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Had a fire in one room (NZ Lockwood home) and renovated the entire interior due to smoke damage. Changed my main panel (150 amp) to a 200 amp and balanced the load to all breakers. No addition to load (@ 150).

Had a 1/0 feed to 150 amp main. The master electrician whom I've hired to sign off on the work says the NEC Code calls for a 2/0 for 200 amp main, but the load remains the same, so I disagree. The local power authority agrees with me do I hire another ME?

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  • So you have a 150A main breaker in a 200A loadcenter? – ThreePhaseEel Dec 30 '18 at 6:55
  • Also, are the wires going into the main breaker copper or aluminum? – ThreePhaseEel Dec 30 '18 at 6:56
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    Need information: ampacity of main breaker in your current panel..... – Billy C. Dec 30 '18 at 12:30
  • If you change the main breaker size the load is irrilevent unless at a supervised location (comercial/industrial) you need to upsize the feeder. Although the feeder can be 83% of the main I have never tried to get by with less than that on residential. – Ed Beal Feb 5 '19 at 15:03
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According to 2017 NEC Article 310.15 (B)(7). The sizing of the conductors can permitted to be sized in accordance with 301.15(B)(7) (1)through (7). Meaning it must meet the requirements of (1)-(7). If it does meet those requirements. Then a 200A service feeders taken from Table 310.15(B)(16) column 2 could be a 2/0.

The calculation would be 200A (which is normally a 3/0)*.83 (310.15(B)(7)(1))= 166A. Using Table 310.15 column 2 the next largest conductor would be a 2/0 (175A).

I really can't be more specific but you would have to detail all of the requirements of 310.15 (B)(7) (1)-(7). I am not sure what you mean with "local power authority" but if it is your authority having jurisdiction (AHJ) aka the electrical inspector. Then they have the final say about what is to be installed. I suggest you have your contractor and AHJ and reach a conclusion.

Hope this helps.

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  • local power authority may be the electric utility. Which should care about safety but may be looking more at the actual loads. Also possible the utility is only supplying 150 Amp service and therefore doesn't care what you have in your house since by their calculations you're only going to get 150 Amps anyway. But if that's the case, I assume there would need to be a 150 Amp breaker somewhere. – manassehkatz-Moving 2 Codidact Dec 30 '18 at 15:14
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    @manassehkatz - If it is an electric utility company then they really have no say of any equipment or conductors being installed after the metering device. They operate on their own set of rules. But if they are the AHJ which might be true in something like a rural area, then they have every right to tell you what to do. I am just showing how a master electrician use the code to calculate his feeder to a 2/0. – Retired Master Electrician Dec 30 '18 at 15:16
  • I agree! But the phrasing "local power authority" makes me think that maybe the utility cut off service after the fire and that OP had a conversation with them regarding getting service restored. But is just a guess because it sounds sort of like "AHJ" but not quite. – manassehkatz-Moving 2 Codidact Dec 30 '18 at 15:19
  • I don't know of any AHJ that would allow two small of a feeder, just because the old panel was 150 means nothing, now with a 200 someone might try to add a whole house tankless water heater and now we might have another fire. + – Ed Beal Feb 5 '19 at 15:08
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Your rationale is faulty, but your conclusion is lucky.

Normal or provisioned load figures cannot be used as an excuse to not put proper overcurrent protection on a wire. Breakers are for abnormal operation. Normal/average operation cannot be your only defense against overload.

Your main breaker needs to be sized to protect the service entrance wires.

Your 1/0 copper wire is good for 150 amps.

However, since it serves 100% of the house's load, you get to derate it by 83% based on NEC 310.15B7. That gives 180.7 amps. You need a 180.7A main breaker.

However, since that size is not made (nor is 180), you get to round up to the next available breaker size. That is 200A.

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