De-rating Question

[andrew.tkelly]

Someone once told me you can't derate a main breaker on a residential panel down to 90 amps anymore. 100 amps is the bottom limit. Is there any validity to that statement?

Thanks!

Regards,
Andy

 

[GoldDigger]

In some areas and uses the minimum service is 100A. Whether that means you cannot put a 90A breaker on a 100A POCO service will be hotly debated.

Sent from my XT1585 using Tapatalk

 

[don_resqcapt19]

230.79(C) specifies a 100 amp minimum rating for the service disconnect for dwelling units. If you have a 90 amp main breaker in a service panel with a bus rating of 100 amps or more, what is the rating of the service disconnect?

 

[ggunn]

230.79(C) specifies a 100 amp minimum rating for the service disconnect for dwelling units. If you have a 90 amp main breaker in a service panel with a bus rating of 100 amps or more, what is the rating of the service disconnect?

The rating of the service disconnect is the rating of the OCPD. In this case, it's 90A.

 

[Smart $]

The rating of the service disconnect is the rating of the OCPD. In this case, it's 90A.

What if you had a 100A unfused SUSE disconnect with a 60A MCB panelboard immediately adjacent thereto on the load side?

Code clearly separates Service Disconnecting Means and OCPD. That said, I agree it is the device the service entrance conductors connect to that must be rated 100A. The debate is whether we can use the equipment rating vs. the device rating. See definition of Disconnecting Means.

 

[ggunn]

What if you had a 100A unfused SUSE disconnect with a 60A MCB panelboard immediately adjacent thereto on the load side?

Code clearly separates Service Disconnecting Means and OCPD. That said, I agree it is the device the service entrance conductors connect to that must be rated 100A. The debate is whether we can use the equipment rating vs. the device rating. See definition of Disconnecting Means.

If the panelboard has 60A main breaker and that's the first OCPD in from the meter, I'd call it a 60A service. It determines the ampacity of the service conductors, doesn't it?

 

[andrew.tkelly]

What does MCB and SUSE mean? Still getting the hang of the jargon.

 

[GoldDigger]

What does MCB and SUSE mean? Still getting the hang of the jargon.

MCB = Main Circuit Breaker. As contrasted to MLO, Main Lug Only.
SUSE = Suitable for Use as Service Equipment.

 

[charlie b]

If the panelboard has 60A main breaker and that's the first OCPD in from the meter, I'd call it a 60A service. It determines the ampacity of the service conductors, doesn't it?

The rating of the first OCPD is not related to the code requirement under discussion - 230.79(C). What matters is the rating of the service disconnecting means. If you have an unfused disconnect serving that role, and if it is rated at 100 amps, then the ratings of the downstream panel and its MCB don't come into play.

 

[Smart $]

If the panelboard has 60A main breaker and that's the first OCPD in from the meter, I'd call it a 60A service. It determines the ampacity of the service conductors, doesn't it?

No, it does not. The lesser of the service entrance conductors ampacity rating or the disconnect rating determines the service rating. Being 100A is required, you have to run at least 100A conductors. You could even add another 60A MCB panelboard.

 

[jaggedben]

What determines the 'service rating' is not made explicit anywhere in the code to my knowledge. But it is also moot with regard to the topic because none of the requirements refer to the rating of the service.

With respect to 230.79(C), I think that using a 100A rated fused disconnect with fuses rated less than 100A gives one a stronger argument in favor of compliance than using a 100A panelboard with a less than 100A rated circuit breaker. That's because the fuses are not the disconnecting means and can be changed out without changing the disconnecting means, but the same can't really be said of the circuit breaker. But I also think that the code is vague enough that an AHJ's opinion matters more than mine.

 

[Smart $]

What determines the 'service rating' is not made explicit anywhere in the code to my knowledge. But it is also moot with regard to the topic because none of the requirements refer to the rating of the service.

I agree 'service rating' is not made explicit anywhere in Code. In fact, it is not even a bona fide Code term. But I do think we all see it as the maximum operating current the service equipment can provide under Code.

The pertinent section to back up my statement...

(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).

So what are 'specific installations'??? :blink:​

 

[jaggedben]

I agree 'service rating' is not made explicit anywhere in Code. In fact, it is not even a bona fide Code term. But I do think we all see it as the minimum operating current the service equipment can provide under Code.

FIFY. As for whether the code clearly establishes the point...

The pertinent section to back up my statement...[/QUOTE]

So if I install a 100A disco fed with 100A service conductors, can I still put smaller fuses in it?

More to the point of the OP: If I have an existing 100A panelboard with 100A MCB fed by 100A service conductors, can I downsize the MCB to 90A? It is notable that the section of 230 that deals with overcurrent protection does not refer to the same minimum requirements. But see mycomment above about AHJs' opinions.

 

[Smart $]

FIFY. As for whether the code clearly establishes the point...

You did not fix it. How I stated it is correct. Service rating is a well-established trade term.

So if I install a 100A disco fed with 100A service conductors, can I still put smaller fuses in it?

More to the point of the OP: If I have an existing 100A panelboard with 100A MCB fed by 100A service conductors, can I downsize the MCB to 90A? It is notable that the section of 230 that deals with overcurrent protection does not refer to the same minimum requirements. But see mycomment above about AHJs' opinions.

Yes to the first question.

No to the second if it is a dwelling service... because the main circuit breaker is the service disconnecting means device. That it is also the OCPD is irrelevant.

 

[jaggedben]

You did not fix it. How I stated it is correct. Service rating is a well-established trade term.

I got mixed up and thought you were referring to the 230.79 requirements.

Still vague to me if the service rating of a service where where everything is rated 100A except the fuses is not still 100A. I mean, just change the fuses...

 

[Smart $]

...
Still vague to me if the service rating of a service where where everything is rated 100A except the fuses is not still 100A. I mean, just change the fuses...

The fuses do not determine the service rating (providing they are of a compliant rating). The service would only supply up to the fuse rating, but it is still a 100A service.

The only thing that is vague to me is if an unfused disconnect is used and OCPD is immediately adjacent, are the conductors between the switch and OCPD service conductors or feeders. I lean heavily towards service conductors. Then there's the question whether or not we can 'branch' to more than one OCPD. Food for thought.

 

[kwired]

The fuses do not determine the service rating (providing they are of a compliant rating). The service would only supply up to the fuse rating, but it is still a 100A service.

Now complicate it with 2 to 6 service disconnecting means consisting of two 400 amp breakers and two 125 amp breakers but ampacity of service conductors is only 750 amps and load calculation is only maybe 450 amps.

I know not quite what the OP is about, but that was an actual install I once did. To get more on topic I guess a hypothetical though probably impractical dwelling unit service disconnecting means consisting of two 30 amp breakers and two 60 amp breakers, total load calculation of 85 amps service conductors with ampacity of 90 amps (before applying 310.15(B)(7). Should be legal.

 

[matt123]

This installation is going to have 2 single phase 7kw inverters. Balancing as best as possible has come up as an idea but in the end are we going to need to change to 3 phase inverters?

 

[matt123]

This installation is going to have 2 single phase 7kw inverters. Balancing as best as possible has come up as an idea but in the end are we going to need to change to 3 phase inverters?

sorry wrong thread***

 

[Smart $]

Now complicate it with 2 to 6 service disconnecting means consisting of two 400 amp breakers and two 125 amp breakers but ampacity of service conductors is only 750 amps and load calculation is only maybe 450 amps.

I know not quite what the OP is about, but that was an actual install I once did. To get more on topic I guess a hypothetical though probably impractical dwelling unit service disconnecting means consisting of two 30 amp breakers and two 60 amp breakers, total load calculation of 85 amps service conductors with ampacity of 90 amps (before applying 310.15(B)(7). Should be legal.

Bringing up 310.15(B)(7) is a pertinent point in this discussion. You can compliantly have 83A_or the earlier equivalent_service conductors and have a 100A service... but the service disconnecting means device must still be rated 100A.