Motor Branch Circuit Short Circuit OCPD Sizing Question

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Hello,

Does anyone have a calculation example for sizing the branch circuit fuses (Class RK-5 Time Delay FRS-R-xxx) when the control transformer is tapped from the load side of the branch circuit overcurrent protective device? For anyone with the 2008 Handbook my circuit is exactly like the one in Exhibit 430.15 (correct version). I have not found a example calculation in the Handbook or my Stallcups books. Everyone shows the calculation for just the motor load not including the transformer load.

I have been over section 430.53 C, and my question is how to size the fuses in the safety disconnect switch with consideration that the other load is a transformer (all be it a small one). 430.52 indicates the max allowable fuse at 175% of the full load current from table 430.250, and then my understanding is I would add the rating of the transformer load per 430.53C4 (125% of primary amps). My concern lies it that a transformer also has high inrush as a percentage of its normal load, and when pulling in the coil while the motor is starting will I exceed the value? My control transformer is only 100VA with a primary that is less than 2 amps, so section 430.72C4 allows the transformer primary OCPD to be up to 500%. Since the motor is only a 3 phase 1/2hp 460V I am talking the difference between using a nonstandard 2.5 amp fuse and a standard 3 amp fuse (I know it is really splitting hairs since I can use the next largest standard and be in compliance). I would just like to make sure I am accounting for the transformer correctly in my design calculations.

My system components are:
1/2 HP 3 phase motor 460VAC Continuious duty 1.15SF Nameplate 0.9A Table Value 1.1A Nema Design Code J
100VA single phase transformer 480/240 Primary (using 480) 240/120 Secondary (using 120)
Class RK5 time delay fuses for branch short circuit protection.

My calculations look like:
1.1A X 175% = 1.925 which is approximate 2A max fuses if it were stand alone
100VA / 460 = 0.217 A primary current on transformer
1.925 + (0.217 X 125%) = 2.19A (a 2.5A fuse)
or
1.925 + (0.217 X 500%) = 3.01A (a 3A fuse) Not sure transformer would ever inrush 500%

Any help would be appreciated.
 

JoeStillman

Senior Member
Location
West Chester, PA
Welcome to the forum.

Are we talking about a fusible-disconnect type combination starter with the control transformer, contactor and overloads all in one enclosure?
 
Welcome to the forum.

Are we talking about a fusible-disconnect type combination starter with the control transformer, contactor and overloads all in one enclosure?

The setup is a separate fused disconnect, standard size 0 three pole nema 1 enclosed motor starter, and separate transformer. All are individual components with their own enclosure. All three are mounted to the side of a belt conveyor.
 

Smart $

Esteemed Member
Location
Ohio
A transformer is not a load.

A dual element time delay fuse can be rated up to 225% of motor FLA.

1.1 FLA ? 225% = 2.5A rounded to tenths place.

That's the value I'd use.
 
A transformer is not a load.

A dual element time delay fuse can be rated up to 225% of motor FLA.

1.1 FLA ? 225% = 2.5A rounded to tenths place.

That's the value I'd use.


It was my understanding that you can only go up to the 225% if the motor will not start due to inrush current limitations but that it was exclusive to the motor current, maybe my misunderstanding. I guess the motor would not have sufficient current if the transformer was "stealing" from it. I believe you are correct in that the transformer itself is not a load, but I was evaluating it based on the fuse in question being on the 480V side, and the load that is connected on the secondary should not exceed the primary current value, though it could reach it if the coil bound during contactor pull in.
 

Smart $

Esteemed Member
Location
Ohio
It was my understanding that you can only go up to the 225% if the motor will not start due to inrush current limitations but that it was exclusive to the motor current, maybe my misunderstanding. I guess the motor would not have sufficient current if the transformer was "stealing" from it. I believe you are correct in that the transformer itself is not a load, but I was evaluating it based on the fuse in question being on the 480V side, and the load that is connected on the secondary should not exceed the primary current value, though it could reach it if the coil bound during contactor pull in.
Technically correct... but as mentioned 430.53(A) permits up to 15A.

Again, a transformer is never a load. Under Code, when a single motor is connected on the primary side, you can only consider the actual other loads... and even then, what effect they have on maximum SC/GFPD rating is quite limited or nonexistent. The smallest motor must always be protected on a branch circuit.

That said, if you put additional branch protection inline for the motor only... your original branch circuit turns into a combo feeder/branch circuit (otherwise quite rare under Code)... then the rules change.

Nonetheless, in this case, we're only talking a 1/2 HP motor... and we've already covered being allowed up to 15A.
 
Smart $,

Thanks for all the feedback. I guess the way I should have presented it would be a blank question in that if I wanted to account for the load on the branch circuit from motor controller coil, how would I do that in a calculation format (the calculation method being most important). I can get the rated coil wattage, but there is not a lot of information in the code for sizing this type inductive load other than looking at the control transformer section or taking it as a standard continuous load and using the 125% multiplier (the two ways I posted my original calculations). Supposing I were to drop out the control transformer and design around a 460V coil (just standard 3 wire control) on the starter, would that math then look like? Only difference with this than with the transformer is that only 1 leg of the three phase see's elevated loading rather than 2 legs using a single phase transformer setup. I was not sure if 430.53(A) applied as this is not a multi-motor installation, but having the window to go up to 15A would mean that none of this makes a difference. My goal was to calculate the actual device load on the branch circuit, with my assumption being that if any of the legs required higher fusing due to the coil that they all would (which may be a bad assumption). Perhaps it is standard industry practice to ignore this coil load in the calculations as it is typically minimal when supplied with the correct voltage and functioning properly. I am always looking at getting the math correct even if it does not change the end result in how I would fuse it, as one time it might make a difference.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Technically correct... but as mentioned 430.53(A) permits up to 15A.

Again, a transformer is never a load. Under Code, when a single motor is connected on the primary side, you can only consider the actual other loads... and even then, what effect they have on maximum SC/GFPD rating is quite limited or nonexistent. The smallest motor must always be protected on a branch circuit.

That said, if you put additional branch protection inline for the motor only... your original branch circuit turns into a combo feeder/branch circuit (otherwise quite rare under Code)... then the rules change.

Nonetheless, in this case, we're only talking a 1/2 HP motor... and we've already covered being allowed up to 15A.

430.53(A) specifically calls out "several" motors. I don't see how you can apply it to a single motor.

However, if one were to change the fuses out to a 15A CB, that would be Ok but I don't think you can use a 15A fuse.
 

Smart $

Esteemed Member
Location
Ohio
Regarding motor branch circuit SC/GFPD rating, that's pretty much how it is. Your options no matter what the conditions, are limited to Part IV (430.51 through 58).
 

Smart $

Esteemed Member
Location
Ohio
430.53(A) specifically calls out "several" motors. I don't see how you can apply it to a single motor.
See following...
430.53 Several Motors or Loads on One Branch Cir-
cuit.
Two or more motors or one or more motors and other
loads
...
The control transformer is not a load... but it powers a load that is not the motor. :D
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If your transformer load is as large or larger than your motor load, then it is not a "motor starter" any longer, it is a control and power distribution panel so different rules for OCPD sizing will apply. The assumption in a "combination motor starter" panel is that the CPT is sized for the necessary coil power of that starter, with maybe up to 100VA more on the secondary side for associated control loads, such as interposing relays, timers, added pilot lights etc. (that's why you often see that additional 100VA as a "standard" option). So if you have a Size 0 starter, it will not need a 500VA CPT for that coil, in fact it will never need more than about 25VA, but standard CPTs start at 40-50 VA. So the load expressed by the basic control circuit load of that starter is generally insignificant to the fuse sizing if following the rules for protecting that motor. Any more than that and you are obviously using the CTP for other loads beside the motor control circuit, so it becomes more complex to figure it out, as in the rules for a custom control panel. At that point I generally use a Main Non-Fused disconnect switch and separate fuses for the motor starter and CPT primary, or use a separate little rotary fused disconnect for the CPT with the handle interlocked with the door.
 

Smart $

Esteemed Member
Location
Ohio
That's right. but is also says that paragraph only applies to "several motors" so it cannot apply to a single motor.
I concede. :ashamed1: (...and thanks for being persistent :happyyes:)

So entertaining 430.52(C) Exception No. 1, the disconnect fuse could be 3A, being the next standard size above the motor FLA ? 175% for dual element... which BTW also exceeds the Exception no. 2 permissive for up to 225% if 175% is insufficient for starting the motor.

Eliminate the fused disconnect and replace with an enclosed MCCB, he can go with a 15A rating, correct?
 

JoeStillman

Senior Member
Location
West Chester, PA
...Perhaps it is standard industry practice to ignore this coil load in the calculations as it is typically minimal when supplied with the correct voltage and functioning properly. I am always looking at getting the math correct even if it does not change the end result in how I would fuse it, as one time it might make a difference.

Yes, it is standard practice. The coil load is insignificant compared to the margin for error in our ability to predict the actual current the motor will draw under load. Have you ever put an ammeter on a motor and found it drawing it's actual nameplate amps?

The coil current is only significant when you are selecting the CPT.
 
Yes, it is standard practice. The coil load is insignificant compared to the margin for error in our ability to predict the actual current the motor will draw under load. Have you ever put an ammeter on a motor and found it drawing it's actual nameplate amps?

The coil current is only significant when you are selecting the CPT.

I was getting the feeling that no one actually looks at the coil load, and most are/were treating it as a single motor installation. I was only interested in it due to the small hp rating. With such a small motor FLC the coil load is a larger percentage relative to the motor load. I have had similar inductive loads (valve solenoids) pull enough current where there could be a problem, but had never really considered it myself before for the motor starters. It seemed more plausable as an issue once I had put in a control transformer in the circuit as the protection level of the transformer was allowed to be (not that it needs to be) set at 500% of the transformer rating, which is almost the same as the motor current (again not that it would actually see that much draw). I guess for now I will abide by the standard practice and call it null and void. RK-5 dual element time delay fuses can overdraw for a few seconds anyhow. For now I believe I will go with a nonstandard size fuse and try to stay below the 225% max indicated by Exception 2. I will let you know if there are any issues.

Thanks to everyone who has posted a response. I will leave the thread open in case someone has additional info to contribute.
 
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