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Entrance conductor oversized


Jim Baird
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I know that generally it doesn't hurt to downsize an overcurrent protection device, but I'm not used to seeing it on the main panel.

150 amp rated panel with 150 amp main at the top has a fat 4/0 al entrance conductor straight from meter.

An addition has a subpanel fed by a 100 amp breaker out of the main.

I just noted this in report, and asked that the sparky who did the addition (within the last yr) verify that the house total load would safely be carried by this old 150 amp main.

I'm used to seeing no thicker wire than is needed just as a matter of easier bending.

Comments?

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Originally posted by Jim Baird

I know that generally it doesn't hurt to downsize an overcurrent protection device, but I'm not used to seeing it on the main panel.

150 amp rated panel with 150 amp main at the top has a fat 4/0 al entrance conductor straight from meter.

An addition has a subpanel fed by a 100 amp breaker out of the main.

I just noted this in report, and asked that the sparky who did the addition (within the last yr) verify that the house total load would safely be carried by this old 150 amp main.

I'm used to seeing no thicker wire than is needed just as a matter of easier bending.

Comments?

The 150-amp breaker is rated to accept a certain range of wire sizes. Was 4-0 aluminum one of them?

- Jim Katen, Oregon

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Usually when I see the four-ought it's to a 200 amp panel. Some of the economy grade spec builders in this area are putting 150 amp panels in, they don't oversize their feeders.

The breaker was large and appeared to accept the large cable. It didn't look funny till I found the breaker ID on the side of the toggle. Till then I assumed it was a 200 amp panel.

This was a existing installation that must have had it's entrance cable replaced at some time prior. It was painted all the way to the box, and if they had gone ahead and upgraded the panel I think it would have required the outside disconnect.

I have never bothered to learn how to calculate the load for a given house. Does anyone here do that?

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Originally posted by Jim Baird

Usually when I see the four-ought it's to a 200 amp panel. Some of the economy grade spec builders in this area are putting 150 amp panels in, they don't oversize their feeders.

Sometimes the electrician just uses what he has. It's cheaper than making an extra trip.

The breaker was large and appeared to accept the large cable. It didn't look funny till I found the breaker ID on the side of the toggle. Till then I assumed it was a 200 amp panel.

If the wire fit in the lug without needing a "haircut" it's probably fine.

This was a existing installation that must have had it's entrance cable replaced at some time prior. It was painted all the way to the box, and if they had gone ahead and upgraded the panel I think it would have required the outside disconnect.

I have never bothered to learn how to calculate the load for a given house. Does anyone here do that?

I do every so often when I suspect that a service is too small. It's pretty easy, anyone with 8th grade math skills ought to be able to do it. There's a calculation worksheet in the back of Code Check Electrical that you can use. I use a similar one that takes me less than a minute to do.

- Jim Katen, Oregon

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