How to size a circuit breaker?

[zog]

Zog,

I bet you got this but I just want to add my reply to help.

?Overload? is different than ?Overcurrent? in the respect of Definitions. The breaker does provide overcurrent protection just specifically not overload protection. The overload protection for the motor and its branch circuit is provided by the OL?s or the thermo-protection on the motor.

The reason a breaker is sized at 250% is to allow startup (lock rotor), if it is sized too small it may trip. The equipment grounding conductor is sized from the OCP in Table 250.122 but it does not have to be larger than the phase conductor so #10 is fine for EGC on a 60A as well.

Thanks, got it, just not used to dedicated circuits, or breakers that dont have programable trips units or relays for shaping the desired Time/Current charteristics.

 

[R2EG]

Breaker function

Breaker function

A breaker is designed to protect the conductors (wires) behind it.

Determining the caliber (amperage) of the breaker depends on your on wire size (gauge). Once we know the caliber of the breaker we have several breaker type curves to choose B, C, D... in Europe we use D type to allow for startup current of motors. In your case it is not a startup problem.

My guess is that with time your conductor & motor are heating up enough to increase resistance, hence intensity (amperage) therefore tripping.

Perhaps the power requirements of the receptor require that you increase the wire size because of the type of cable path (single open, stacked cable tray etc.; heat and induction related issues), length of cable run, and average ambiant temperature.

 

[javajake]

thanks all!

thanks all!

you have been a huge help. Based on your input, I'm posing some questions to the vendor to assure accountability. I do think they selected the 30A CB a little too close to the line, especially given the dramatic changes in environmental conditions (namely the night time cooling and daytime heat). To be sure, though, they appear to be on track and taking a conservative approach to the system component selection for the adjustments.

Can all of the NEC be accessed on line?

 

[javajake]

wire gauge - motor vs. breaker

wire gauge - motor vs. breaker

I've received some conflicting info. A post above suggested that NEC regulations describe gauge selection based on motor ratings (208V motor FLA = 24.2 = #10), but the vendor is stating the NFPA regs require wire gauge selection based on the breaker (so, in this case 40A = #8). I appreciate his conservative stance, but, can any explain which standard guides the decision?

thanks!

 

[benaround]

Jake, You would have a tuff time finding fault with post #17. NFPA is mother

of NEC.

 

[quogueelectric]

I've received some conflicting info. A post above suggested that NEC regulations describe gauge selection based on motor ratings (208V motor FLA = 24.2 = #10), but the vendor is stating the NFPA regs require wire gauge selection based on the breaker (so, in this case 40A = #8). I appreciate his conservative stance, but, can any explain which standard guides the decision?

thanks!

The wire ampacity should be capable of carrying 1.25% of nameplate current cut and dried end of story.

 

[quogueelectric]

you have been a huge help. Based on your input, I'm posing some questions to the vendor to assure accountability. I do think they selected the 30A CB a little too close to the line, especially given the dramatic changes in environmental conditions (namely the night time cooling and daytime heat). To be sure, though, they appear to be on track and taking a conservative approach to the system component selection for the adjustments.

Can all of the NEC be accessed on line?

yes it is all available on line for free.

 

[iwire]

A breaker is designed to protect the conductors (wires) behind it.

In the US the breaker supplying a motor does not necessarily protect the conductors from overload, the breaker only protects the conductors from short circuits.

The conductors supplying a motor are (or can be) protected by the thermal overload installed at the motor controller.

 

[tryinghard]

...they appear to be on track and taking a conservative approach to the system component selection for the adjustments.

Can all of the NEC be accessed on line?

The breaker sized to the maximum allowed, Table 430.250 x 250%, is every bit as safe as Table 430.250 x 125%, the differance is one will hold on start up and one may not. Actually if it does not hold on start up it really is a code violation 430.52(B).

You may size the breaker up to 250% and the next standard size but you cannot be more, there is no good reason to be less with OL protection involved

Check out these sites:NEC 2005 & NEC 2008

 

[sparky59]

did anyone check the actual voltage at the motor when it was running?

 

[javajake]

voltage at the motor

voltage at the motor

yes ... all phases were checked and were 212.
-jj