I am working in a consultant industry, wherein we get the switchgears from different OEMs. I wonder why the switchgears (including fuses and fuse-gears) and control gear assemblies are rated in the specific values for all the manufacturers. And these values do not deviate from the given 'slots'. One reason what I think which governs the factor for rating is the commercial feasibility, loads determined by the utility companies and consumers and safety standards, such UL, IEC. But I am skeptical if this is the only reason. By rating I mean the frame rating and not the trip rating (of course the trip ratings can be fine tuned to the desired load current). Please see attached the ratings of the Rockwell Automation's Allen Bradley breakers.

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    \$\begingroup\$ Are you asking about the general benefits of standardization? Or how is your question different from "why are screws only sold in some sizes" or "why can't I buy three seventeenths of a litre of milk"? \$\endgroup\$
    – TooTea
    Commented Feb 12, 2023 at 9:25
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    \$\begingroup\$ Plot them on a log scale and you'll find them reasonably close to equally spaced. This is quite an old series, also seen on (pre-1940) cameras, with lens stops at f6.3, f8, f10,f12.5,f16 etc. You can see 3 values per doubling. \$\endgroup\$
    – user16324
    Commented Feb 12, 2023 at 14:59
  • \$\begingroup\$ @TooTea Is 'Standardization' followed for every electrical ratings ? I got your answer from 'why are screws only sold in some sizes'. Thank you. \$\endgroup\$
    – DaSnipeKid
    Commented Feb 12, 2023 at 16:42
  • \$\begingroup\$ @user_1818839 What is the relation of their values plotted on a log scale and their ratings being standardized ? It would be helpful if you elaborate by adding an answer. I am really interested in knowing why should we plot them on log scale. \$\endgroup\$
    – DaSnipeKid
    Commented Feb 12, 2023 at 16:43
  • \$\begingroup\$ Consider sets such as these: en.wikipedia.org/wiki/Preferred_number \$\endgroup\$ Commented Feb 14, 2023 at 20:14

3 Answers 3


The fuses and switches you see follow the R10 Renard series. This series of 10 numbers (1.00, 1.25, 1.60, 2.00, 2.50, 3.15, 4.00, 5.00, 6.30, 8.00), for each order of magnitude, provides an approximately-equal geometric spacing for ten component values per decade.

Why geometric spacing, and not, say, equal arithmetic spacing between successive values (e.g. 2, 4, 6, 8, 10)? It comes down to the tolerance and error allowance for each component. For R10 specifically, each value is about 1.26 times the previous value; a manufacturing process that provides a 15% tolerance can't actually make its nominally-valued components more precisely than this anyway. You might want a 117 A breaker, but any given 125 A ± 15% breaker might actually trip at 117 A (or 143 A) anyway. Better to design a system such that you know the breaker will trip before damage occurs -- either by using your 125 A breaker with components that can tolerate 150 A, or by using a 100 A breaker with your 117 A load.

Conversely, if we did use the 2, 4, 6, 8, 10 series, we'd have a mismatch: 50% tolerance in the first gap, but only 12% tolerance in the final gap. This is not particularly useful -- either the manufacturing process results in 50% tolerance and there's a lot of overlap at the higher digits, or the process can do 12% tolerance and some of the range is not covered.

A related concept is the E series of preferred numbers that you may have seen on resistors and capacitors. Capacitors with 20% tolerance usually come in the E6 range; brown four-stripe (5%) resistors frequently come in hobbyist starter kits following E12, and the blue ones match E24 or E48.

These "preferred number" patterns are everywhere. Convenient multiples across orders of magnitude are found in electronics, yes, but also (for example) in currency denominations. American 1¢, 5¢, 10¢, 25¢, 50¢, $1, $2, etc. follows this idea unevenly, but other (more sensible) currencies like the Australian dollar or the Euro follow a 1-2-5 pattern consistently. You can typically buy milk (or other groceries) in 0.25 L, 0.5 L, 1 L, 2 L, and 4 L. The A-series of paper sizes (e.g. A4) are each half the previous number's size. In photography, camera aperture F-stops and ISO film speeds both fit the bill. I'm sure you'll start seeing them everywhere.

It turns out it's really useful to say "I need the next one to be X% bigger" and these patterns just fall out of there.

(Addendum, the math):

Specifically for R10, the values are approximately \$10^{n/10}\$, where \$n\$ is an integer between 0 and 9. The values are then rounded for convenience.

Value Nominal Exact
1 1 1.000
2 1.25 1.259
3 1.6 1.585
4 2 1.995
5 2.5 2.512
6 3.15 3.162
7 4 3.981
8 5 5.012
9 6.3 6.310
10 8 7.943

Any \$n\$-step evenly-spaced geometric sequence can be calculated using the general formula

$$ n^{(i-1)/n} $$

where \$n\$ is the number of steps, and \$i\$ is an integer between \$1\$ and \$n\$ inclusive.

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    \$\begingroup\$ Side note: in some usage the values are rounded. For example, you will generally find 6A, 13A, and 32A breakers/fuses for low-precision installation use, although an order of magnitude higher, 63A, 125A, and 315A are used instead. On the other hand, small electronics fuses generally are rated at e.g. 3.15A, 6.3A etc. \$\endgroup\$ Commented Mar 28, 2023 at 7:07

I am mostly answering from a US perspective:

The ratings of fuses and non-adjustable circuit breakers are defined in the National Electrical Code:

NEC Table 240.6(A)

These are very roughly a geometric series (though not the Renard series). The general concept is explained well in other answers and the Wikipedia article on Preferred numbers.

At least historically, these ratings were approximately aligned with the nominal ampacity of standard wire gauges and the allowable overcurrent protection settings of standard distribution transformers.

But you asked about frame sizes, not trip settings:

At least at the lower ratings, it makes sense for manufacturers to offer switch and circuit breaker frame sizes and that align with standard trip or fuse values dictated by NEC so that each frame has one fixed-trip option that utilizes the full rating of the frame. (Fixed trip is very common in ratings 800A and below.)

Although manufacturers generally choose frame sizes from this list, they don't all offer the same selection of frame sizes.

Your excerpt from Rockwell gives 125A, 225A, 250A, 400A, 800A, 1200A, 2000A, 2500A, and 3000A.

Here are the frames listed in the molded-case circuit breaker catalog from Eaton:

  • EG-Frame (15–125 Amperes)
  • JG-Frame (63–250 Amperes)
  • LG-Frame (250–630 Amperes)
  • NG-Frame (320–1200 Amperes)
  • RG-Frame (800–2500 Amperes)

For each frame, either the minimum or maximum rating (or both) aligns with a value in the NEC.

Schneider Electric's Square D product includes these frames:

  • 125 A B-Frame
  • 150 A H-Frame
  • 250 A J-Frame
  • 250 A Q-Frame
  • 400 A LA/LH
  • 600 A L-Frame
  • 800 A M-Frame
  • 1200 A P-Frame
  • 3000 A R-Frame

These options all align with the NEC values, but are a different set from those offered by the Rockwell or Eaton.

The choices made by each manufacturer are largely commercial or practical in nature: Physical package sizes must be selected to enclose the required conductor and bus sizes related to each ampacity; certain trip ratings are more common in certain markets, so a frame may be offered to align with that, etc.

Eaton's product range includes some values that don't align with an NEC rating so it is most likely selected to harmonize with IEC offerings, so that the same product could be offered in different global markets.


Intersection of preferred numbers, as some have said already. When the circuit breaker will trip is based on the amount of overcurrent, how long the overcurrent condition has existed, the design of the breaker itself, and the ambient temperature. It's not exact, which is why circuit breaker datasheets have trip curves instead of a single +/- %. This has big implications for selective tripping schemes, where you want a malfunctioning device to trip its own branch without affecting other branches. Say you have a motor on a 50 A breaker and a control circuit on a 1 A breaker; you wouldn't want to have both powered from a 51 A breaker since it's very possible that the 51 A breaker could (because of manufacturing tolerances) trip before the 50 A breaker. The preferred numbers space the values out at certain percentages so you don't have confusing situations like this.

The other "preferred numbers" situation arises from the installation. The adage is that the circuit breaker is there to protect the wiring. Wire is generally available in certain sizes with certain ampacities, not an infinite range. Rockwell states that their 125+ A breakers are rated for 75 C wire. Looking at ampacity tables, we see 100 A for 2 gauge wire, 150 A for 1/0, 175 A for 2/0, and so on. If you're protecting the wire, it doesn't make sense to have breaker ratings that are too low to use the full capacity of one gauge of wire while being unsafe for the next smaller size.


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