Power　line　communication　(PLC)　is　an　attractive　solution　to　facilitate　the　home　area　network　(HAN).　According　to　the　indoor　power　line　norm　in　China,　an　approximation　of　channel　capacity　is　proposed　based　on　the　bottom-up　channel　model,　which　is　helpful　evaluating　the　transmission　capacity　by　using　the　information　of　transmission　distance　and　number　of　branches.　The　decode-and-forward　relay　scheme　is　employed,　and　boundary　condition　indicates　the　number　of　relays　needed　under　different　scenarios　is　studied　based　on　the　approximation　equation　of　channel　capacity.　Finally,　the　optimal　position　of　relay　in　the　power　network　condition　of　all　branches　uniformly　and　nonuniform　distributed　are　analyzed　respectively.