section 4.10 on solar panels.

Anderson connectors

Often used in automotive or mobile applications where quick connections and disconnections are common.

They are available with different nominal currents and wire thicknesses.

Make sure that the current rating matches the currents present in your system when it is fully loaded.

They will increase the resistance of the cable if they are located between the battery and the inverter. In that case, limit or avoid its use.

Cigarette lighter connectors

Used in simple automotive applications.

They do not have the capacity to carry high currents.

Note that the car circuit may only have a low-rating fuse.

Be careful to insert the plug correctly and deep enough; otherwise, the connector may become hot and melt.

Limit or avoid its use.

Battery clamps

They are only for temporary connections.

They do not usually have a high enough current rating.

They should never be used permanently in an electrical system.

Limit its use or avoid it altogether.

Fuses and Circuit Breakers

A fuse is an electrical safety device. Protects an electrical circuit from high currents.

The fuse is placed in the power cord of an electrical device. When a current greater than its rated current passes through the fuse for a certain period of time, the fuse will blow. Once the fuse has blown, no current passes through the circuit.

Situations can occur where the current is higher than expected when an electrical device has a fault or when there is a short in the electrical circuit.

The fuse protects from:

Severe overload – when more current than nominal passes through the system.

Short circuit – when a driver comes into contact with another driver by accident.

How does a fuse work?

There are three types of fuse mechanisms, which are:

Wire fuse

Thermal fuse

Magnetic fuse

Traditionally, the fuse contains a wire or strip of metal that melts as soon as it is passed through by an unacceptably high current. When the fuse wire melts, the electrical circuit is broken, and no more current can pass through it.

Once the fuse has blown; it will need to be replaced with a new one to get the circuit to work again. These fuses are single use only. Once they have melted, they cannot be reset. They have to be replaced with a new one.

Another type of fuses are automatic fuses, often called circuit breakers or miniature circuit breakers (CBs or MCBs). These devices interrupt the flow of current when a high current is detected. Sometimes they are reconnected when the high current situation has passed or has to be manually reset. Unlike traditional fuses, they do not need to be replaced.

These fuses work in two ways: thermal or magnetic or a combination of the two.

The thermal breaker contains a bimetallic strip that heats up when current passes. When heated, it bends and thus interrupts the path of the current.

The magnetic breaker contains an electromagnet that is sensitive to high currents. When a high current  passes, the electromagnet creates a magnetic field strong enough to interrupt the flow of current.

Location of DC fuses: Each electrical consumer that is connected to a battery must have a fuse. The fuse is placed on the positive wire. Each electrical consumer needs its own fuse. Regardless of the nominal power of the equipment. Batteries can produce very high currents that can cause a fire. If the electrical consumer develops a fault and is short-circuited internally, a very high current will flow, which could create a risk of fire. A DC circuit usually has a main battery fuse and then branches out to each of the electrical consumers. Each electrical consumer has an independent fuse.

Location of AC Circuit Breakers: Circuit breakers are located near the utility or generator entry point in the electrical panel. The AC circuit breaker is located in the current-carrying conductor or the current-carrying conductor and neutral. One or two-pole circuit breakers are used. There is usually one main circuit breaker for each AC source, and after it, the supply branches into several groups. Each group has a circuit breaker, which protects a group of AC electrical consumers.

Location of PV group breakers: It is necessary to put a fuse between the PV group and the solar charger. Check with local authorities, as regulations may differ depending on the type of application and the country.

Fuse holder

The fuses must be placed in the fuse holder. The fuse holder holds the fuse securely in place. And in some cases, they also provide electrical insulation. Circuit breakers are normally mounted on a DIN

rail. Fuses and circuit breakers are usually located in an electrical panel, preferably in a closed box.

Fuse classification and selection of the correct fuse

There are four criteria for selecting a fuse:

Nominal current

Nominal voltage

Speed

Type.

It is important to choose the correct fuse, which fits the circuit and matches the power consumption of the equipment on that circuit.

The fuse rating is listed on the fuse itself or can be found on its datasheet or in its specifications.

Rated current

If there is only one consumer on that circuit; the fuse will have to be set to the rated current of that appliance or to the rated current of the cable, whichever is less.

If there are several electrical consumers in the circuit; the fuse will have to be adjusted to the rated current of the circuit wiring.

Nominal voltage

The nominal voltage of the fuse must be equal to or greater than the maximum voltage expected in the system.

The fuse must be specifically rated for the corresponding type, DC and/or AC.

Most DC fuses are suitable for 12 and 24 V but are not necessarily suitable for 48V or more.

Note that not all fuses or circuit breakers can be used in both types of circuits: AC, and DC. If the fuse can be used in both AC and DC, the AC nominal voltage is usually higher than DC.

Note that how circuit breakers are wired in the DC circuit is important as they may not be one-way.

Speed

The speed of a fuse is the time it takes to open when a power failure occurs. This depends on the material of the fuse, its mechanism, the current and the temperature.

There are fast and slow blow fuses:

Slow fuses are typically used in DC applications that can be found in automotive and marine circuits. These circuits contain electrical consumers with a high starting currents, such as motors, or devices with capacitors, such as an inverter. The slow blow fuse will withstand a high and short duration starting current, allowing a motor to start.

Fast blow fuses use in AC applications. Appliances that consume AC are often sensitive to changes in the flow of electricity, so they need a fast-acting fuse to protect them. But in some cases, an AC appliance can have a high starting current. This is what happens with electric motors, such as refrigerators, air conditioners, and compressors. In these situations, a slower fuse will be needed.

Fuse speed rating:

FF Very fast acting (Flink Flink).

F Fast acting (Flink).

M Medium speed actuation (Mitteltrage).

T Slow acting (Trade).

TT Very slow acting (Trade Trade).

Fuse indications

The fuses have their ratings written. But information may be missing. A good place to look for more information is the fuse specifications, which can be easily found on the Internet or can be obtained from the supplier. 

Continue in part 2.

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