Outbuilding Disconnect Rating

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devnull

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Bolingbrook,IL
Background.
Outbuilding is fed with a single phase mwbc protected by a double pole 30A breaker
Panel in building has the following circuits
1 - 15A furnace circuit
2 - 15A lighting circuits
3 - 20A receptacle circuits
1 - 30A mwbc for air compressor.

225.39 (B) says a 60 amp disconnect, which doesn't make sense to me on a protected 30A feed.

225.39 says I can combine the ratings of the disconnects, so if I use two 30A single pole breakers and
tie the handles together (225.33(B) handle ties and 225.38(B) for Simultaneous Opening of Poles) I
meet the requirements. The part that has me confused is how can this meet the requirements, 30+30=60,
but a single double pole 30 amp breaker doesn't? Is my thinking wrong and I've missed someplace
that says that each branch of the mwbc needs a 60 amp disconnect or can you add the 30A from each branch
of the double pole breaker to get to 60A? The code says combining the ratings of all the switches or circuit
breakers for determining the rating of the disconnecting means shall be permitted. It doesn't say to differentiate
between branches it just says combine so 30+30=60.

I'm probably just overthinking this but I'd rather see the additional protection of two handle tied 30A single
pole breakers ( or a single 30 amp double pole breaker) than a 60A breaker for the disconnect or combining
the two lighting circuits to get back under the 6 hand movement rule.

Long time lurker, first time poster.
 
225.39 makes no sense to me but it only refers to the rating of the disconnect not the conductor size. Technically you can install a dp 60 amp breaker in the panel at the detached structure and back feed it if the panel allows back fed breakers.

However what is the calculated load for the rest of the circuits? It seems like they may an issue when the compressor and all the lights are on but maybe not.
 
Dennis Alwon said:
225.39 makes no sense to me but it only refers to the rating of the disconnect not the conductor size. Technically you can install a dp 60 amp breaker in the panel at the detached structure and back feed it if the panel allows back fed breakers.

However what is the calculated load for the rest of the circuits? It seems like they may an issue when the compressor and all the lights are on but maybe not.
Click to expand...
For me the clue that resolves the apparent two-breaker vs. one two pole breaker contradiction comes in the fact that the sentence refers to the disconnect being sized to the rated load.
The rated load (in kVA) for a 120V breaker will be half the rated load of a two pole 240V breaker of the same amperage.

So when you combine two 20A single pole breakers you get the same rated load capacity as a two-pole 20A 240V breaker.

Since we are used to calculating load in VA and then converting to amps before we look at the breaker size, it is easy to fall into the trap of adding the amperage values of the breakers rather than their load capacity.
 
Dennis Alwon said:
BTW, the wire to the outbuiding is called a feeder and not a multiwire branch circuit.
Click to expand...

Yes it is, it's been a while since I've actually had to do any real work, the upside is I found my copy of the 1999 code book. I seem to have converted a lot of brain gray matter into gray hair since then.


Dennis Alwon said:
225.39 makes no sense to me but it only refers to the rating of the disconnect not the conductor size. Technically you can install a dp 60 amp breaker in the panel at the detached structure and back feed it if the panel allows back fed breakers.

However what is the calculated load for the rest of the circuits? It seems like they may an issue when the compressor and all the lights are on but maybe not.?
Click to expand...

The compressor is intermittent use only and only has problems when the building is ice cold and the pressure relief valve hasn't taken the load off of it.

It seems to me that 225.39 was written with good intent, but without taking real world applications into account. It should be clarified how the ratings of of the switches/breakers should be summed, per ungrounded conductor or all ungrounded conductors total and maybe take into account the ampacity of the feeder and feeder protection breakers.
 
It isn't clear. To me, I'd take all the breakers out of the panel and pick them up one at a time and read their rating. Keep adding numbers until you're done -- 15+15+15+20+20+20+30 = 135A disconnect, so this is large enough.

A problem you have, is you have more than 6 handles. So either you need a backfed main in that panel (60A or larger), or you need to turn some of those single pole circuits into double pole MWBC's to make 6 or less handles.
 
Another problem is if the op is not under the 2014 NEC. The panel must be rated for service equipment and most main lug panels state that they are rated for service equipment only when a main breaker kit is installed. BS I know.........
 
I disagree it is 'BS', you know the reason for that requirement has to do with neutral bonding ability of the disconnect.

Now that the rules changed a few cycles ago preventing new installations from using the neutral as grounding, this bonding ability is less important but the rule had / has a purpose. It was not just BS. :)
 
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I don't believe it needed to say it had to be service rated. The rule was there before the 2005 but it lay there having no meaning since they changed the code on conductors to detached structures. Sure it had some meaning back then
 
iwire said:
I disagree it is 'BS', you know the reason for that requirement has to do with neutral bonding ability of the disconnect.

Now that the rules changed a few cycles ago preventing new installations from using the neutral as grounding, this bonding ability is less important but the rule had / has a purpose. It was not just BS. :)
Click to expand...

I think the point is most of these panels say "Suitable for use for Service Equipment when not more than 6 overcurrent devices installed and not used as a lighting and appliance panelboard, or when a main breaker is installed".

This has nothing to do with the neutral bonding ability -- either the panel has that or it doesn't and the number of breakers is irrelevant. There were some other SUSE rating limitations, especially with panels with more than 12 slots or circuits. But it was really this whole lighting and appliance panelboard thing that messed it up. That went away in 2008 or 2011. But the panel manufacturers didn't change their labeling and it still caused confusion. The 2014 solution should fix this problem.
 
But, even though the NEC reason behind it no longer exists, we are still required to follow labelling instructions, right?
No confusion there.
:)
 
GoldDigger said:
But, even though the NEC reason behind it no longer exists, we are still required to follow labelling instructions, right?
No confusion there.
:)
Click to expand...

Yes, but the panel does not need to be service rated in 2014 so the install could be compliant by making a few sp breakers dp breaker using handle ties.
 
suemarkp said:
I think the point is most of these panels say "Suitable for use for Service Equipment when not more than 6 overcurrent devices installed and not used as a lighting and appliance panelboard, or when a main breaker is installed".

This has nothing to do with the neutral bonding ability -- either the panel has that or it doesn't and the number of breakers is irrelevant. There were some other SUSE rating limitations, especially with panels with more than 12 slots or circuits. But it was really this whole lighting and appliance panelboard thing that messed it up. That went away in 2008 or 2011. But the panel manufacturers didn't change their labeling and it still caused confusion. The 2014 solution should fix this problem.
Click to expand...
Lighting and appliance panelboard is no longer defined in NEC though, they need to change labeling if they haven't already, and some places are still on older codes, so labeling changes possibly need to specify if it does comply with 2011 NEC requirements. If they can push AFCI's like they did I don't think this labeling is too much to ask for.
 
Just wanted to thank everyone for their feedback. Gave me a few things to think about and
I think I'm going to just go with the 2 single pole breakers and a handle tie. I'm going to interpret
the code like it's written, "Where the branch circuit or feeder disconnecting means consists of more
than one switch or circuit breaker, as permitted by 225.33, combining the ratings of all the switches
or circuit breakers for determining the rating of the disconnecting means shall be permitted." We all
know what they really want is 60 amp disconnect per ungrounded conductor, but that's not what
they put into code, no place do they say I can't add the values of the disconnects on different
ungrounded conductors. In my specific application it also just makes sense to use 30A breakers at the
panel because it's being fed with a 30A protected feeder.

As an aside the panel I'm using is UL listed as a panelboard. It does have the label about no more than
6 breakers or use as a lighting or appliance panelboard unless a main breaker is used. If a main
breaker is used a hold down kit is required, which might not be possible with two single pole breakers,
so I'm forced to use a 60A duplex breaker instead of 30A breakers which are the size that makes
sense to me.

Again thanks for everyone's comments.....
 
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