Nameplate Rated Current

JuliaTruchsess

JuliaTruchsess

Member
Location
Newtown, CT, USA
Occupation
Engineer
I'm confused by the nameplate ratings on some busway tap boxes I'm seeing. They have 30A 80%-rated breakers but the nameplate says 30A - shouldn't it be 24A?

I also see dual-output units with two sets of 30A breakers with each drop cord sized for 30A, and they're still rated 30A - shouldn't they be rated 48A?

My understanding is that the nameplate should reflect the total allowable maximum current that the device can deliver, not the breaker rating.
 
A 30 amp breaker is listed to carry 30 amps. If the load is continuous then it's required to be loaded to no more than 80% of its rating.
 
ptonsparky said:
It's up to the installer to make the appropriate call.
What's there today may not be there tomorrow.
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Understood from an installer/electrician's viewpoint, but my question is about what the applicable standard(s) dictate to the equipment's manufacturer. Standards have labeling requirements, I want to know if the ratings I'm seeing are compliant.
 
JuliaTruchsess said:
Standards have labeling requirements, I want to know if the ratings I'm seeing are compliant.
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So if I'm understanding what you're saying the equipment with a 30 amp circuit breaker is labeled 30 amps? DO you have any photos or model/part numbers for the equipment in question?
 
JuliaTruchsess said:
Understood from an installer/electrician's viewpoint, but my question is about what the applicable standard(s) dictate to the equipment's manufacturer. Standards have labeling requirements, I want to know if the ratings I'm seeing are compliant.
Click to expand...
The breaker in the bus tap is the rating of that type of equipment and always has been. 100% rated breakers are very rare, but even if you had a 100% rated 30 amp breaker in the bus tap, it would be 30 amp bus tap.
 
Same thing applies if you have a fusible bus plug rated 30 A, it will only accept fuses up to 30A and that is the maximum rating for loads that are non continuous in their operation.
If the load is expected to operate for 3 hours or more the rating is only good for 80% of the nameplate, because thats how fuses and breakers are rated, not because of the bus plug. Same principal applies to a 20A breaker in a kitchen outlet, rated for 20A max non continuous or 16A for loads 3hours or more. Standard practice for most all overcurrent protection devices.
 
There is no such thing as an “80% rating” for breakers. It’s a common misconception. ALL breakers are rated to carry 100% of their rating in free air. It’s how they are used where the differences arise.

Breakers protect conductors. Conductors must be sized for 125% of the continuous (defined as 3 hours or more) load. So if your continuous load is 80A, you must use conductors rated for 100A. You protect 100A rated conductors with a 100A breaker. Ergo the breaker is never USED at more than 80% of its continuous load rating. BECAUSE that is known, breaker panel manufacturers assume that breakers, mounted next to each other, will not have to endure more than 80% of their rated load, so the heat rise of the assembly will be based on that.

If you want a “100% rated breaker”, you will find that (for the most part) they are stand-alone enclosed breakers or bussed Main breakers in switchboards, switchgear and MCCs (because the bussing carries away the heat).
 
Jraef said:
There is no such thing as an “80% rating” for breakers. It’s a common misconception. ALL breakers are rated to carry 100% of their rating in free air. It’s how they are used where the differences arise.

Breakers protect conductors. Conductors must be sized for 125% of the continuous (defined as 3 hours or more) load. So if your continuous load is 80A, you must use conductors rated for 100A. You protect 100A rated conductors with a 100A breaker. Ergo the breaker is never USED at more than 80% of its continuous load rating. BECAUSE that is known, breaker panel manufacturers assume that breakers, mounted next to each other, will not have to endure more than 80% of their rated load, so the heat rise of the assembly will be based on that.

If you want a “100% rated breaker”, you will find that (for the most part) they are stand-alone enclosed breakers or bussed Main breakers in switchboards, switchgear and MCCs (because the bussing carries away the heat).
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Nice explanation.
 
Jraef said:
Breakers protect conductors. Conductors must be sized for 125% of the continuous (defined as 3 hours or more) load. So if your continuous load is 80A, you must use conductors rated for 100A. You protect 100A rated conductors with a 100A breaker. Ergo the breaker is never USED at more than 80% of its continuous load rating.
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You have the causation exactly backwards. An 80A conductor is rated for 80A continuous and does not itself require upsizing for a continuous load. It is the enclosed breaker, which is working in a thermal environment different from the free air testing environment, which many not hold at its full rated continuous current. Therefore the breaker is required to be upsized by a factor of 125%, and now the conductor has to be upsized to still be protected by the breaker.

Cheers, Wayne
 
wwhitney said:
You have the causation exactly backwards. An 80A conductor is rated for 80A continuous and does not itself require upsizing for a continuous load. It is the enclosed breaker, which is working in a thermal environment different from the free air testing environment, which many not hold at its full rated continuous current. Therefore the breaker is required to be upsized by a factor of 125%, and now the conductor has to be upsized to still be protected by the breaker.

Cheers, Wayne
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This is my understanding as well, that the conductor ampacity ratings are continuous ratings, and the 80%/125% stuff is all driven by the breaker ratings.

I'd like to see data on the thermal time constant of normal building wire. https://electrical.theiet.org/media...-ratings-for-cables-in-thermal-insulation.pdf gives some data from the UK.

-Jonathan
 
Seems like the standards for busway marking specify the ampacity they are marked with is the continuous current, the NEC refers to that in 368.20. I agree that the OCPD is probably the limiting factor.
 
wwhitney said:
You have the causation exactly backwards.
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winnie said:
the conductor ampacity ratings are continuous ratings
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Have our engineers lost their marbles?

NEC table ampacity wont match “continuous load” ampacity, unless exceptions in 210.19(A)(1) apply, subject to 110.14(C) equipment temperature limits.

And, checking wire temperature for 110.14(C) with Equation 310.15(B), is invalid if CCC’s, or non-continuous loads deviate from default-table conditions.
 
ramsy said:
Have our engineers lost their marbles?
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No.
The NEC says to size your conductor to the load, including adjustments and deratings.
Once you have conductor you chose the over current protection device.

Generally, the only time you need to adjust the size requirement for protective devices is if the conductor is run at 100% loading for more than 3 hours. Of course for any general NEC condition there may be exceptions.
 
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