120% and derating a main service breaker

Status
Not open for further replies.

Smart $

Esteemed Member
Location
Ohio
I don't see that as changing anything...yes the complete system must be listed as service equipment, but the service disconnect is the breaker and the rating of the service disconnect is the rating of the breaker.
So if you have a 100A SUSE [unfused] safety switch as the disconnecting means, and an immediately adjacent 60A MCB panelboard, is the service disconnecting means rated 100A or 60A?
 

Carultch

Senior Member
Location
Massachusetts
So if you have a 100A SUSE [unfused] safety switch as the disconnecting means, and an immediately adjacent 60A MCB panelboard, is the service disconnecting means rated 100A or 60A?

What does SUSE mean? Not familiar with that term.

As for your situation, I would expect that the OCPD governs the service size rather than the disconnect ampacity. Because the OCPD governs what current can actually flow. The disconnect has automatic action that is sensitive to the current flowing through it, provided that it is less than its rating.

I see oversizing of an unfused disconnect the same way that I see oversizing conductors. It is simply a stronger link in the chain than it needs to be.
 

Smart $

Esteemed Member
Location
Ohio
What does SUSE mean? Not familiar with that term.

As for your situation, I would expect that the OCPD governs the service size rather than the disconnect ampacity. Because the OCPD governs what current can actually flow. The disconnect has automatic action that is sensitive to the current flowing through it, provided that it is less than its rating.

I see oversizing of an unfused disconnect the same way that I see oversizing conductors. It is simply a stronger link in the chain than it needs to be.
SUSE = suitable for use as service equipment

See 230.66 quoted in earlier post.

The issue here is 230.79 Rating of Service Disconnecting Means. Code does not specifically say this is the service rating.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
So I install a service panel that is rated at 225 amps with a 100 amp main breaker. What is the rating of the service disconnect?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
SUSE = suitable for use as service equipment

See 230.66 quoted in earlier post.

230.66 is really irrelevant.

The issue here is 230.79 Rating of Service Disconnecting Means. Code does not specifically say this is the service rating.

Right. We aren't talking about the service rating or the service equipment rating.

So I install a service panel that is rated at 225 amps with a 100 amp main breaker. What is the rating of the service disconnect?

100A

I think Smart$ has pointed out an interesting loophole in the code, which is that if one uses a switch (i.e. fused disconnect) as the service disconnecting means, then the code does not explicitly require the fuses to meet the minimums in 230.79. I can think of at least one project I've been involved in where, if the AHJ would buy the argument, a 50A breaker could be replaced with a 100A fused disconnect with 50A fuses, and avoid the huge cost of upgrading underground service conductors. Fortunately I'm not the one on the hook for making a bad decision on that project. :happyno:

I don't buy the argument that a circuit breaker's rating can be less than the 230.79 requirements just because some other part of the service equipment is so rated.
 

iwire

Moderator
Staff member
Location
Massachusetts
2if one uses a switch (i.e. fused disconnect) as the service disconnecting means, then the code does not explicitly require the fuses to meet the minimums in 230.79.

No, but it does require the conductors to be.

230.42 Minimum Size and Rating.

(B) Specific Installations.
In addition to the requirements
of 230.42(A), the minimum ampacity for ungrounded conductors
for specific installations shall not be less than the
rating of the service disconnecting means specified in
230.79(A) through (D)

I doubt any AHJ would allow fuses rated lower than the required disconnecting means for the particular installation.
 
Last edited:

Smart $

Esteemed Member
Location
Ohio
So I install a service panel that is rated at 225 amps with a 100 amp main breaker. What is the rating of the service disconnect?
The service disconnecting means is rated 225A.

The service rating also depends on the ampacity of the supply conductors and the OCPD rating, both of which must not be less than the continuous-factored calculated load. In this case, conductor ampacity has not been stated, so it is not less than 91A [240.4(B)] and not more than 100A.
 

Smart $

Esteemed Member
Location
Ohio
230.66 is really irrelevant. ...
It is relevant. The service disconnecting means must be identified as SUSE and must also be listed. The main breaker itself does not meet that requirement. While the panelboard itself is listed per the standard main breaker rating. So sorry if that introduces a conflict into everyone's way of thinking... :weeping:
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
It is relevant. The service disconnecting means must be identified as SUSE and must also be listed. The main breaker itself does not meet that requirement. While the panelboard itself is listed per the standard main breaker rating. So sorry if that introduces a conflict into everyone's way of thinking... :weeping:
We are not going to agree on this one....I stand by my statement that the service disconnect is the device that physically disconnects the connection to the utility and that is the breaker it self in the case of a breaker panel. In the case of a disconnect, it is the knife switch.
 

Smart $

Esteemed Member
Location
Ohio
We are not going to agree on this one....I stand by my statement that the service disconnect is the device that physically disconnects the connection to the utility and that is the breaker it self in the case of a breaker panel. In the case of a disconnect, it is the knife switch.
Fine by me. It wouldn't take much for me to see things your way. Just a few words actually.... :D
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It is relevant. The service disconnecting means must be identified as SUSE and must also be listed. The main breaker itself does not meet that requirement. While the panelboard itself is listed per the standard main breaker rating. So sorry if that introduces a conflict into everyone's way of thinking... :weeping:

I'm with Don, I don't buy this line of argument whatsoever.

First, the code does not require the service disconnecting means to be identified as SUSE. That requirement applies to the equipment, not the disconnecting means per se. Now if you have a panelboard that is listed SUSE and has a label that says that model(s) #XXX circuit breaker can be installed as a disconnecting means, then that logically entails that the breaker is suitable for use as the service disconnecting means when used in that equipment. But that does not entail that the breaker needs its own SUSE label (again, code doesn't require that) or that the breaker is SUSE when used in equipment that is not SUSE.

Second, one cannot install service equipment that has a listed rating less that the disconnecting means inside it. And the section iwire quoted pretty much entails that the rest of the line side elements must also meet the minimum requirements.

All we are left with is your original point, that the code forgets to lay those requirements on the OCPD, which only means anything when it's a separate fuse. I agree with iwire that it would be a hard argument to drive with any AHJ, even though I agree you've correctly pointed out what the code says (or rather, doesn't say).
 

Smart $

Esteemed Member
Location
Ohio
I'm with Don, I don't buy this line of argument whatsoever.

First, the code does not require the service disconnecting means to be identified as SUSE. That requirement applies to the equipment, not the disconnecting means per se. Now if you have a panelboard that is listed SUSE and has a label that says that model(s) #XXX circuit breaker can be installed as a disconnecting means, then that logically entails that the breaker is suitable for use as the service disconnecting means when used in that equipment. But that does not entail that the breaker needs its own SUSE label (again, code doesn't require that) or that the breaker is SUSE when used in equipment that is not SUSE.
I understand what you are saying, but I see it as a construed interpretation, but not one from explicit , difficult-to-misconstrue wording. So I ask, where does Code actually state what you are saying? ...because I don't see it.

Article 230 Part V is titled Service Equipment — General, so it covers all service equipment whether it be a switch or circuit breaker, a cabinet, an enclosure, anything forming part of the service other than what is excluded by Code. Actually this requirement can be extended to things like bugs (lugs, splice/tap blocks, etc.) if you go by the Article 100 definition of equipment but I believe many just overlook the SUSE and listing requirement for seemingly irrelevant equipment.

Now just because Article 230 Parts VI & VII cover the service disconnecting means and overcurrent protection, note the Part titles begin with Service Equipment. These titles can be taken as two opposing views. It either includes or excludes Part V requirements. Since it is all service equipment, Part V applies to all, just as general requirements apply to various sections throughout Code. It appears you are saying the SUSE requirement does not apply to the service disconnecting means or overcurrent protection. I can definitely not agree with that and further supports my view of these devices being an integral part of the SUSE equipment.

Second, one cannot install service equipment that has a listed rating less that the disconnecting means inside it. And the section iwire quoted pretty much entails that the rest of the line side elements must also meet the minimum requirements.
This is a matter of consequence stemming from your interpretation. I'm not going to argue these finer points when I'm not in agreement with the more general concept.

All we are left with is your original point, that the code forgets to lay those requirements on the OCPD, which only means anything when it's a separate fuse. I agree with iwire that it would be a hard argument to drive with any AHJ, even though I agree you've correctly pointed out what the code says (or rather, doesn't say).
As I mentioned to Don, I have no problem agreeing with your position... if Code actually made it clearly the intent. Unfortunately, I simply do not see it at this point in time.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I understand what you are saying, but I see it as a construed interpretation, but not one from explicit , difficult-to-misconstrue wording. So I ask, where does Code actually state what you are saying? ...because I don't see it.

I think the only code section that hasn't been mentioned that plays into the logic in my post #34 is 100.3(B). Otherwise they have all been mentioned already in this thread.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
As I mentioned to Don, I have no problem agreeing with your position... if Code actually made it clearly the intent. Unfortunately, I simply do not see it at this point in time.
When it is not clear it is up to the AHJ, and I am am member of our Electrical Commission and most of the members will vote with me so I represent the AHJ:D
 

Zee

Senior Member
Location
CA
I've gotten away with the type of configuration in question only when the existing main breaker was ..... grandfathered.

If the panel label doesn't specify the size, then next time just label the 90A breaker as 'existing' on your planset, and let it somehow be existing before the AHJ ever shows up to inspect. ;)

Funny...ya know it's weird how often I run into 200 Amp MSP's, already "pre-configured" with (E) 2p175Amp main breakers. Somehow this seems to happen only when I am installing over 40 amps of PV.
I am really quite fortuitous.:roll:
What are the odds!
--------------------------
On a serious note, and I know this is an old thread, I'd like to hear what was resolved.
---------------------------
OP saw three options. I would add a fourth...... depending on total DC kW vs inverter rating.
Option 4: Get an inverter that is 16 A max. This means the 3800 W.
It is possible, given the 30A breaker mentioned, that th einverter chosen had been the ridiculously wastefully sized 4 kw or similar inverter. That is a hair over the 16 A output @ 125% = 20 A cutoff....yet requires a 2p30 pv breaker.
Also maybe a smaller inverter can be overloaded quite a bit with the existing kW.
It may be a minor loss in output and a major $$ savings.
Sometimes, I'd rather put 5 kW-STC, say, on a 3800 inv. than deal with a new service.....
 
Status
Not open for further replies.
Top