View Full Version : Standard Breaker Sizes?

December 26th, 2004, 02:02 PM
When it comes to sizing OCPDs for motor circuits, is it my imagination or do most people ignore the list of standard breaker sizes (NEC 240.6)? (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100A, etc.)

It seems that they tend to replace the list of Standard Breaker Sizes in NEC 240.6 with their own list of Standard Standard Breaker Sizes where the 25A, 35A, and 45A sizes are omitted. Granted, these sizes are less common as they don't correspond to specific conductor ampacities, they are still included in the 240.6 list.

For example, NEC 430.52 states that an inverse-time breaker can be sized up to 2.5X the motor's full load current. A 3HP single phase 230V motor has a full load current of 17A. This results in a breaker calculated at 42.5A which is rounded up as per NEC 240.4(B) and NEC 240.6 to 45A.

However, many electricians will tell you that the NEC allows you to use a 50A breaker in this case as they feel that 50A is the next standard size.

If you read 240.4(B) and 240.6, this is clearly not the case.

Am I being too rigid here? What are your experiences with these situations?

How many round down in these cases instead and confirm that the breaker holds on startup?


December 26th, 2004, 02:16 PM
How many round down in these cases instead and confirm that the breaker holds on startup?

HomerNot me. Even if the rounded down breaker holds on the startup of a new installation, it has been my experience that things only get worse as the motor ages. If this installation was done by a maintenance electrician permanently assigned to a certain factory, then rounding down would be acceptable. Rounding down and then leaving the installation for years may result in unnecessary downtime in future years. I think (per your example) that I would put in a 50 amp breaker for the period of time that it takes to order the 45 amp breaker (if you insisted on using a breaker). It would take a little while to order a 3 pole 45 amp breaker. I much prefer fuses for protecting motors.

You're right... 25, 35, 45, 70, 80, and 90 are often skipped. I think (I know) that a lot of guys get "standard sizes" and "standard in-stock sizes" confused.

December 26th, 2004, 02:21 PM
... I much prefer fuses for protecting motors...I've been called a dinosaur for holding the same beliefs! :D

Homer the Brontosaurus

December 26th, 2004, 02:26 PM
Not really. Fuses are pretty high tech now. You can get a specific "boutique" fuse that is expressly designed for a certain application. Leaf through a fuse catalog and look at all the graphs and such for all the different types of fuses. You can get a fuse with a lot more precise characteristics to suit a certain application much easier than you could ever find a breaker to do the same thing (and at a much less expensive price). I'm not skilled enough to be able to make sense of all the fuse specs to select one on my own, but the manufacturers have been tons of help to me in the past for selecting the proper type of fuse. I like fuses for protecting motors and compressors, transformers, and larger rectifiers and inverters. Save the breakers for everything else.

December 26th, 2004, 02:32 PM
Have you tried the new combined OCPD, Overload, and disconnect switch devices that are being pushed now?

These are supposed to be 'the future'. This old-timer is still skeptical for now.


December 26th, 2004, 03:04 PM
Have you tried the new combined OCPD, Overload, and disconnect switch devices that are being pushed now?

Can you provide a link? I'm not sure I've ever seen or heard of one, except for the "combination motor starters" that have been used for a hundred years. You say "device", so I'm picturing something quite small, as opposed to the quite large combination motor starter. I suppose this device you speak of would be electronic in nature. If it came from a respected manufacturer like Allen-Bradley or Telemechanique, I'd give it a look see.

December 26th, 2004, 03:51 PM
Can you provide a link? ... If it came from a respected manufacturer like Allen-Bradley or Telemechanique, I'd give it a look see.Here's a link to the Allen-Bradley Bulletin 140M (http://www.ab.com/industrialcontrols/products/iec_motor_control/motor_protection_breakers_and_protectors/140m.html) devices that include disconnecting means, overload and short-circuit protection.

Here's a link to the Allen-Bradley Bulletin 140M (http://www.ab.com/en/epub/catalogs/12768/229240/229254/229469/3100798/229517/) devices without overload protection (disconnect and short-circuit only) for use with VFD applications.

The only thing not wrapped up in the package is the contactor.


December 26th, 2004, 06:40 PM
Okay, they're ringing a bell now. I've never seen this in the larger frame sizes. I've only seen them in the C, D, and F sizes. The only experience I have with them is with models produced by Klockner-Moeller, which came in on some equipment manufactured in Germany. I only wired this equipement up, so I don't know how the combo control performed over the next few years.

My knee jerk reaction is to say that I don't like adjustable controls, such as these. Some maintenance personnel in places where these will be installed will crank on the dials when problems crop up without thinking it through. This may cause them a very expensive problem down the line. With a definate fuse or breaker size and a definate overload size, there's a lot less room for error. It is for this same reason that I don't like the flimsy IEC style overloads.

Homer, when you're making up the prints for equipment with adjustable trip and overload settings, do you note what the setting should be on the print? I've seen far too many prints that don't have these settings marked on the print. It makes troubleshooting other people's monkey-business a lot harder without these notations. I've even seen machine prints with a fuse symbol, but no noted fuse size or type. Grrr...

December 26th, 2004, 07:02 PM
These Allen-Bradley 'motor protectors' have a fixed short circuit trip point and an adjustable overload trip point over a narrow range, so they're really no different than having a separate breaker, disconnect, and overload.

You just size these based on the motor full load current.

One thing that I've found is that you can have trouble with VFD's. The full load current to the VFD is larger than the FLA of the corresponding motor so the sizing doesn't end up matching. If you pick a unit with a high enough trip point for short circuit current, then you won't be able to dial the overload down low enough. That's why I have used the units without the overload function in combination with VFDs that are UL listed as not requiring any additional overload protection.

Also, I've noticed that you really need to use 3-phase line reactors in series with the VFDs if they're protected by these things. If the impedance is too low back to your distribution transformer, you'll trip these suckers every time you power up the panel. The VFD inrush to charge the capacitors is a dead short for a cycle or two and if your available fault current is too high, the magnetic trip goes on inrush.

Professional drawings will have the O/L settings, fuse type and value, as well as any jumper and DIP switch settings.


December 26th, 2004, 07:23 PM
Yeah, I thought that almost all VFD's in common use permitted you to program in the overload setting. I guess there are some that do not. I've just never run across them.

I guess the line reactors on the VFD's have other benefits too. Seems like they ought to reduce the harmonics a bit. I saw 3 phase reactor on a 200 hp VFD that was in sheet metal and looked like a regular transformer. One person commented, "What's the big transformer for?". I can't explain why, but after the reactor was installed on this big motor, the motor sounded pretty sweet. When it ramped up, it sounded almost "spaceship like". I guess the reactor was cleaning up the power a bit.

December 27th, 2004, 05:24 PM
Just thought I would mention that breakers are not allowed to be used as an overload device but rather only for short circuit for a motor.

Fuses may be used as an overload device as long as it stays between the minimum and maximum of 430.32.

Breakers or fuses used as short circuit may be sized much larger such as the 250% higher than the full load current rating of the motor but it is best only to size it as large overcurrent device as necessary to ensure reliable production of that motor. The wording says maximum overcurrent device. This more or less means if you have a magnesium direct drive fan then 30 amp probably will carry a 28 flc motor. However if a cast iron direct drive fan may need a 90 amp breaker on that same motor.

YOu should only oversize for motor overcurrent device how much you need to in order to ensure reliability of the motor production.

Sizing up or sizing down to next listed overcurrent device is dictated again by the NEC. Feeders must be sized down to the next lower overcurrent device for maximum sizing of that breaker or fuse yet branch circuits may be sized up to the next higher overcurrent device for sizing of that maximum sizing of that breaker of fuse.

Fuses or overload starters or overload components or built in thermal within the motor must protect that circuit and that motor from overheating or overamping. The breaker can not protect that motor for anything but short circuit.

Hope this helps


December 27th, 2004, 08:47 PM
Just thought I would mention that breakers are not allowed to be used as an overload device but rather only for short circuit for a motor.I don't believe that it was ever implied that breakers could be used for motor overload protection.

I did state that the Allen-Bradley Bulletin 140M devices provided a combination of short-circuit, overload and disconnecting means. These devices are UL 489 listed for short circuit protection and UL 508 listed for overload protection.

As an old-timer, I still like to use fuses for short circuit protection and use an overload relay with my contactor.

As for using fuses for overload protection, it's not really practical as you would need to size them so small based on 430.32 that they would likely blow on motor start.


December 28th, 2004, 07:38 PM
Many read these posts. You guys understood each other. I just wanted to clear any misunderstanding with those others reading your conversation between you two. Playing it safe.

Fuses are allowed to be sized for overcurrent and overload but are limited to the overload ratings limited for that type motor.

However say a motor starter with an overload is required not to be sized any larger than the same overload ratings as fuses.

Now If you are talking in rush concern only time delay fuses are allowed to be used as overload protection and will hold pretty close to being as accurate as heaters are in amp monitering action. Nontime delay fuses are only allowed for use as a short circuit not for overload same as inverse time breakers.

Now you get into instatanious breakers industrial size you have a different animal all together. These are the type with a dial on the breaker not inverse time breakers normally used.

Time delay fuses react pretty close to same tolorance level as the heaters work in accuracy and tolerant to inrush of motor starts. That is why the NEC recognized time delay fuses as overload and overcurrent.

Just added info