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When sizing motor over-current protection, CSA 22.1 28-200 3a states that a circuit breaker can be sized up to 250% of the rated full load amperage (FLA) of the motor.

E.g. 100hp, 575VAC motor draws 99A (CSA 22.1 Table 44): 99A * 250% = 247.5A

Usually I would've rounded up and selected a 250A circuit breaker. I just noticed today that in the over-current example on page 662 that they always size the the next lowest standard size and specified a 225A circuit breaker. This makes sense since a 250A circuit breaker is past 250% of FLA. However, Rule 28-200 4c goes on to state that if the circuit breaker above isn't large enough for the motor to start that you can go up to 300% of the motor FLA (297A). Therefore, 250A isn't in violation.

My question:

  1. As a rule of thumb, when sizing motor branch over-current protection, should you round up or down to the next standard breaker size? If rounding up, do you need to prove that the motor couldn't start with the smaller sized breaker?
  2. Does the answer to #1 change when considering feeder over-current protection?
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  • \$\begingroup\$ It's been a few years so my memory may be off, but I seem to remember that at the place I used to work at, we used the CEC and I was told you size the wire to the motor, then you size the breaker to the wire. I was told you don't size the breaker to the motor. I seem to remember being told it wasn't to protect the motor from damage; It was to protect the building from burning down and that the wiring insulation would melt and burst into flames before the motor would. This was a few years ago though so my memory may be off. \$\endgroup\$ – DKNguyen Feb 22 at 20:49
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    \$\begingroup\$ Better to bump up the breaker class from B to C or D, rather than the current, to avoid tripping on starts. \$\endgroup\$ – Brian Drummond Feb 22 at 22:22
  • \$\begingroup\$ @BrianDrummond, so I understand, you would suggest "rounding down" and then changing from a class C to a class D breaker, for example, if tripping was still an issue? \$\endgroup\$ – C. Lange Feb 23 at 0:34
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    \$\begingroup\$ That was a general point. If the calculation really shows 247A, choose 250A. None of the values in the calculation have a tolerance as small as 1%. But I'd then choose 250A class D rather than 300A class C for example. \$\endgroup\$ – Brian Drummond Feb 23 at 12:58
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This rule, and the similar version in the NEC, is based on there being another way of protecting the motor from running thermal overload, either an Over Load Relay (OLR) or the motor having internal thermal protection (TP). The branch breaker is then really only providing the SHORT CIRCUIT protection.

The reason for the 250% rule was to ensure that the instantaneous (magnetic ) trips in an Inverse Time (Thermal-Magnetic) circuit breaker would be high enough to allow for the motor inrush current without nuisance tripping. Most thermal mag breakers have their magnetic trips set for 10x the thermal rating, so a 250A breaker has a magnetic trip setting of 2500A. More recently many mfrs have taken to making the mag trip settings adjustable, but will still max out at 10x the breaker rating. But if your motor can start with a lower mag setting, that provides better protection for the motor.

The later addition of the 300% rule came about because a decade or so ago when "Energy Efficient" motors were mandated, there were issues with some of the techniques used to increase efficiency also causing a significant increase in magnetizing inrush current, and the 250% rule no longer worked. So the allowance is that IF you can demonstrate that 250% will not allow the motor to start, you can bump it to 300%, basically to get a higher trip threshold on the magnetics.

In your case because the motor FLC is 99A, the magnetizing inrush is likely to be only around 1500-1700A max, so no need to go with the higher size. In fact a 225A breaker would work as well.

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  • \$\begingroup\$ This answers my question wonderfully. I wasn't aware that the 300% came afterwards for energy efficient motors -- it just made me wonder why they wouldn't set the initial rule at 300%. \$\endgroup\$ – C. Lange Feb 27 at 22:54
  • \$\begingroup\$ Yes. Here in the US it happened I think somewhere around the 2002 NEC, maybe 2005, I can't remember any more. But the CEC usually changes at the same time. \$\endgroup\$ – JRaef Feb 27 at 22:57
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It depends on the motor rating and load to full speed time. Start current can be 3 to 8x rated current and start-up time is highly load dependent.

Thus they have 3 classes and 3 categories of solutions depending on motor class and "heavy" load.

125/150 175 200 A

enter image description here

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  • \$\begingroup\$ How would you determine if a motor is a "heavy start" or an "optimal start"? Is this something you would get from the manufacturer, or is it calculated? Additionally, why not just go straight to the "heavy start" option? Cost? \$\endgroup\$ – C. Lange Feb 23 at 0:30
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    \$\begingroup\$ That seems be indicated by motor type and class. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 23 at 1:44
  • \$\begingroup\$ Between the motor class and type, as well as breaker types, mentioned by @Brian Drummond, it seems to me that there's quite a bit more here to learn! Not as simple as a rule of thumb. Thanks for your input. \$\endgroup\$ – C. Lange Feb 24 at 22:52
  • \$\begingroup\$ The stored rotational kinetic energy / second compared with rated power may be the best rule of thumb to indicate Heavy and thus higher starting current and breaker rating. I don't know this, but that's how I would approach this. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 24 at 23:10
  • \$\begingroup\$ Another factor is duty cycle of motors and temperature different between motor and thermal relay ! \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 24 at 23:30
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Don't know about CSA standards, but in EU (machines were exported to US also) we use motor protection switch. It's a device made for motor overload protection.

Keep in mind that using a 250A circuit braker will also impose to use wires with a gauge for 250A. Also it may happen that if the motor is overloaded at 250% the circuit braker won't never disconnect it leaving the motor to melt down.

Maybe you have read some standart that is refereing at some small motors, but the right way IMO is to use motor protection devices.

https://www.amazon.co.uk/Siemens-3RV2041-4MA10-Protection-Screw-Type-Connection/dp/B0796X54RX

enter image description here

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  • \$\begingroup\$ We also use motor circuit protectors/motor starters in Canada. I would disagree with your second point though. The 250% above FLA is for starting current. The motor starter you linked has an instantaneous trip of 1300 A but you're not going to size your wires to 1300 A. \$\endgroup\$ – C. Lange Feb 23 at 0:23
  • \$\begingroup\$ @C.Lange Sure not. When using motor protection circuit braker, you use gauge for rated current, say 99A. The trip charactersitics of the device allows high overload for short time since it simulates the windings temperature of the motor. The main element is a bimetallic switch with heating winding. A fuse is different technology where the conductor breaks is the energy exceeds melting/evaporating energy thus the characteristics is different. \$\endgroup\$ – Marko Buršič Feb 23 at 8:39
  • \$\begingroup\$ It is a magnetic or thermal; response relay? The idea , I think is to have the relay thermally matched to the thermal over temp of the motor under overstress conditions? overload at speed or excess load on start or phase imbalance or overvoltage etc but this depends on intermittent or continuous duty \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 24 at 23:40
  • \$\begingroup\$ electronics.stackexchange.com/questions/218075/… your link indicates a 10 second 600% overload relay (class 10) and surge currents can be 5 to 8x rated current so expected startup time is key. or in other words ratio of kinetic to static energy at rated load assumiing >96% efficient \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 24 at 23:44
  • \$\begingroup\$ I think a thing to point out here is the question I asked was specifically related to the over-current protection (circuit breakers) but the motor starter you linked is both over-current and overload protection. If you're not using a motor starter (which is both over-current and overload), the circuit breaker has to be followed by a thermal overload. This overload is set max 125% of rated current and protects from overheating due to high continuous amp draw. \$\endgroup\$ – C. Lange Feb 25 at 2:14

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