Motor Breaker Sizing

[Cold Fusion]

Philly -

Just to keep from jumping around, my discusson is limited to Non-strange stuff motors and instantaneous trip CB used in listed combination starters.

I'm assuming here, you are referring to the ratings of the contactors and not the breakers correct? ...

No, I'm refering to the breaker. The breaker is the disconnect. 430.110 is refering to the rating of the disconnect.

... So as an example lets say I had a motor with a FLA of 7A. If I was going to select a circuit breaker would I start with 2.5* this value and select maybe a 15A breaker. ...

The motor FLA doesn't matter. It's the motor FLC, from T430.150.

And, I don't know where this 2.5X FLC comes from. If one were picking an Inverse Time Breaker from T430.52, then 2.5X FLC is where you would start. But we are not.

So the guiding principle for the breaker rating is 43.110. And it says .... (this is a test, you get to fill this in)

... Could you technically select a 7A breaker which would be adjustable between 21A and 70A thus allowing you to get to 800%. Or should you use a breaker that allows you to get to 1300% at least?

Technically: 7A doesn't ever show up in T430.150, so I don't know

Practically: I'd tell AB what the motor hp was, and use their standard bucket configuration. And they might tell me to choose between the energy efficient or not energy effecient.

However, for an example, I'll pick 230V, 1.5hp, 6.0FLC. The rating of the CB has to be at least 1.15X or 6.9A. So a 7A rating is fine - a 15A is also fine. Now look at the instantaneous settings. Since I don't know if the motor will be an energy efficient or not, and I get to use the engineering evaluation section of 430.52.C.3, I'll pick the smallest one with settings that can get up to 17X (102A)

cf

 

[philly]

However, for an example, I'll pick 230V, 1.5hp, 6.0FLC. The rating of the CB has to be at least 1.15X or 6.9A. So a 7A rating is fine - a 15A is also fine. Now look at the instantaneous settings. Since I don't know if the motor will be an energy efficient or not, and I get to use the engineering evaluation section of 430.52.C.3, I'll pick the smallest one with settings that can get up to 17X (102A)

cf

Is there not a 10A breaker betwen 7A and 10A?

So what you are saying is after you satisfy the 1.15% rating you will choose the lowest size breaker that will allow you to get to 1700%. That seems like a good rule of thumb.

 

[Cold Fusion]

Just happen to be working on a similar thing and came across this attached chart on AB MCP breakers.

Is there not a 10A breaker betwen 7A and 10A? ...

Assuming you meant between 7A and 15A, AB jumps from 7A to 15A.

... So what you are saying is after you satisfy the 1.15% rating you will choose the lowest size breaker that will allow you to get to 1700%. ...

Looking at the chart I attached (stay with the 460V column), on the low end (3A, 7A, 15A), the current rating is 2X the upper Hp FLC. The available instantaneous settings are nominally 10X the lower Hp FLC to 20X the upper Hp FLC.

So I'd say your thumb rule is okay. But like all thumb rules it occasionally will end up looking like my left thumb nail - mangled and regrowing, simple case of not paying attention

For example, look at the available mag trip settings for a 150L, 150H, 250, all for a size 4. If you were sizing for a 100hp in a Size 4, you would want to look at the motor specs before ordering the bucket

Could be the one you would want is the 150H instead of the 250. The 150H has a wider setting range.

cf