How to wire a bus or motorcoach

How To Wire (or re-wire) a bus or motorcoach

DC Wiring Guide

I know what you’re thinking.  “How is this lunatic going to cover such a complex topic as – how to wire a bus or motorcoach– in one post?

Well – you’re right – We won’t be able to cover every situation or every possible setup on every bus or motorcoach.  And if all this info is new to you, you’re probably best hiring a professional electrician to do it for you.  But, we’ll try anyway to explain some of the general theory and best practices in hopes it will help.

In this guide, we’ll stick with the 12Vdc power distributions systems, as opposed to the engine or gauge wiring.

A few notes before we start:

  • (+) Positive wires are red (in our guide, your bus or motorcoach may have other colors)
  • () Negative wires are black (or maybe yellow on your bus or motorcoach)
  • Current is measured in Amps (A)
  • Potential difference is measured in Volts (V)
  • Current flows through the wires (like water through a pipe).  Too much current can heat up the wiring to the point of starting a fire
  • Voltage does not “flow” it is a measurement of potential to do work.  Like water pressure in a pipe

In a bus or motorcoach, electricity is stored in two or more batteries.  The starting batteries are charged by your engine’s alternator or auxiliary battery charger.  They can hold an enormous amount of energy, capable of pushing hundreds, or even a thousand amps (more than your entire house uses)… so care must be taken, and proper circuit protection should not be ignored.

Greatly generalizing the topic here, but you usually run into two types of batteries in the size of bus or motorcoach we deal with:

  1. Starting Battery – Has a high current rush capacity
  2. Deep Cycle Battery – Capable of deep discharge without harm

The two setups we most often run into is:

  1. Prime Mover – 2 starting batteries, and 2 house batteries
  2. Prime Mover and Generator – 2 starting batteries (prime mover), 1 starting battery (generator), and 2 house battery

Every non-engine wire (EVERY ONE) should be circuit protected with a fuse or circuit breaker

Batteries have a positive and negative.  For current to flow (which does the work) a complete circuit must be made from positive back to the negative.  Any break in the circuit anywhere will stop the load from operating (which you probably already know or you wouldn’t be reading this to try and fix your wiring issue.

Generic Battery Image

A normal battery might be able to push 800A or more current

A normal battery might have 70-80AH (amp hours) of capacity. Meaning it can run a 1A load for 70 to 80 hours, or a 10A load for 7 to 8 hours before it is discharged.

Bus and Motorcoach Wiring Diagram Step 1

In nearly all cases your bus or motorcoach wiring system should have a marine-grade main battery disconnect switch.  This allows you to open the switch turning everything off at once.  In this case, We’ve shown a 1-2-BOTH type battery switch.

Both battery positives are run through this switch, and you can use it to select which battery you want to output, similar to an A-B switch.  But a 1-2-BOTH marine battery switch also allows you to parallel both batteries.  The “Both” setting might be used when you are running your engine and want to charge both batteries from the alternator, or if you need to parallel the batteries in an emergency to help start your engine if your start battery becomes too depleted.

Remember to turn your battery switch to the “house circuit” when your engine is not running, so you are only drawing down your deep cycle house battery meant for that purpose.

We’ve changed the diagram a bit now to show the start battery running through a marine battery switch

A Double Pole ON/OFF/COMBINE battery switch is a great choice for a single-engine, two battery bus or motorcoach wiring system.  It allows your house and start batteries to remain isolated except for emergency conditions.

1-2-Both Battery Switch

Used to shut off everything and prevent parasitic discharge  from draining your battery

Single Battery Disconnect
Bus & Motorcoach Wiring Diagram Step 2

Battery Switch Bypass Loads

It’s pretty standard in bus or motorcoach wiring to bypass the main battery switch for one thing.  This way, even if your battery switch is off those items will remain to have power. A stereo memory line might be a “bypassed” load.

Notice the fuse shown – this needs to be circuit protected with an inline fuse.  I’m also showing the negative return wiring for a water pump in this step.

Bus & Motorcoach Wiring Diagram Step 3

Get the Source to the Switch Panel

The next step is to get the power from the house battery up to the switch panel where we can use it to do some good.  Two conductors – a positive from the battery switch (with a fuse) and a negative from the ganged together battery negatives should be run to where the central switch panel is.  You should use marine grade primary wire for this.

This is sometimes long wiring run on a bus or motorcoach.  Plus these two conductors will carry the current of all your electrical loads combined, so they are typically fairly beefy cables.  8AWG – 0AWG is in most cases recommended for buses and motorcoaches over 30ft.

Remember these are all generalities, there are many valid reasons to make exceptions

Keep in mind that the longer your wiring run from the battery to switch panel is, the more voltage drop you’ll have.  Prevent voltage drop by using larger cable.

The power cables will be run to your switch panel and your tinned negative bus bar.  The best switch panels include waterproof resettable circuit breakers with all the connections pre-made to make them work, that’s how it is shown here.

Note, you’d need to insert a fuse block before the panel, then individual conductors from each fuse to each panel (we really recommend including circuit breakers in your panel if you have space, it will really make your life easier installing and maintaining your new custom switch panel).

The main house battery positive conductor will feed directly into the new switch panel.  The main battery negative should go to a negative buss bar (like this one), where all your buses or motorcoaches load negatives will eventually be attached. The negative buss bars can be connected to the chassis (for negative ground vehicles) also allowing for more paths to the negative terminal of your battery and creating redundancy.

Negative Buss Bar

Example negative bus bar.  Note, this is different than a terminal block – all the screws are “bused” together.

Bus and Motorcoach Wiring Diagram Step 4

Install Terminal Block as Breakout Point

If you get your bus or motorcoaches switch panel fully wired, then you’ll have an easy to install wiring harness coming off pre-installed with heat shrink labels, and ring terminals.  This is meant to land on a terminal block like this one.

Each switch output gets it’s own gang on the terminal block, and with the labels right there it makes a handy breakout point for troubleshooting or adding items down the road.  These are the positives of course – the “switch legs” – and all that’s needed is to crimp a #8 ring terminal on the positive load wiring that runs out around your bus or motorcoach to the various loads.

We’re showing one output from the terminal block here for the manual water pump switch.

Switch Panel Interface with Terminal Block

This is how a fully wired switch panel would interface with a terminal block.  With the heat shrink labels, it’s easy to hook up your load wiring and troubleshoot.

Run Load Wiring to the Terminal Block and Buss Bar

From here the rest of the wiring is straightforward.  Just hook up your existing bus or motorcoach wiring infrastructure to the terminal block and buss bar.  Positives to the terminal block, and negative to the bus bar.

Most are terminated with standard #8 ring terminals.  The positives of course must be installed on the correct gang associated with the respective switch for that load.  The negatives can go on any screw on the buss bar, they are just trying to get back to the negative post on the battery.

Red ( + Positive) Wire Roll

From here on our it’s just a + and – wire run to each load.

Bus and Motorcoach Wiring Diagram Step 6

24-Volt DC Applications

For those that have a 24VDC system, the same principals apply. Utilizing a Vanner system allows for 24VDCand 12VDC electrical systems to be used. To properly wire from the Vanner system the 24VDC system is wired directly to the batteries as displayed in the diagrams above. the 12 VDC system is similar but instead of the positive terminal being connected directly to the terminal before the disconnect, the disconnect is connected to the 12VDC terminal on the Vanner 24-volt – 12-volt converter.

AC Wiring Guide

  • (Neutral) Common wires are White (in our guide, your bus or motorcoach may have other colors)
  • Hot wires are Black or Red on your bus or motorcoach
  • Current is measured in Amps (A)
  • Potential difference is measured in Volts (V)
  • Current flows through the wires (like water through a pipe).  Too much current can heat up the wiring to the point of starting a fire
  • Voltage does not “flow” it is a measurement of potential to do work.  Like water pressure in a pipe

The Electrical Source: Pick Your Power

In a bus or motorcoach, electricity is transferred or generated from two or more sources.  The sources are typically a Generator, an Inverter and Shore Power.  These sources will provide power current at 110Vac or 220Vac… so care must be taken, and proper circuit protection should not be ignored.

These power sources have a common and a hot wire or wires.  For current to flow (which does the work) a complete circuit must be made.  Any break in the circuit anywhere will stop the load from operating (which you probably already know or you wouldn’t be reading this to try and fix your wiring issue.

Most Generators and Shore Power will provide 220Vac to your bus or motorcoach. 220Vac is typically supplied through the use of 3 wires with the addition on a Ground Wire. The White wire is the Common wire. The Black and Red wires are the Hot wires and the Green wire is the Ground wire.

While technically not a power source, Inverters convert the potential energy store in batteries to energy compatible with your appliances. Invertors typically only supply 110Vac and as such not provide power for all of the electrical needs of your bus or motorcoach. It is beneficial to understand the power consumption of all electrical items that you plan on using and for what duration when choosing an inverter that meets your needs.

Generator
Shore Power Cable
Pure Sine Wave Inverter
Bus and Motorcoach Wiring Diagram Step 1 AC

The Electrical Source: Switch to the Source

In nearly all cases your bus or motorcoach wiring system should have a marine-grade transfer switch.  This allows you to select the correct power source for all house devices all at once.  In this case, We’ve shown a Rotary AC Selection Switch.

The Common wires are run through this switch, and you can use it to select which power source you want to output, similar to an A-B switch.

We’ve changed the diagram a bit now to show the power sources running through a rotary switch

A Double Pole GEN/OFF/SHORE transfer switch is a great choice for a bus or motorcoach with only a generator and shore power connections.  It allows your generator and shore power to remain isolated.

Others methods of transferring power include the use of automatic transfer switches that automatically deliver the power from the preferred method of power from others. Another method is the use of source plugs being manually plugged into the supply side of the distribution/breaker panel.

Used to select your power source and isolate the power sources from each other.

Automatic Transfer Switches can also be utilized to automatically switch the power supply for the bus or motorcoach.

Bus andMotorcoach Wiring Diagram Step 2 AC

Get the Source to the Breaker Panel

The next step is to get the power from the source up to the breaker panel where we can use it to do some good.  Four conductors – a Common (white) and a Ground and 2 Hot wires( red and black) You should use marine grade primary wire for this, not standard house wire since this is a vehicle, these wires require flexibility due to movement while traveling, and expansion and contraction from heat and cold cycles.

This is sometimes long wiring run on a bus or motorcoach.  Plus these four conductors will carry the current of all your electrical loads combined, so they are typically fairly beefy cables.  8AWG – 0AWG is in most cases recommended for buses and motorcoaches over 30ft.

Remember these are all generalities, there are many valid reasons to make exceptions

Remember to size your breaker panel to the correct service needs.

Panel to the Load (110 volt)

Panel to the Load (220 volt)

Complete Slide Progression

Information provided by New Wire Marine

Converted to Bus and Motorcoach Applications by Phoenix Coach Technologies