Minimum disconnect rating

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paul32

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Minnesota
A few questions on this subject. 15A minimum for 1 circuit, 30A for two 2-wire circuits, etc. I believe a multiwire branch circuit qualifies for the 15A minimum, right?

1. Why would anyone use the two 2-wire circuits which would need a 30A disconnect and ground rod, rather than just a 15 or 20A multiwire circuit.

2. Is the 30A thing assuming they are on one phase? If the two circuits are on different phases you'd never need 30A.

(I guess I'm missing the point of the 30A minimum for 2 circuits.)

3. In general, what is the rating of the disconnect when more than one (up to 6) breakers used with no main? Is it sum of the breakers on a phase, rating of the panel, or something else? A 60A main lug panel with two 15A breakers installed. Is that a 60A disconnect? Code reference for this one please.
 
Re: Minimum disconnect rating

I guess my question 3 could be taken a different way. I know the 60A requirement, I was asking how to determine what the actual rating of a disconnect is. If it has a single main breaker, it is the rating of that breaker. If the disconnect is 2 to 6 breakers, it's rating is what? In other words, how do you know you are meeting the 60A requirement?
 
Re: Minimum disconnect rating

Paul,

Some things are better left alone!

If I had 6 - 200 amps breakers for means of disco for 6 buildings each calculated for 100 amps each, I would have 1200 amps of breakers, 600 amps of calculated load. Two sets of 350's.

But yet a 600 amp service.

Feeders however ?????????????Code appears to be silent.

Jim
 
Re: Minimum disconnect rating

paul32...with the 2-6 breakers at the sub-panel and no main, you would determine the?size of the feeder based on the size of the conductors feeding it from its source and the overcurrent device protecting this feeder.

shortcircuit2
 
Re: Minimum disconnect rating

We are trying to find out what size disconnect you would have, not feeder size. The code doesn't say that I need a 60 amp feeder to my detached garage, I however need a 60 amp rated disconnect at my garage, nearest the point of entrance.
 
Re: Minimum disconnect rating

Exactly as John has posted, you need to calculate the load and size the conductors and disconnect accordingly.


What I have experienced from my inspections and asking the electricians how they size their subpanels/disconnects, they usually just guess from past experience.

Calculations scare the *rap out of the average electrician.
 
Re: Minimum disconnect rating

Indoor feeders no longer have to have a minimum rating based on the number of branch circuits. Outdoor feeders still have to have minimu rating based on the number of branch circuits.

If an indoor feeder runs a bunch of itty bitty motors you can run a 15 amp feeder and use small fuses for each motor.
 
Re: Minimum disconnect rating

Originally posted by mc5w:
Indoor feeders no longer have to have a minimum rating based on the number of branch circuits. Outdoor feeders still have to have minimu rating based on the number of branch circuits.
Click to expand...
Are you referring to 225.39?

That has nothing to do with the size of the feeder, only the rating of the disconnecting means. :p
 
Re: Minimum disconnect rating

Pierre, Ok, calculate the load and then the answer has to be at least 60A. :)

Seems like the disconnect for a feeder to another building would be determined by the feeder rating, but no code reference to that. That makes the most sense to me, and I suspect the adding the multiple disconnects for a service is there because there is no feeder rating.

Nobody has addressed my questions 1 and 2. I'm still missing the point of the 30A minimum. I assume if an existing 30A feeder to a building had 2 branch circuits, a 3rd couldn't be added since that requires 60A. So if you can't upgrade the 2 circuit case, why not use a multiwire. But even using the feeder to 2 15 or 20A circuits, why would the feeder need to be 30A.
 
Re: Minimum disconnect rating

The only feeder minimum sizes I can find are in 215.2(2). The requirements here actually make sense.

I can't resolve what the rating of a disconnect to a detached structure is either, especially when using a panel with multiple grouped "disconnect" breakers. I would argue its the rating if the enclosure and not a sum of installed breakers.
 
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