diff --git a/.changeset/validate-phases-sitemeter.md b/.changeset/validate-phases-sitemeter.md
new file mode 100644
index 00000000..7297f286
--- /dev/null
+++ b/.changeset/validate-phases-sitemeter.md
@@ -0,0 +1,5 @@
+---
+"ftw": patch
+---
+
+Config validation now rejects `fuse.phases` above 3. Dispatch only reads three phase currents while the aggregate power limit used every configured phase, so a larger value overstated the usable fuse budget. It also rejects more than one driver with `is_site_meter: true`; the first match previously won without warning. Existing invalid configs must be corrected before the next restart: set `fuse.phases` to 1, 2 or 3 and keep exactly one site meter.
diff --git a/go/internal/config/config.go b/go/internal/config/config.go
index f07a38f8..2287c056 100644
--- a/go/internal/config/config.go
+++ b/go/internal/config/config.go
@@ -1698,6 +1698,12 @@ func (c *Config) Validate() error {
 	if len(c.Drivers) > 0 && siteMeters == 0 {
 		return errors.New("at least one driver must be is_site_meter: true")
 	}
+	// SiteMeterDriver() returns the first match, so a second is_site_meter
+	// entry was silently ignored — the operator thinks meter B is the site
+	// boundary while dispatch trusts meter A. Make the ambiguity an error.
+	if siteMeters > 1 {
+		return fmt.Errorf("exactly one driver may set is_site_meter: true (found %d)", siteMeters)
+	}
 
 	if c.Site.ControlIntervalS < 0 {
 		return errors.New("site.control_interval_s must be >= 0")
@@ -1726,6 +1732,14 @@ func (c *Config) Validate() error {
 	if c.Fuse.Phases <= 0 {
 		return errors.New("fuse.phases must be > 0")
 	}
+	// The phase-current safety model ([3]float64 arrays, meter_l1..l3_a
+	// metrics) covers 1..3 conductors. A larger value used to be silently
+	// truncated to 3 by the dispatch freshness gate while MaxPowerW still
+	// counted every configured phase. That overstated the usable aggregate
+	// fuse budget. Reject it instead.
+	if c.Fuse.Phases > 3 {
+		return errors.New("fuse.phases must be 1, 2 or 3")
+	}
 	if c.Fuse.Voltage <= 0 {
 		return errors.New("fuse.voltage must be > 0")
 	}
diff --git a/go/internal/config/validate_site_test.go b/go/internal/config/validate_site_test.go
new file mode 100644
index 00000000..259d4f67
--- /dev/null
+++ b/go/internal/config/validate_site_test.go
@@ -0,0 +1,75 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func loadSiteValidationConfig(t *testing.T, yaml string) error {
+	t.Helper()
+	path := filepath.Join(t.TempDir(), "config.yaml")
+	if err := os.WriteFile(path, []byte(yaml), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	_, err := Load(path)
+	return err
+}
+
+func TestLoadRejectsMoreThanThreeFusePhases(t *testing.T) {
+	err := loadSiteValidationConfig(t, strings.Replace(minimalYAML, "max_amps: 16", "max_amps: 16\n  phases: 4", 1))
+	if err == nil || err.Error() != "fuse.phases must be 1, 2 or 3" {
+		t.Errorf("Load error = %v, want fuse phase validation error", err)
+	}
+}
+
+func TestValidateAcceptsOneToThreePhases(t *testing.T) {
+	for phases := 1; phases <= 3; phases++ {
+		c := &Config{
+			Site: Site{SmoothingAlpha: 0.3},
+			Fuse: Fuse{MaxAmps: 16, Phases: phases, Voltage: 230},
+		}
+		if err := c.Validate(); err != nil {
+			t.Errorf("phases=%d: unexpected error: %v", phases, err)
+		}
+	}
+}
+
+func meterDriver(name string, siteMeter bool) Driver {
+	return Driver{
+		Name:        name,
+		Lua:         "drivers/test.lua",
+		IsSiteMeter: siteMeter,
+		Capabilities: Capabilities{
+			Modbus: &ModbusConfig{Host: "192.168.1.10", Port: 502},
+		},
+	}
+}
+
+func TestLoadRejectsDuplicateSiteMeters(t *testing.T) {
+	yaml := strings.Replace(minimalYAML, "api:\n", `
+  - name: second-meter
+    lua: drivers/second-meter.lua
+    is_site_meter: true
+    capabilities:
+      mqtt:
+        host: 192.168.1.154
+api:
+`, 1)
+	err := loadSiteValidationConfig(t, yaml)
+	if err == nil || err.Error() != "exactly one driver may set is_site_meter: true (found 2)" {
+		t.Errorf("Load error = %v, want duplicate site meter validation error", err)
+	}
+}
+
+func TestValidateAcceptsSingleSiteMeter(t *testing.T) {
+	c := &Config{
+		Site:    Site{SmoothingAlpha: 0.3},
+		Fuse:    Fuse{MaxAmps: 16, Phases: 3, Voltage: 230},
+		Drivers: []Driver{meterDriver("a", true), meterDriver("b", false)},
+	}
+	if err := c.Validate(); err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+}