120v control transformer on 3 Phase motor circuit

[MD84]

You should power this circuit through auxiliary contacts.

Your 480:120 xfmr is required to have protection. At 50VA puts it under 2A on primary. Being a 2-wire secondary you can use primary only protection... but it cannot be greater than 300%, which is 50VA÷480V×300%=0.3125A.

Are you certain you only need a 50VA xfmr? What's the rating of the louver motor?

Thank you for the feedback.

The louver motor is 30W.

There is a separate intake louver for each exhaust fan. We ran circuits utilizing the aux contact for those. The exhaust fan louvers are co-located to the motor. Utilizing the trf will save us running new circuits.

Does article 725 apply? That is where I was getting 500% (430.72(c)(4) . 500vs300% is really negligible but I was more curious.

 

[MD84]

The circuit will be zero seq balanced but line magnitudes/phase angles will differ

what starts the fan?
120 v control circuit?

the 300% primary protection is fine
using the contactor aux contacts or an interposing relay is a good idea

Fan starts with "smart" motor starter. It takes 480 and has a black box which has some controls, settings, and probably a CPT. The contactor coil is 24v.

We ran a 120v circuit through the starters for the intake louvers.

 

[MD84]

Motor operated louvers typically require open and close limit switches and motor reversing controls. You have not mentioned any, so I want to make certain these are covered also. If you open the louver using fan motor ON as an enabler, how does the louver close when the fan motor is OFF.

The louver motor when energized opens the louvers and stores energy in a spring. When fully opened an internal limit switch stops the motor. When power is removed the spring returns the louvers to closed passively.

 

[MD84]

it is common for small actuators like what the OP describes to use an impedance protected gear motor which is constantly energized to hold the louvers open and a strong spring return to "unwind" the motor and close the louver or damper when the motor is deenergized.

A transformer which is also impedance protected against secondarhy short circuit (like a doorbell transformer) might need only a secondary fuse of a size which protects the wiring. That is, much larger than .3A.
The question is whether such a transformer is available for 480V input.
It may be necessary to use a transformer larger than 50VA just to make the fusing possible with a standard size.

We have some installed now. It looks like a doorbell trf. It is mounted to a 4" cover.

 

[MD84]

You can buy any motor starter with a 50VA CPT pre-wired by the factory. You typically don't even need 50VA for the starter coil, but that's as small as they come anyway. So just tap off of that secondary, run it through an aux contact of the starter and be done with this. Seems like a lot of hand wringing over a simple issue.

The GC supplied the materials. We are responsible for wiring. The drawings show a note for a 120v CPT but none were supplied. I am sure we would have specified a different system however we need to use what was supplied.

We chose the most economical solution given the scenario. I am just curious about protection requirements of the CPT.

I appreciate the help. Perhaps it is internaly protected I will check.

 

[MD84]

Sorry about sideways picture.

 

[Ingenieur]

Looks like a class 2 with an internal cb
run the 120 from a 15A pnl cb

 

[kwired]

Fan starts with "smart" motor starter. It takes 480 and has a black box which has some controls, settings, and probably a CPT. The contactor coil is 24v.

We ran a 120v circuit through the starters for the intake louvers.

I've seen some of those damper/valve motors that are 24V, and they have optional internal transformer that can be installed if you want to supply them with other voltages - particularly Honeywell "Modutrol" motors.

 

[Jraef]

Sorry about sideways picture.

Oh yeah, that's one of those Greenheck Fan Starters. That "Smartstart" unit takes any voltage from 100-600V as control power, so it didn't need a CPT, (as long as everyone is comfortable with the contactor coil being 480V), that's why you didn't get one.

That little RIB transformer is a Class 2 device, meaning it is "Inherently Limited" so you can use it without any fusing. If you want to fuse it anyway, you can use up to 500% of the primary current and no secondary fuse. It's intended for being used exactly as you as planning here.

*But since those buttons on the Smartstart unit go through the door, you don't need any other control devices so the 480V control is not so bad.

 

[don_resqcapt19]

Oh yeah, that's one of those Greenheck Fan Starters. That "Smartstart" unit takes any voltage from 100-600V as control power, so it didn't need a CPT, (as long as everyone is comfortable with the contactor coil being 480V), that's why you didn't get one.

That little RIB transformer is a Class 2 device, meaning it is "Inherently Limited" so you can use it without any fusing. If you want to fuse it anyway, you can use up to 500% of the primary current and no secondary fuse. It's intended for being used exactly as you as planning here.

*But since those buttons on the Smartstart unit go through the door, you don't need any other control devices so the 480V control is not so bad.

Where does the code say that? I see 725.127 as requiring a primary OCPD.

725.127 Wiring Methods on Supply Side of the Class 2 or Class 3 Power Source
Conductors and equipment on the supply side of the power source shall be installed in accordance with the appropriate requirements of Chapters 1 through 4. Transformers or other devices supplied from electric light or power circuits shall be protected by an overcurrent device rated not over 20 amperes.

Of course is likely that the motor branch circuit OCPD would be suitable to protect the transformer.

 

[Jraef]

Where does the code say that? I see 725.127 as requiring a primary OCPD.


Of course is likely that the motor branch circuit OCPD would be suitable to protect the transformer.

Oops, you're right. I was thinking Table 11A, but that is only for when the source is below 150V, this is 480V. It's been so long since I worked on anything that used line voltage control that I got sloppy there.

 

[don_resqcapt19]

Oops, you're right. I was thinking Table 11A, but that is only for when the source is below 150V, this is 480V. It's been so long since I worked on anything that used line voltage control that I got sloppy there.

I thought that Tables 11A and B only applied to the output of the Class 2 device, not the supply....but I have always been confused by those tables.

 

[MD84]

Thanks for the feedback. I will consider a fuse to protect the transformer. Good to know the 20A branch circuit breaker is acceptable for protection in this case.

How do I determine the transformer is class 2? Is this based solely on its VA and voltage? Should this be listed by the manufacturer or marked on the unit?

 

[Ingenieur]

Thanks for the feedback. I will consider a fuse to protect the transformer. Good to know the 20A branch circuit breaker is acceptable for protection in this case.

How do I determine the transformer is class 2? Is this based solely on its VA and voltage? Should this be listed by the manufacturer or marked on the unit?

should be on the xfmr or instruction sheet
picture is blurred
what is the model number ?

the xfmr most likely has a cb
no fuse required

this looks like it http://www.kele.com/Catalog/18 PowerSupplies/PDFs/TR50VA004US Data Sheet.pdf

 

[MD84]

I do not think it has a cb nor class 2.

It is TR50VA018

http://www.functionaldevices.com/pdf/datasheets/TR50VA018.pdf

 

[Ingenieur]

I do not think it has a cb nor class 2.

It is TR50VA018

http://www.functionaldevices.com/pdf/datasheets/TR50VA018.pdf

it is UL class 2
no cb
no 120 primary either

exchange for a 120 w/cb

 

[Ingenieur]

Fan starts with "smart" motor starter. It takes 480 and has a black box which has some controls, settings, and probably a CPT. The contactor coil is 24v.

We ran a 120v circuit through the starters for the intake louvers.

if you ran a 120 ckt why the need for the xfmr?
just tap a run to the 120 actuator and interpose aux/run contacts

 

[Jraef]

OK, I'm screwing up royally here.

Class 2 is going to be limited to 30V or less on the output, so there is no such thing as a Class 2 transformer with a 120V output, sorry.

I was led astray by looking some catalog info and the fact that the 120V output transformers said they were UL listed under UL-5085-1, which I knew is the standard that covers Class 2. But I looked at the definitions themselves in the UL spec and Class 2 is limited to 30V on the output side. That UL spec does also include larger transformers with higher voltages, but they wouldn't be Class 2. A Class 3 transformer can be as high as 150V output, but the exceptions allowing for not having primary protection wouldn't apply, so it's a moot point.

Bottom line, you need protection on the primary as previously mentioned. If that breaker is 20A or less, you're good to go. If not, you will have to find somewhere to put one, it won't fit in that box with the starter.

http://www.functionaldevices.com/pdf/datasheets/TR50VA008.pdf

 

[Ingenieur]

The 3 data sheets posted are all marked UL class 2 ?
all have 120 secondaries

Power for Class 2 and Class 3 circuits is limited either inherently (in which no overcurrent protection is required) or by a combination of a power source and overcurrent protection.

The maximum circuit voltage is 150VAC or VDC for a Class 2 inherently limited power source and 100VAC or VDC for a Class 3 inherently limited power source. The maximum circuit voltage is 30VAC and 60VDC for a Class 2 power source limited by overcurrent protection, and 150VAC or VDC for a Class 3 power source limited by overcurrent protection.

 

[MD84]

if you ran a 120 ckt why the need for the xfmr?
just tap a run to the 120 actuator and interpose aux/run contacts

We have intake louvers on the opposite side of the building which we ran the 120v circuit. The exhaust louvers are located opposite the intake and are in the same vent as the fan/motor. The idea was to use the 480:120 trf in the exhaust fan vent. This allows control of the louvers without running another circuit. We already have the 480v circuit in place and the installation of the 480:120 trf is rather quick and cheap vs running another Raceway in this situation.