Minimum current ampacity

Other than the requirement for carrying LRA on fire pumps, I can't readily recall any place in the NEC that controls the minimum size of OCPD.

For sure, I regularly used smaller than 125% primary side protection of 75kVA transformers so I could stay in a 100A disconnect.
 
ggunn said:
Although if it were to pull 23A through a 20A breaker and never ran continuously for several hours at a time, the breaker might never trip.
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Actually it could pull 115% forever, per UL listing of minature molded case breakers. OCPD trip curves are based at a specific ambient temperature.
 
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AC\DC said:
now this equipment is in 440- I don’t have my book I am at an event.. it this by default consider continuous use?? For the over current protection device. if it doesn’t state it in 440 this overrides 220.
General rules are chapter 1 through three apply and 400 can modify
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90.3 Code Arrangement

This code is divided into the introduction and nine chapters, as shown in Figure 90.3. Chapters 1 through 4 apply generally. Chapters 5 through 8may supplement or modify the requirements in Chapters 1 through 8.
Chapter 9 consists of tables that are applicable as referenced.
Informative annexes are not part of the requirements of this code but are included for informational purposes only.
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Master Nater said:
i used to take a continuing education class with an old-school master electrician by the name of fred bender. the guy was the president of the electrical board in the state of nebraska. he was an incredibly well-versed and super-intelligent guy.

one year, he did a motor class and was pretty spot on when someone asked him about what OCPD to install on a motor when we were given the possibility of a 250% on an inverse time breaker. he said, "which one do you install? you install the biggest one you can. why not? do you think the 40a breaker will clear slower than a 25a breaker on a short circuit? nope. do you think the breaker will protect the motor, in the event of overload? NOPE. there's no reason not to put in the biggest one that the code allows."

overlooking the possible price difference, it was hard to disagree with his logic. he was right. the breaker will still function in the event of a short circuit, and protect the insulation on the conductors. and the overload/VFD settings that provide the OL protection are the only things gonna protect the motors, if they go into overload.

the only reason i see not to go to the 25a is just stubborn pride. but, i get it. i've been there too. :)

have a good night.
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Makes a lot of sense.
 
Yeah, modify could’ve been the bad term well, like 240 call out that 310.15 circuit size 14 through 10 can be modified based on these sections so they don’t broadly modify it but they do modify it
 
AC\DC said:
Got a Mini Split with a minimum current capacity of 23 and a Max overcurrent protection of 25 I installed the number 10 and I put it on a 20 amp breaker
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Could you send us an actual model number, or picture of the rating label?

Note every Mini-split I've taken a meter to has pulled less than 50% of it's nameplate rating continuously.
And had essentially zero startup current.
(Not that startup current would matter for a breaker.).
These units, at least the ones used here, all have solid state controls and probably linear motors that soft start, so startup current is not an issue.

Perhaps bigger HVAC units still have startup current as a factor, dunno.
 
brycenesbitt said:
Could you send us an actual model number, or picture of the rating label?

Note every Mini-split I've taken a meter to has pulled less than 50% of it's nameplate rating continuously.
And had essentially zero startup current.
(Not that startup current would matter for a breaker.).
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That matches my experience with the minisplits in my home

I stand by the idea that when we measure the load on these units, we are measuring in a random operating condition rather than the worst case operating condition, so I suspect that our measurements will tend lower than worst case scenario.

But I have 2 additional thoughts: 1) The MCA already includes a 125% factor for continuous loads. 2) These minisplits are generally rated for use on both 208 and 240V systems, but have a single MCA.

My guess is that the MCA is determined at the minimum acceptable supply voltage, which is probably 15%-20% lower then typical measurement conditions.

Based on these two factors, I'd expect the worst case operating condition current measurement to be perhaps 65% of MCA.
 
208 voltage through 240 makes a good point cause it’s gonna be the worst case.
All things dumb I bet if I go in the specs and actually do my own numbers I come it way under what there ampacity, but as you can tell for not changing the 20 to 25 in a house that’s two minutes from my shop. I’m either extremely stubborn or lazy or both
 
Just speculating but isnt some foriegn wire size like 13 or something and some of the reason they have to come up with 25A breaker? I installed my first a while back and read the instructions because of that and looked at the labels. Seems the run was quite low, maybe 8 or 9A and MCA 20@20. I had read a little here and was aware some called for 25 and checked before I got to it. I stock breakers,,, never stocked a 25, never installed one.
 
winnie said:
I stand by the idea that when we measure the load on these units, we are measuring in a random operating condition rather than the worst case operating condition, so I suspect that our measurements will tend lower than worst case scenario.
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I stand by the idea that when I measure the device I work my hardest to put it into the worst case operating condition possible.

And mini-splits are not the only offender. A lot of the nameplates are really lazy, as if the regulatory guys doing the nameplate have zero idea what's inside the appliance, and don't care. The nameplates are vastly overstating the normal operating condition, kind of like drug expiration dates are more about re-ordering profit than any objective expiration ( https://pmc.ncbi.nlm.nih.gov/articles/PMC7040264/ "With a large and expensive stockpile of drugs, the military faced tossing out and replacing its drugs every few years. What they found from the study is 90% of more than 100 drugs, both prescription and over-the-counter, were perfectly good to use even 15 years after the expiration date.")
 
We see it suggested here to use max breaker allowed by law usually followed by the question or statement,,,, how much difference d we think we would see in fault between a 30 and 40,,, so I believe the 25 doesnt have squat to do with circuit conductor size but the machine and wire connecting the units. Following this theory of max what would a 30 hurt? (not advocating it) mjust sayin.
 
brycenesbitt said:
I stand by the idea that when I measure the device I work my hardest to put it into the worst case operating condition possible.

And mini-splits are not the only offender. A lot of the nameplates are really lazy, as if the regulatory guys doing the nameplate have zero idea what's inside the appliance,
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I somewhat agree.

Unless you are controlling the outdoor temperature and the supply voltage during your testing, no matter how hard you try you are not forcing the worst case operating conditions that the MCA has to account for.

However we have a clear example of nameplate laziness that I've given in this discussion: having a single MCA value for systems designed for two nominal voltages. 240V and 208V systems are two different things and should have different MCA values.

I don't believe this is regulator laziness, since I don't think that the regulation agency creates the nameplates.

I recall the drug thing. As I recall liquid drugs did generally expire but dry thing like pills had a huge actual shelf life. I wouldn't base my drug safety decisions on my recollection, however.
 
winnie said:
I somewhat agree.

Unless you are controlling the outdoor temperature and the supply voltage during your testing, no matter how hard you try you are not forcing the worst case operating conditions that the MCA has to account for.

However we have a clear example of nameplate laziness that I've given in this discussion: having a single MCA value for systems designed for two nominal voltages. 240V and 208V systems are two different things and should have different MCA values.
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I am not sure its lazy but if the conductor is fine at 208 then going to work at 240 and difference not enugh to allow a smaller one so it gets a single listing. If it changed the required wire size might be different.
 
I was in a mobile park and noticed they had AC and 2 different size units and they had a scheme with 10 SO to wire both. I think it was legal size and ocpd for both and they simply simplified it and didnt have to fuss with breaker to wire size questions,,, not sure, could have saved some wire cost but someone figured out this was probably cheaper at the time.
 
brycenesbitt said:
Could you send us an actual model number, or picture of the rating label?

Note every Mini-split I've taken a meter to has pulled less than 50% of it's nameplate rating continuously.
And had essentially zero startup current.
(Not that startup current would matter for a breaker.).
These units, at least the ones used here, all have solid state controls and probably linear motors that soft start, so startup current is not an issue.

Perhaps bigger HVAC units still have startup current as a factor, dunno.
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Is common to see actual running current be well below rated current. But how often are you measuring this current in worst possible conditions? Have to remember the more heat the refrigerant is moving the more it will load the compressor. Then degradation of the system, maintenance issues, and simple wear and tear can contribute as well. Also have to factor in that it is usually going to have more capacity than needed unless max temperature design extremes are met or exceeded. Unit can draw much more current when in cooling mode on 100 plus degree day then it will when in heating mode on a 10 degree day.
 
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