diff --git a/consensus/common_test.go b/consensus/common_test.go
index 377f1929f..dd6fce660 100644
--- a/consensus/common_test.go
+++ b/consensus/common_test.go
@@ -347,7 +347,7 @@ func consensusLogger() log.Logger {
 }
 
 func randConsensusNet(nValidators int, testName string, tickerFunc func() TimeoutTicker, appFunc func() abci.Application, configOpts ...func(*cfg.Config)) []*ConsensusState {
-	genDoc, privVals := randGenesisDoc(nValidators, false, 10)
+	genDoc, privVals := randGenesisDoc(nValidators, false, 30)
 	css := make([]*ConsensusState, nValidators)
 	logger := consensusLogger()
 	for i := 0; i < nValidators; i++ {
diff --git a/consensus/reactor_test.go b/consensus/reactor_test.go
index e6f5b4b19..bcf97f94b 100644
--- a/consensus/reactor_test.go
+++ b/consensus/reactor_test.go
@@ -180,7 +180,7 @@ func TestReactorVotingPowerChange(t *testing.T) {
 		t.Fatalf("expected voting power to change (before: %d, after: %d)", previousTotalVotingPower, css[0].GetRoundState().LastValidators.TotalVotingPower())
 	}
 
-	updateValidatorTx = dummy.MakeValSetChangeTx(val1PubKey.Bytes(), 100)
+	updateValidatorTx = dummy.MakeValSetChangeTx(val1PubKey.Bytes(), 26)
 	previousTotalVotingPower = css[0].GetRoundState().LastValidators.TotalVotingPower()
 
 	waitForAndValidateBlock(t, nVals, activeVals, eventChans, css, updateValidatorTx)
@@ -194,8 +194,8 @@ func TestReactorVotingPowerChange(t *testing.T) {
 }
 
 func TestReactorValidatorSetChanges(t *testing.T) {
-	nPeers := 9
-	nVals := 6
+	nPeers := 7
+	nVals := 4
 	css := randConsensusNetWithPeers(nVals, nPeers, "consensus_val_set_changes_test", newMockTickerFunc(true), newPersistentDummy)
 
 	logger := log.TestingLogger()
diff --git a/docs/app-development.rst b/docs/app-development.rst
index 9574f8630..e373bcff1 100644
--- a/docs/app-development.rst
+++ b/docs/app-development.rst
@@ -408,11 +408,12 @@ Additionally, the response may contain a list of validators, which can be used
 to update the validator set. To add a new validator or update an existing one,
 simply include them in the list returned in the EndBlock response. To remove
 one, include it in the list with a ``power`` equal to ``0``. Tendermint core
-will take care of updating the validator set. Note you can not update more than
-1/3 of validators in one block because this will make it impossible for a light
-client to prove the transition externally. See the `light client docs
+will take care of updating the validator set. Note the change in voting power
+must be strictly less than 1/3 because otherwise it will be impossible for a
+light client to prove the transition externally. See the `light client docs
 <https://godoc.org/github.com/tendermint/tendermint/lite#hdr-How_We_Track_Validators>`__
-for details on how it tracks validators.
+for details on how it tracks validators. Tendermint core will report an error
+if that is the case.
 
 .. container:: toggle
 
diff --git a/state/execution.go b/state/execution.go
index f2cc3a4ee..d626ee0fc 100644
--- a/state/execution.go
+++ b/state/execution.go
@@ -122,18 +122,15 @@ func execBlockOnProxyApp(txEventPublisher types.TxEventPublisher, proxyAppConn p
 }
 
 func updateValidators(currentSet *types.ValidatorSet, updates []*abci.Validator) error {
-	// ## prevent update of 1/3+ at once
-	//
-	// If more than 1/3 validators changed in one block, then a light
-	// client could never prove the transition externally. See
-	// ./lite/doc.go for details on how a light client tracks
-	// validators.
-	maxUpdates := currentSet.Size() / 3
-	if maxUpdates == 0 { // if current set size is less than 3
-		maxUpdates = 1
+	// If more or equal than 1/3 of total voting power changed in one block, then
+	// a light client could never prove the transition externally. See
+	// ./lite/doc.go for details on how a light client tracks validators.
+	vp23, err := changeInVotingPowerMoreOrEqualToOneThird(currentSet, updates)
+	if err != nil {
+		return err
 	}
-	if len(updates) > maxUpdates {
-		return errors.New("Can not update more than 1/3 of validators at once")
+	if vp23 {
+		return errors.New("the change in voting power must be strictly less than 1/3")
 	}
 
 	for _, v := range updates {
@@ -174,6 +171,42 @@ func updateValidators(currentSet *types.ValidatorSet, updates []*abci.Validator)
 	return nil
 }
 
+func changeInVotingPowerMoreOrEqualToOneThird(currentSet *types.ValidatorSet, updates []*abci.Validator) (bool, error) {
+	threshold := currentSet.TotalVotingPower() * 1 / 3
+	acc := int64(0)
+
+	for _, v := range updates {
+		pubkey, err := crypto.PubKeyFromBytes(v.PubKey) // NOTE: expects go-wire encoded pubkey
+		if err != nil {
+			return false, err
+		}
+
+		address := pubkey.Address()
+		power := int64(v.Power)
+		// mind the overflow from int64
+		if power < 0 {
+			return false, fmt.Errorf("Power (%d) overflows int64", v.Power)
+		}
+
+		_, val := currentSet.GetByAddress(address)
+		if val == nil {
+			acc += power
+		} else {
+			np := val.VotingPower - power
+			if np < 0 {
+				np = -np
+			}
+			acc += np
+		}
+
+		if acc >= threshold {
+			return true, nil
+		}
+	}
+
+	return false, nil
+}
+
 // return a bit array of validators that signed the last commit
 // NOTE: assumes commits have already been authenticated
 /* function is currently unused
diff --git a/state/state_test.go b/state/state_test.go
index 1df300c7f..36e8f6975 100644
--- a/state/state_test.go
+++ b/state/state_test.go
@@ -134,7 +134,7 @@ func TestValidatorSimpleSaveLoad(t *testing.T) {
 // TestValidatorChangesSaveLoad tests saving and loading a validator set with
 // changes.
 func TestValidatorChangesSaveLoad(t *testing.T) {
-	const valSetSize = 6
+	const valSetSize = 7
 	tearDown, _, state := setupTestCase(t)
 	state.Validators = genValSet(valSetSize)
 	state.Save()
@@ -171,16 +171,14 @@ func genValSet(size int) *types.ValidatorSet {
 // with changes.
 func TestConsensusParamsChangesSaveLoad(t *testing.T) {
 	tearDown, _, state := setupTestCase(t)
-	const valSetSize = 20
-	state.Validators = genValSet(valSetSize)
-	state.Save()
 	defer tearDown(t)
 
 	// change vals at these heights
 	changeHeights := []int64{1, 2, 4, 5, 10, 15, 16, 17, 20}
 	N := len(changeHeights)
 
-	// create list of new vals
+	// each valset is just one validator
+	// create list of them
 	params := make([]types.ConsensusParams, N+1)
 	params[0] = state.ConsensusParams
 	for i := 1; i < N+1; i++ {
@@ -247,18 +245,21 @@ func makeParams(blockBytes, blockTx, blockGas, txBytes,
 	}
 }
 
-func TestLessThanOneThirdOfValidatorUpdatesEnforced(t *testing.T) {
+func TestLessThanOneThirdOfVotingPowerPerBlockEnforced(t *testing.T) {
 	tearDown, _, state := setupTestCase(t)
 	defer tearDown(t)
 
 	height := state.LastBlockHeight + 1
 	block := makeBlock(state, height)
 	abciResponses := &ABCIResponses{
-		Height:   height,
-		EndBlock: &abci.ResponseEndBlock{ValidatorUpdates: []*abci.Validator{{PubKey: []byte("a"), Power: 10}}},
+		Height: height,
+		// 1 val (vp: 10) => less than 3 is ok
+		EndBlock: &abci.ResponseEndBlock{ValidatorUpdates: []*abci.Validator{
+			{PubKey: crypto.GenPrivKeyEd25519().PubKey().Bytes(), Power: 3},
+		}},
 	}
 	err := state.SetBlockAndValidators(block.Header, types.PartSetHeader{}, abciResponses)
-	assert.NotNil(t, err, "expected err when trying to update more than 1/3 of validators")
+	assert.Error(t, err)
 }
 
 func TestApplyUpdates(t *testing.T) {