120v control transformer on 3 Phase motor circuit

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Ingenieur said:
Since class 2, 120 V with no internal cb it must be internally limited
it would seem to me you can just hook it up with no ocp?

A sc would be protected by the 480 ckt?
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I do not see where it is class 2. Seems like a short on the primary would be cleared by the branch ocpd. A short on the secondary would be limited by the impedance of the trf. SC current on the secondary would likely be <10A.
 
MD84 said:
I do not see where it is class 2. Seems like a short on the primary would be cleared by the branch ocpd. A short on the secondary would be limited by the impedance of the trf. SC current on the secondary would likely be <10A.
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Much less than 10 A
the data sheet you posted
upper right hand below the UL

page1image9832
Class 2
 
I see it now. Thanks

Maybe more like <5A. I was taking a conservative approach at 5% impedance. It is probably closer to 10%

Maybe I will get a variac and check it myself.
 
Jraef said:
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
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Ingenieur said:
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.
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I am uncertain how that xfmr is listed Class 2. According to Table 11(A), the power source of over 30V through 150V cannot have a VA rating greater than 0.005A x Vmax. In this case, that'd be 0.6VA.

???
 
Ingenieur said:
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.
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It is not power "for" a Class 2 or 3 circuit....it is power from the Class 2 or 3 circuit. Nothing in Tables 11A or B address input power to the Class 2 or 3 power source.
 
Smart $ said:
I am uncertain how that xfmr is listed Class 2. According to Table 11(A), the power source of over 30V through 150V cannot have a VA rating greater than 0.005A x Vmax. In this case, that'd be 0.6VA.

???
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my guess typo
should be 0.5

same table limits current rating to 5 mA
 
don_resqcapt19 said:
It is not power "for" a Class 2 or 3 circuit....it is power from the Class 2 or 3 circuit. Nothing in Tables 11A or B address input power to the Class 2 or 3 power source.
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It's not power 'to'
what they are saying is that FOR a class 2 circuit power is limited
it is understood that it is the load or output/secondary side
 
don_resqcapt19 said:
And that has nothing to do with the question of the primary OCPD.
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Never said it did

if we assume the inherently limited nature of the xfmr as seconday protection
primary ocp 250% of 50 va / 480 vac next higher std rating
0.25 or 0.30 A fuse

if we assume the inherent limit is not sec protection
primary ocp 167%
fuse next up from 0.18 or 0.20

725 says prim or input power wiring must conform to chapts 1-4
if he ran a dedicated pwr/ltg ckt all he would need is a 20 cb in the source pnl

he did not, he ran wires from each starter to the actuatior
imo chapt 4 xfmr <600 v and <9 A applies
a decision of whether it is considered to have sec protection needs made
the 2 options are detailed above
imo it does have sec protection so 250% applies
 
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Ingenieur said:
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.
Click to expand...

Iggy, we are not blind.:D
 
Ingenieur said:
Never said it did

if we assume the inherently limited nature of the xfmr as seconday protection
primary ocp 250% of 50 va / 480 vac next higher std rating
0.25 or 0.30 A fuse
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I agree that the primary protection can be up to 250% of the primary current, but don't agree that you can round up to the next size OCPD. Note 1 to Table 450.3(B) does not apply to the primary OCPD for transformers with currents less than 9 amps.

if we assume the inherent limit is not sec protection
primary ocp 167%
fuse next up from 0.18 or 0.20 ...
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I don't agree (assuming primary current of less than 2 amps)....if there is no secondary protection the primary can be protection can be 300%. We would also have to add secondary protection at no more than 167% of the secondary current. Again, no rounding up of the either OCPD is permitted.

It really doesn't make any sense to me that you are permitted to have a larger primary OCPD where there is no secondary protection.
 
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don_resqcapt19 said:
I agree.


I don't agree....if there is no secondary protection the primary can be protection can be 300%. (assuming primary current of less than 2 amps). It really doesn't make any sense to me that you are permitted to have a larger primary OCPD where there is no secondary protection.
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Where is the <2A 300% ocp defined?
not doubting it, just haven't seen it
edit
Ok see it, table 450.3(B)
300% it is


looked at 450
450.1 exception 4
class 2 devices are exempted

725.127 class 2 supply side wiring
must comply with ch 1-4 ( which includes 450)
 
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Ingenieur said:
Where is the <2A 300% ocp defined?
not doubting it, just haven't seen it
edit
Ok see it, table 450.3(B)
300% it is


looked at 450
450.1 exception 4
class 2 devices are exempted

725.127 class 2 supply side wiring
must comply with ch 1-4 ( which includes 450)
Click to expand...
No matter what the cut sheet says, there is no way a transformer with a 120 volt, 50VA output is a Class 2 device, so 725 does not apply.
 
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