sub panel, main breaker question

[Bill Dufour]

I will be installing a 200 amp subpanel in the shop. It will only be supplied by 50-70 amps or so. Can I use the existing 200 amp main breaker rather then buy a new lower amp one? To buy a main switch seems to cost more then a breaker does to fit the same panel location serving as a main disconnect.
Bill D

[Maurice Mcmurry]

For sub panels I have always used a main lug rather than a main breaker panel.

[Zachary Hoyt]

In my new workshop I used a used 100 amp panel from the house. I only had 50 amps feeding it. My electrical inspector told me to backfeed the panel through a 50 amp breaker and just leave the 100 amp breaker at the top in place but not connected. It may be different where you are, though.

[Tom M King]

The wire to that panel is protected by the properly sized breaker for the wire on the feed end. The main breaker in that panel just becomes a disconnect.

[Charlie Velasquez]

Yes, you can.
Done all the time.

Circuit breakers need to protect the wiring.
The wires feeding your sub will be protected by the breaker in your main.
The individual circuits in your ….

Never mind, I type too slowly.
I see Tom has already said this.

[glenn bradley]

Answers will be geographical. Consult the NEC; I believe we are currently complying with the 2022 edition in SoCal. These things are very easy to comply with once you know what version your state, county, and town are following. Deviations can result in insurance companies not paying if things go sideways.

[Jim Becker]

It will work fine as a disconnect (and it's nice you'll have one at the shop end...my shop disconnect is up at the house where the service splits), but I agree with Glenn that the local jurisdiction is where the final word lands.

[Bill Dufour]

Logically a over sized breaker is as good or better then a switch rated for even more amps before it fails. But logic does not always apply to rules.
Bill D

[Maurice Mcmurry]

Looking at the code, turns out mine with main lugs is in violation because it is in a detached building. It was inspected and passed. That is not surprising because our town voted in favor of not requiring the city engineer to have a degree in engineering.

[Charlie Velasquez]

Looking at the code, turns out mine with main lugs is in violation because it is in a detached building. It was inspected and passed. That is not surprising because our town voted in favor of not requiring the city engineer to have a degree in engineering.

Depends on year code you're area was using. The disconnect rule was started in the 2020 edition. Prior to that it was the rule of six. Must be able to disconnect power with no more than six throws.

[Jim Becker]

Yes, many jurisdictions remain on the 2017 (?) version of the NEC for their reference...mine included.

[Maurice Mcmurry]

Depends on year code you're area was using. The disconnect rule was started in the 2020 edition. Prior to that it was the rule of six. Must be able to disconnect power with no more than six throws.

OK! It was 2010 and I have 6 breakers. Good to know. Thank you Charlie.

[Charlie Velasquez]

OK! It was 2010 and I have 6 breakers. Good to know. Thank you Charlie.

Just as an aside, if you had had more than six breakers, and included several 120 volt circuits, you could bring it into compliance by getting approved handle ties and linking a couple of 120’s (or going to 240 volt breakers, one circuit on each leg).

You would maintain the over-current protection but have to put up with both circuits going off if either circuit trips a breaker.

[Bill Dufour]

In my new workshop I used a used 100 amp panel from the house. I only had 50 amps feeding it. My electrical inspector told me to backfeed the panel through a 50 amp breaker and just leave the 100 amp breaker at the top in place but not connected. It may be different where you are, though.

That seems like a poor idea. In an emergerncy a fire fighter would assume the top centered breaker, not in line with the lower ones was the main breaker.
Everyone knows the top most breaker is the main breaker without reading the old fadded labels..
Bill D

[Maurice Mcmurry]

That seems like a poor idea. In an emergerncy a fire fighter would assume the top centered breaker, not in line with the lower ones was the main breaker.
Everyone knows the top most breaker is the main breaker without reading the old fadded labels..
Bill D

I have used convertible panels. They come with "permanent" labels. Relying on labels will keep you compliant in most cases. In an emergency, in smoke, with a BA pack on, expecting a fire fighter to read the labels is asking a lot. I am wishing I had not gotten by with the rule of 6 back in 2010.

I have been trying to help my father in law sort our a failed inspection in Boston for over 30 years. A big, single function disconnect right inside the front door is required. We eventually received permission to use a shunt rather than rewire the entire main service. That was over 20 years ago. Eventually that work will have to be done. Now we are going to have to replace the entire service, under ground, out to the street, upgrading from 200 amps to 400 to meet the current requirements. I sure wish F.I.L. and his electrician had gotten this sorted out in 1987.