NEC motor protection help

can someone help me out on this.

The problem is simple. A 1hp 220V motor with 6.8A FLA

I summarized the wire size and protection in the table below.



I have two issues:

1 - did i size up the overload protection correctly since the FLA with 1.15 multiplier still resulted in a value less than FLC?

2- the required protection is so small. is there a overload protection device for 10AMP? In any case, i thought the device would need to be UP TO A MAX of 9.5AMP?

So first things first.  

Motor Short Circuit MUST BE sized based on NEC FLA tables in 430.Motor Overload MUST be sized off Motor Nameplate.  Not the FLA given in 430  tables.

This is dictated by NEC 430.6(1) and (2).  You can get into circumstances where Short Circuit and Overload do not match due to the range of values permissible by different base numbers and different multiplies.

How the values you provided relate to 430 Part XIV tables and nameplate... not sure.  But if the question has those terms, you need to use them.  You also miss important information like OCP type for the Short circuit protection, confirmation its a separate or integral overload, how its started, etc etc.  

Cable seems correct.

Short Circuit seems correct assuming the 250% is because you have a I2t Breaker (inverse time) and not an MCP (instantaneous trip) which tends to be much more common in today motor starter world.  

Overload is one or the other.  Its not a range.  Usually its 115% of Nameplate as that encompasses 99% of most usual cases... however not enough information to confirm which number you should be using.    

I've been up since 2 AM with a screaming child though... someone should double check me here.  

sayed said:

2- the required protection is so small. is there a overload protection device for 10AMP? In any case, i thought the device would need to be UP TO A MAX of 9.5AMP?

Click to expand...

This in particular tells me you may not be familiar with Motors.

So... an overload IS NOT a breaker or fuse (normally). 

Its an entirely different class of device, completely separate from the breaker that acts as a short circuit protection device in this case.   

In the rest of the code, the breaker / fuse serves as both Overload and Short circuit protection.  With motors, that is broken down into separate devices.  The breaker you are familiar with only protects the cable against short circuit, and is sized according (typically much larger then would otherwise be a allowed for a load.).

The overload (while this type I will describe isnt commonly used anymore) is called a bi-metallic overload basically a spinning gear that when is heated causes a material holding the gear in place to melt and the gear to turn.  When the overload is in its melty state, the circuit can be broken by the gear and requires either an automatic or a manual reset depending on what you ordered and its configuration.  (Now adays solid state overloads are the bees knees.)  



Overloads come in sizes down to fractional amps up to hundres of amps.  Whereas standard OCP breaker / fuse is only common whole numbers and you usually dont see any below 15Amps for breakers but fuses can be fractional amps.

On really small motors, the breaker is permitted to be the Short Circuit and Overload protection... but that is again moving outside your question.  

edits in italics

Szar said:

So first things first.  

Motor Short Circuit MUST BE sized based on NEC FLA tables in 430.Motor Overload MUST be sized off Motor Nameplate.  Not the FLA given in 430  tables.

This is dictated by NEC 430.6(1) and (2).  You can get into circumstances where Short Circuit and Overload do not match due to the range of values permissible by different base numbers and different multiplies.

How the values you provided relate to 430 Part XIV tables and nameplate... not sure.  But if the question has those terms, you need to use them.  You also miss important information like OCP type for the Short circuit protection, confirmation its a separate or integral overload, how its started, etc etc.  

Cable seems correct.

Short Circuit seems correct assuming the 250% is because you have a I2t Breaker (inverse time) and not an MCP (instantaneous trip) which tends to be much more common in today motor starter world.  

Overload is one or the other.  Its not a range.  Usually its 115% of Nameplate as that encompasses 99% of most usual cases... however not enough information to confirm which number you should be using.    

I've been up since 2 AM with a screaming child though... someone should double check me here.  

Click to expand...

 
FLC is code (table)

FLA is actual (nameplate)

anyways, i thought because 1.15xFLC < FLA that we have to multiply up to a max of 1.40 x FLC for overload protection?

sayed said:

 
FLC is code (table)

FLA is actual (nameplate)

anyways, i thought because 1.15xFLC < FLA that we have to multiply up to a max of 1.40 x FLC for overload protection?

Click to expand...

What part of the code are you following here?  

Szar said:

What part of the code are you following here?  

Click to expand...

430.32 (C)

Selection of Overload Device. Where the sensing element
or setting or sizing of the overload device selected in accordance
with 430.32(A)(1) and 430.32(B)(1) is not sufficient to start the
motor or to carry the load, higher size sensing elements or incremental
settings or sizing shall be permitted to be used, provided
the trip current of the overload device does not exceed the following
percentage of motor nameplate full-load current rating:

Motors with marked service factor 1.15 or greater 140%
Motors with a marked temperature rise 40°C or less 140%
All other motors 130%

i followed the above, which i guess i screwed up anyways. I should have multiplied by 1.3 in my calculations

sayed said:

430.32 (C)

Selection of Overload Device. Where the sensing element
or setting or sizing of the overload device selected in accordance
with 430.32(A)(1) and 430.32(B)(1) is not sufficient to start the
motor or to carry the load, higher size sensing elements or incremental
settings or sizing shall be permitted to be used, provided
the trip current of the overload device does not exceed the following
percentage of motor nameplate full-load current rating:

Motors with marked service factor 1.15 or greater 140%
Motors with a marked temperature rise 40°C or less 140%
All other motors 130%

i followed the above, which i guess i screwed up anyways. I should have multiplied by 1.3 in my calculations

Click to expand...

An engineer will  never ever use that section.  ("Never" is a dangerous word, but I'm comfortable saying that here.)

"is not sufficient to start the motor or to carry the load"

The only way you jump into this section is if you designed to (A)(1) and (B)(1) and the motor still failed to run properly.  Repeatably.  And the overload itself was the problem... which is usually never the case when sized appropriately.     

At this point in the diagnosing, is a field technician's / electrician's domain who is doing the commissioning and startup and cursing the engineer for their "shitty" drawings.  (Unless you do commissioning too, then your blaming the designer who obviously messed up your intent.)

As a test problem it would basically have to be stated that the overload was sized according to 430.32(A)(1) and (B)(2) and still failed to run.  Very important and self evident information in the problem that almost answers itself for you, and unlikely to be a test problem.