Alternator Type and Power/GND/Field wiring¶
Alternator type¶
Before wiring, determine whether your alternator is P-type (high drive) or N-type (low drive).
Quick reference — alternator type¶
| P-type / high drive | N-type / low drive | |
|---|---|---|
| Field wire to | FIELD_FOR_P_TYPE | FIELD_FOR_N_TYPE |
| Jumper | BAT+ ↔ FIELD_FOR_N_TYPE | FIELD_FOR_P_TYPE ↔ GND |
| Regulator switches | B+ onto field | Ground onto field |
| External field fuse | Required | Required |
Expand either type below for its full conduction path; what the board's input protection tolerates is in the collapsed section at the bottom of this page.
P-type alternator (high drive)
A P-type alternator has the field winding's far end internally bonded to the alternator case (chassis ground). The single field wire emerging from the regulator-side brush is switched to B+ by the regulator to excite the field.
- Field wire to: field-positive terminal (FIELD_FOR_P_TYPE)
- Jumper installed: BAT+ input to drain (BAT+ ↔ FIELD_FOR_N_TYPE)
- External field fuse: required, sized to the alternator's rated field current — see Fusing
Field current path (output on):
battery (+) → board BAT+ input → jumper → Q3 → field-positive terminal (FIELD_FOR_P_TYPE) → field wire → field winding → alternator case → engine ground → battery (−)
N-type alternator (low drive)
An N-type alternator has the field winding's far end internally tied to B+ (typically at the alternator's diode trio or an internal B+ tap on the rectifier). The single field wire emerging from the regulator-side brush is switched to ground by the regulator to excite the field.
- Field wire to: field-negative terminal (FIELD_FOR_N_TYPE)
- Jumper installed: source to ground (FIELD_FOR_P_TYPE ↔ GND)
- External field fuse: required, sized to the alternator's rated field current
Field current path (output on):
battery (+) → alternator B+ stud → internal tap → field winding → field wire → field-negative terminal (FIELD_FOR_N_TYPE) → Q3 → jumper → board ground (GND) → battery (−)
Note
Field current never enters the regulator's input protection stage, in either configuration, so the board's protected supply rail sees no field current. When the output switches off, the field's freewheel current returns to the battery through the BAT+ wire.
Wire, connectors, and current ratings¶
This regulator's 10 A design ceiling gives margin for most installations.
Connector ratings. The PCB-side connector is the Cixi Kefa Elec KF2EDGK-5.08-04P-AA, rated 15 A at 320 V. The mating plug is the Kangnex WJ2EDGRC-5.08-04P-14-00A, rated 10 A at 300 V. The lower of the two — 10 A continuous — is the system's design limit. For installations under 10 A field current (the typical case) you are operating within all ratings comfortably. For installations exceeding 10 A, see High-current installations below.
Wire. 14 AWG marine-grade stranded tinned copper, Type II or Type III stranding per ABYC E-11. (12 gauge will fit too, but it's tight) An alternator rarely operates at full PWM duty in practice, so voltage drop across the field wiring is not a meaningful concern — a small drop just means the regulator commands slightly higher duty to compensate. Size wire for safety and code compliance, not voltage drop.
Ampacity is per ABYC E-11, stranded tinned copper, single conductor. Voltage drops are one-way on a 12 V system, and are a sanity check only — even the high end of this table doesn't meaningfully degrade output.
| Wire size | Ampacity, engine space | Ampacity, elsewhere | Drop: 10 A, 12 ft | 15 A, 12 ft | 10 A, 24 ft | 15 A, 24 ft |
|---|---|---|---|---|---|---|
| 16 AWG | 10 A | 15 A | 0.48 V | 0.72 V | 0.96 V | 1.45 V |
| 14 AWG | 15 A | 20 A | 0.30 V | 0.45 V | 0.60 V | 0.90 V |
| 12 AWG | 20 A | 25 A | 0.19 V | 0.29 V | 0.38 V | 0.58 V |
| 10 AWG | 30 A | 40 A | 0.12 V | 0.18 V | 0.24 V | 0.36 V |
NEC chassis-wiring tables allow ~15% higher ampacities, but design to ABYC if possible.
Wire termination. Plain stranded wire under the screw — no ferrule required. Strip back no more than 5 mm (0.2 in) from the insulation — you should not be able to see bare copper once the wire is inserted into the connector and screw tightened. See Short hazard below!
Jumper. The supplied Phoenix Contact EBP 3-5 (p/n 1733172) with the middle blade snapped out (already done by X Engineering) gives a 2-position bridge with 10.16 mm prong spacing — matching the regulator's terminal layout. Each blade is 2.0 mm wide × 0.5 mm thick. Phoenix rates the bridge at 12 A continuous.
The jumper blade and the harness wire share a single wire entry hole at one terminal (BAT+ for P-type, GND for N-type). The 0.5 mm-thick blade lies flat against the back wall of the hole; the 1.6 mm-diameter wire stacks on top of it. The screw cage clamps both.
To install:
- Insert the jumper blade flat into the wire entry hole, gray plastic angled down and away (towards the wall/bulkhead) to make the most space for wires.
- Strip no more than 5 mm from the harness wire and push it fully into the same hole, on top of the blade.
- Torque the screw to 0.5–0.6 N·m.
- Tug-test the wire firmly. If it pulls free, something is wrong — re-seat and repeat.
Short hazard at the bridged-over terminal¶
Uncontrolled full-field hazard — inspect carefully during installation
Bare copper of an excessively stripped wire can accidentally touch the jumper's broken-off middle blade stub. In both P-type and N-type configurations, this is the field wire, and the bridge carries battery voltage or GND. Contact shorts the field wire straight to the bridge, bypassing the regulator's field switch entirely. The field locks on at full drive, the regulator cannot shut it off, and the field fuse will not blow (full-field current is normal current). The alternator runs unregulated and drives the system into overvoltage. This is obviously extremely dangerous, and at that point only a BMS can save the battery and possibly the boat.
High-current installations — over 10 A field
Only needed for unusual loads: dual alternators in parallel from one regulator, very large frames (300 A+), or industrial/commercial alternators.
Warning
Running the Kangnex connector above its 10 A rating is at your discretion and shortens connector life. For sustained operation above 10 A, source a higher-rated mating plug and do your own rework.
If you proceed:
- Wire: 12 AWG marine-grade tinned stranded.
- Jumper: Above 10 A the EBP bridge runs near its 12 A rating with no thermal margin. Splice the jumper function into the harness instead — P-type: a short 12 AWG branch off BAT+ into FIELD_FOR_N_TYPE; N-type: BAT− to FIELD_FOR_P_TYPE. Y-splice inside heat-shrink near the regulator end. The EBP is out of the high-current path; only your wire and the connector body carry load.
- Fuses: both need re-speccing for the currents you actually run — the 10 A parts on this page are the standard build. The 80 V rating stays; only the ampere rating changes.
Fusing¶
Two fuses are required:
1. Field circuit fuse — on the field wire, all installations. Size to the alternator's rated maximum field current — typically 10 A standard. Confirm against the alternator's spec sheet or make a measurement with a current meter at 99% field if you have reason for concern.
Both alternator types
The field circuit never runs through the regulator's input protection. In N-type mode, field current flows from the alternator's B+ stud through the field winding into the regulator's GND. In P-type mode the bridge takes field current from the BAT+ terminal ahead of the board's current sensing. Either way the field fuse is what protects the field wiring and winding; without it they are unprotected against a fault.
2. Battery feed fuse — on the BAT+ wire near the battery. 10 A recommended: it protects the connector and sits below the board's own overcurrent trip. Sizing to the wire instead per ABYC E-11 (15 A for 14 AWG, 20 A for 12 AWG) is acceptable if you judge the risk. Protects against harness shorts between battery and regulator; ABYC requires it on any conductor connected directly to battery positive. Take the feed from a fused distribution point downstream of the battery bank's main fuse rather than straight off the post, so the available fault current stays within the fuse's 1000 A interrupting rating. In P-type mode the feed fuse also carries the field current (the bridge takes field current from the BAT+ terminal), so its rating must be at least the field fuse's. In N-type mode the field's freewheel current returns through the BAT+ wire, so keep the feed fuse at 10 A there too.
Which fuse to buy — 80 V rated, on every voltage class¶
Buy an 80 V DC rated ATO blade fuse, the same part on 12, 24, 36 and 48 V. Ordinary blade fuses are rated 32 V DC and the marine "high voltage" ones stop at 58 V; above its rating a fuse can hold an arc instead of clearing the fault, and a 48 V bank absorbs near 58 V.
- Recommended — OptiFuse ANR80-UL-10A: 10 A, 80 V DC, 1000 A interrupting, UL listed, tested to SAE J1284 and ISO 8820-3, −40 to +85 °C.
- Equivalent — Littelfuse 166.7000.5106, the FKS ATO 80 V: same ratings, cULus recognized, arc-quenching filled.
Other ampere ratings come from the same two families: 15 A is ANR80-UL-15A or 166.7000.5156, 20 A is ANR80-UL-20A or 166.7000.5206.
The holder carries its own rating, and most in-line blade holders are marked 32 V DC whatever wire is on them. Use the OptiFuse LPR-02B-14R — 125 V DC, IP67, 4 in of 14 AWG lead each end, 20 A. A 12 AWG build takes the LPR-02B-12R, 30 A.
Quick reference — wire and fusing¶
If you're not sure whether your installation falls into "high-current," it almost certainly doesn't. Build to the standard column.
| Aspect | Standard (under 10 A field) | High current (10–15 A field) |
|---|---|---|
| Wire | 14 AWG tinned stranded | 12 AWG tinned stranded |
| Wire termination | Bare stranded under screw | Bare stranded under screw |
| Jumper | EBP 3-5 with middle blade removed | Wire splice into harness |
| Field fuse | Sized to alternator spec, typically 10 A | Re-spec to the alternator's rated field current |
| BAT+ fuse | 10 A | Re-spec to the wire and connector you fit |
| Fuse voltage rating | 80 V DC, every voltage class | 80 V DC, every voltage class |
| Fuse holder | LPR-02B-14R (14 AWG, 20 A) | LPR-02B-12R (12 AWG, 30 A) |
| Within connector ratings? | Yes | No — at user discretion |
Next step¶
Continue to Data Cables & Pinout for the four data-cable pinouts, then install the sensors: Current Sensor and Temperature Sensors.
Board input protection — what happens in each fault
The BAT+ input runs through an automotive protection controller (TPS48000-Q1) driving back-to-back MOSFETs, with a TVS diode and a crowbar diode across the input terminals. The field's return diode lands on the BAT+ input terminal, ahead of those MOSFETs, so the field always has a path back to the battery whether the protection stage is conducting or has tripped.
Battery leads reversed. The crowbar diode conducts and clears the BAT+ fuse. The board does not power up and is not damaged. Replace the fuse and correct the wiring before trying again — on an N-type installation the field is fed from the alternator, not from the fuse, so a blown fuse does not stop the field if the engine is run with the leads still reversed.
Bus overvoltage — a load dump, a BMS opening under charge, or a failed external charger. The controller trips somewhere between 60 V and 65 V and isolates the board in about 5 µs. The field's stored energy returns to the battery through the return diode rather than into the board, so the alternator de-excites while the board is dark. Power restores automatically once the bus falls back below about 58 V.
Short circuit or sustained overload on the BAT+ wiring. The controller trips at about 15 A after a 14 ms delay and isolates the board, then retries about every 23 s. A short that is still there on retry leaves the board off. The delay is deliberate: brief current peaks from field PWM must not trip it.
Battery voltage collapses. Below about 5 V at the input the controller shuts down and stays off until the bus recovers, so a flat or disconnected bank leaves the board off rather than browning out.
Field wire shorted to ground or to B+. The field switch is downstream of the input protection, so nothing in this stage sees that fault, and neither fuse does either — full-field current is normal current. This is the failure the short hazard section above is about: strip the wires short and inspect the bridged-over terminal.
Battery connection opened while the alternator is charging. Breaking the charging path under load is a load dump: the alternator's own output has nowhere to go, and the spike hits everything on the bus. The board's TVS clamps what reaches its input, but the alternator's rectifier is what usually pays for it. Shut down charging before opening a battery switch or a terminal.
Lightning nearby. Induced surges conducted along vessel wiring are clamped by the TVS and cut off by the overvoltage trip. A direct strike is beyond what any of this survives.
Field circuit
Field current does not pass through the input protection stage in either wiring type — see Fusing above. The field fuse is the protection for the field circuit.
For trip levels, timing, and the circuit itself see the Input Protection hardware doc.