What does the cut-off wiring planner do?
A remote cut-off uses a small relay to open one wire in the vehicle when a tracker says so. The wiring is simple. The consequences are not: a cut at the wrong moment can stop an engine on a highway. This planner covers three things in one place: whether the relay and the fuse suit the load, which relay pin goes to what, and the rules for when a cut is allowed.
It has three tabs. Relay and fuse checks ratings, coil drive and fuse. Wiring and fail-safe draws a generic relay and shows what the vehicle does if the tracker loses power. Safe-cut rules turns your limits into one clear policy sentence.
It is generic on purpose. It does not describe the pins or the output of any Tracom device. Those come from your device manual, and you enter them here.
Why does a cut-off need a plan?
- A relay that is too small overheats. A fuse that is too big does not protect the wire.
- A tracker output that is too weak for the coil can fail without any warning.
- A normally open contact on an engine circuit cuts the engine when the tracker loses power, including on the road.
- A cut with no rules is a decision nobody wrote down. Write it down first.
What inputs do you need?
| Input | Where to find it | Tip |
|---|---|---|
| Load current | Pump or circuit datasheet, or a clamp meter | A motor draws more when it starts. Use that figure. |
| Relay rating and coil | Relay body and datasheet | Check the coil voltage as well as the contact rating. |
| Tracker output | Your device manual | Type (ground or supply) and the current limit. Never guess it. |
| Wire capacity | Wire maker table | Depends on conductor size, length and temperature. |
| Safe-cut limits | Your own policy, local law, your insurer | The defaults are cautious starting points, not rules. |
| Parameter | Default |
|---|---|
| Safety factor on the load | 1.5×typical |
| Fuse factor on the load | 1.25×typical |
| Coil current | 150mAtypical |
| Tracker output limit | 200mAtypical |
| Output margin | 20%typical |
| Highest speed for a cut | 5km/htypical |
| Standstill before the cut | 30stypical |
| Driver warning | 10stypical |
Typical starting values. Verify against the relay maker's datasheet and your device manual. The relay terminal numbers follow DIN 72552-2, current edition July 2014 (source and date checked 3 November 2026). The meaning of each terminal sits in the paid text, so check the markings on your relay. The safe-cut limits are this tool's own cautious defaults.
What do the relay pins mean?
| Pin | Role | Normal use in a cut |
|---|---|---|
| 30 | Common contact | The live side of the wire you cut, fed through a fuse |
| 87a | Normally closed contact | Joined to 30 while the coil has no power. Use it for a fail-safe cut |
| 87 | Normally open contact | Joined to 30 only while the coil has power |
| 86 | Coil plus | Supply, or the tracker output, depending on the output type |
| 85 | Coil minus | Ground, or the tracker output, depending on the output type |
Terminal numbers follow DIN 72552 on most automotive relays. Always check the markings on the relay in your hand, because layouts and numbering can differ.
How is the plan calculated?
A diode across the coil absorbs the voltage spike when the coil switches off. Put the stripe (cathode) on pin 86 and the other end (anode) on pin 85. Some relays include one, so check the datasheet.
Fail-safe means the vehicle keeps working when something goes wrong. With the load on 87a, a power loss at the tracker, a pulled fuse or a restart leaves the coil without power, the relay falls back, and the circuit stays connected. With the load on 87 the opposite happens.
When is a cut allowed?
The tool does not decide this for you. It helps you write your own rule. A careful policy usually has these parts:
Standstill only
Accept the command only at or near zero speed, after the vehicle has been still for a set time.
Warn the driver
Sound a warning before the cut, so it is never a surprise.
Limit who can send it
Named, trained people, and a log of each command with a reason.
Know the exceptions
No cut with passengers on board, with dangerous goods, or on an emergency vehicle.
Check the law
Remote cut-off may be regulated where you work. Ask a lawyer and your insurer before you offer it.
Test and record
Test the cut with the vehicle standing and write the result down.
The installation record template has a table for that test and a place for the customer to acknowledge the conditions.
Worked examples
- A 12 V fuel pump drawing 8 A: the contacts need 8 × 1.5 = 12 A, so a 30 A relay is plenty. The suggested fuse is 8 × 1.25 = 10 A, which is a standard size. A 15 A fuse is still below a 20 A wire, so it is accepted. A 150 mA coil on a 200 mA output with a 20 % margin (limit 160 mA) is driven directly.
- A 24 V coil on a 12 V system: the relay may not pull in, so the tool shows a stop result. Use a 12 V coil.
- A normally open contact on a fuel pump: if the tracker loses power, the pump loses power with it, so the tool shows a stop result. Move the pump wire to 87a.
The planner opens with the first example, so you can check each line.
How do you read the results?
- A green lamp means the numbers you entered are consistent with each other. It is not an approval of the installation.
- An amber lamp means something needs a decision or a check.
- A red lamp means do not wire it like this.
- The matrix lists each check with the numbers behind it, so you can see why.
Common mistakes
- Cutting a running engine. Never do it. Allow a cut only at standstill.
- Using the normally open contact on an engine circuit. A tracker fault then cuts the engine.
- Guessing the tracker output. Read the type and the current limit from the manual.
- A fuse bigger than the wire. The wire can overheat before the fuse blows.
- Skipping the diode. The spike can damage the tracker output over time.
Assumptions and limits
This is a planning aid, not a wiring diagram for your vehicle and not an approval. It does not know your vehicle, your relay or your tracker. The factors and the safe-cut limits are typical starting values that you can change. Pin numbers follow DIN 72552 for most automotive relays, so verify them on your relay. Vehicle wiring differs, and some vehicles have security systems that react to a cut wire. Remote cut-off can be regulated, so check local law and your insurer, and have the policy reviewed before you rely on it. The calculations run in your browser. Your inputs are sent to our server, and kept for up to 45 days, only if you ask for the emailed report.