Can you load panelboards at 100% capacity?

[alej27]

If I have a 200-A panel, can I run 200 amps continuously through it? Or can I run up to 160 A (= 200 A * 0.80)?

I think not. Here's my research:

• As we know, most breakers are not rated for 100% operation, and therefore should not have a load greater than 80% of its capacity (or equivalently, the breaker should have a capacity of at least 125% of the load) [art. 215.3 of 2020 NEC]. For example, we can't use a 200-A breaker if the continuous load will be/is 200 A, but we can either install the 200-A breaker and limit the load to 160 A (= 200 A*0.80), or better install a 250-A breaker (= 200 A * 1.25) without limiting the load. This applies for both the branch and the main circuit breakers of a panelboard.
• On the other hand, panelboards should have a capacity of at least the ampacity of its main wires, which were sized for the calculated load [sec. 480.30]. For example, if the calculated load of the feeder was 153 A, and the ampacity of the chosen wires was 200 A THW (since 153 A * 1.25 = 191.25 A and the next THW size has 200 A ampacity), then the panel must be at least 200-A rated.
• Furthermore, the main breaker must not exceed the capacity of the panelboard [sec. 480.36]. For example, we can't use a 250-A main breaker on a 225-A panel, but we can use a 200-A or 225-A main breaker.

Considering these three articles, does this mean we can't have a 200-A continuous load on a 200-A panel with a 200-A main breaker and 200-A feeder? Because in such case, sec. 215.3 would be violated.

In case I'm correct, I've just answered my own question. But I'm asking for you to check if I'm correct. Thanks in advance!

 

[wwhitney]

If I have a 200-A panel, can I run 200 amps continuously through it? Or can I run up to 160 A?

Per UL 67 31.1, the busbar rating of a panelboard is an ampacity and therefore a continuous rating.

However, as you noted, NEC 408.36 requires that the panelboard be protected at its rating in most circumstances. So if you want to use a 200A busbar at 200A continuously, you'd need to protect it with a 200A 100% rated breaker. Most likely this breaker would require its own enclosure. But you could use a 200A 100% rated breaker to protect a 200A MLO panelboard loaded to 200A continuously.

If the panelboard is protected by a regular 200A breaker, then the current through that breaker, and hence on the busbar, would be limited to 200A non-continuous, or 160A continuous, or any interpolated combination of continuous and non-continuous loads.

As we know, most breakers are not rated for 100% operation, and therefore should not have a load greater than 80% of its capacity (or equivalently, the breaker should have a capacity of at least 125% of the load)

That is only true for continuous loads. A 200A breaker is fine for a 200A non-continuous load.

Cheers, Wayne

 

[Dennis Alwon]

Per UL 67 31.1, the busbar rating of a panelboard is an ampacity and therefore a continuous rating.

However, as you noted, NEC 408.36 requires that the panelboard be protected at its rating in most circumstances. So if you want to use a 200A busbar at 200A continuously, you'd need to protect it with a 200A 100% rated breaker. Most likely this breaker would require its own enclosure. But you could use a 200A 100% rated breaker to protect a 200A MLO panelboard loaded to 200A continuously.

If the panelboard is protected by a regular 200A breaker, then the current through that breaker, and hence on the busbar, would be limited to 200A non-continuous, or 160A continuous, or any interpolated combination of continuous and non-continuous loads.


That is only true for continuous loads. A 200A breaker is fine for a 200A non-continuous load.

Cheers, Wayne

I don't believe you will find a 100% rated 200 amp main breaker. I have heard they exist in very large sizes but not the smaller sizes. So to answer your question-- No you can only load the breaker to 160 amps continuous.

 

[wwhitney]

I don't believe you will find a 100% rated 200 amp main breaker. I have heard they exist in very large sizes but not the smaller sizes.

It would be in its own enclosure, so not part of a main breaker panelboard. And it would definitely be a specialty item. For single phase, you might need to use a 3 pole breaker just to get the 100% rating.

For example, this 200A 3 pole 600V breaker says it has "a has a continuous current rating of 100%."

JGL36200C - Circuit breaker, PowerPacT J, 200A, 3 pole, 600VAC, 18kA, lugs, thermal magnetic, 100% | Schneider Electric USA

Schneider Electric USA. JGL36200C - Circuit breaker, PowerPacT J, 200A, 3 pole, 600VAC, 18kA, lugs, thermal magnetic, 100%.

www.se.com

Cheers, Wayne

 

[Jraef]

Per UL 67 31.1, the busbar rating of a panelboard is an ampacity and therefore a continuous rating.

However, as you noted, NEC 408.36 requires that the panelboard be protected at its rating in most circumstances. So if you want to use a 200A busbar at 200A continuously, you'd need to protect it with a 200A 100% rated breaker. Most likely this breaker would require its own enclosure. But you could use a 200A 100% rated breaker to protect a 200A MLO panelboard loaded to 200A continuously.

If the panelboard is protected by a regular 200A breaker, then the current through that breaker, and hence on the busbar, would be limited to 200A non-continuous, or 160A continuous, or any interpolated combination of continuous and non-continuous loads.


That is only true for continuous loads. A 200A breaker is fine for a 200A non-continuous load.

Cheers, Wayne

Beat me to it.

Contrary to popular belief, there is no "80% rule" for breakers found in any regulations. ALL breakers are tested and rated at 100% of their stated ampacity.

What really happens is that you are required to size CONDUCTORS for 125% of the continuous load, then you pick a breaker to protect the conductors. So INDIRECTLY, you end up with the breaker never being used at more than 80% of it's rating (80% is the inverse of 125%). But then because that is how the breakers will be USED, the PANEL design, in terms of cooling capabilities of the breakers when mounted with OTHER breakers in the box, is going to be based on the breakers only seeing 80% of the rating continuously. Ergo, the fact that you end up with an 80% utilization is an indirect artifact of other rules and issues.

That's why, if you want a "100% rated breaker", most of the time it is going to be in a stand-alone enclosure. That is not however universally true. Square D I-Line breaker panels for example CAN have 100% rated breakers in them, probably because of the direct plug-in bus connections, to where that is going to aid in heat dissipation. You can also get Main Breakers in some cases that are 100% rated, but they end up being somewhat separated off from the branch breakers. There are also possible conductor and termination ratings issues that arise when trying to use 100% breakers as well. But basically it comes down to the PANEL design as to limiting the use of the breaker capacity.

 

[wwhitney]

What really happens is that you are required to size CONDUCTORS for 125% of the continuous load, then you pick a breaker to protect the conductors.

I'm going to say that is backwards.

The conductor's ampacity is a continuous rating. So for situations where all the equipment is 100% rated (e.g. a 100% rated breaker, or current limited circuits without any breakers or non-100% rated equipment), the conductor is sized to 100% of the continuous load.

And while breakers are tested to hold at 100% of their rating at 40C ambient, that is done individually in free air. So the concern is that the same breaker in an enclosure, next to other breakers, may nuisance trip at 100% continuous load. Therefore the NEC requires that the breaker is upsized to 125% of the continuous load, unless it is a 100% rated breaker in an appropriate enclosure.

Note also that the conductor sizing rules do _not_ require that the conductor have an ampacity after adjustment and correction that is 125% of the continuous load; 100% of the continuous load will suffice. Of course, the rule on OCPD sizing together with 240.4 will usually result in the ampacity of the conductor being 125% of the continuous load, or slightly less due to 240.4(B).

The conductor sizing rules _do_ require a 125% factor on the continuous load for the termination conductor size check (unless a 100% rated OCPD is used), which is done using the table termination temperature ampacity without any adjustment or correction. It seems to me that this requirement could be deleted from the NEC. All it does is ensure a little additional heat sinking at the breaker termination in some cases, but the breaker is already upsized by a factor of 125%, so I fail to see why that additional heat sinking should be required.

Cheers, Wayne