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T-Former VA rating check???

Member Posts: 2,343
Volt's X amps = VA rating........I just had a brain stumble and forgot the basics.

Mike T.

• Member Posts: 2,343
VA rating for 24 vac t-former.

Need to know the formula to arrive at required VA rating required. Example: 3 HW zone valves rated at .8 amps each =2.4 amps....so 24 vac divided by 2.4 amps equals VA???? of current set up or load. Right or wrong?

Mike T.
• Member Posts: 466
Transformers

a (120V AC)40VA transformer which supplies 24V. 40VA ÷ 24V = 1.66 usable amps available.

a (120V AC)50VA transformer which supplies 24V. 50VA ÷ 24V = 2.083 usable amp available.

Regards,

PR

• Member Posts: 2,343
Thanks Paul

That makes sense......If you take your 1.66 ampsx 24 vac that equals 39.84 va on the rating of the t-former...;-)

Mike T.
• Member Posts: 11,904

Not only do you add up the va ratings but it is a good idea to add some margin for safety. Depending on the type of load there is current inrush involved.

The other factor is wire size especially with low voltage wiring. watch your voltage drop.

ED
• Member Posts: 2,144
T-Former

When figuring the VA rating ,does power factor get added into the formula? V x A x P.F.=watts.? And the VA rating is the same thing as watts.
• Member Posts: 2,343
Techman, ...

I think PF is used in current drawing loads such as motors usually 115vac and higher. from this watts is determined and overall efficiency on the load is determined. Did you purchase that "Ugly's Electrical Reff."??? Great book and it's in there.

See Ya.

Mike T.
• Member Posts: 2,144
Xformer

Mike T. I did and I didn't.I did purchase "Ugly's" but, I didn't read all of it. I'm heading out to the truck right now to go get some reading material-Ugly's. Thank's!
• Member Posts: 2,144
X -Former

O.K. I read Ugly's .The formula IS the formula! So, is power factor more limited to industrial applications? How does P-F affect my units in the 7 1/2 - 50 HP range and smaller commercial / residential units up to 5 HP ? How would I recogonize a problem ? 4 questions, 4 thanks!
• Member Posts: 1,059

I was thinking that power factor comes into play with inductive loads such as transformers, and motors. Resistance loads such as electric heat would not apply.
As to how the power factor would apply to a small control Xformer is of no value. Power factor in regards to a 3 phase motor could raise to cost of operation for that motor. A .80 PF on a 3 phase motor that draws 30 amps and a .95 PF on a 3 phase motor that draws 30 amps the .80 PF motor will cost more to run. Why ? because you have two kinds of power real power(IxE=W) which is what the amp meter sees and apparent power (PF) which is what creates the magnetic field/inductive power. Hope this helps.
Been a long time how did I do. Best Wishes J.Lockard
• Member Posts: 2,144
T-former

Morning!Hi Jim, sounds all right to me! ,so I'll say you did good.But somewhere along the line a bigger t-former is used to get power into our customers buildings. The control T-former's secondary load is almost exclusivly used on resistive loads and being small (75va and lower), I can see the P-F effects being negligable.. I also understand the monitory savings.
One of my refrigeration customers has 4 -5hp3ph units ,4-3hp3ph,and several smaller 1ph units.Should I even care about power factor? How do I recogonize low p-f? Thanks again!(See what you started Mike T.!)(Enjoy!)
• Member Posts: 2,343
SMiles all around.....;-)

The power factor is fixed on a given load. If you were to look in the parts book to replace one of them motors you will find that there are a few selections to choose from. some cost more.....Why? I think if you look at the motor specs some of them are rated more efficient than others, less run cost, more for motor. Check out pg. 13 in the Ugly's Hand book. If you plug in the #'s of your motor, you can get the Idea.

Mike T.

PS: I know Techman,...Look what I started.:-) The last thing I want to do on Football Sunday is try and reassure myself...;-)

Also, just an additional point if you look on further to pg 14, 15 and so on the power factor, HP, and the eff% of the inductive load is set and constant. As a result the #'s may change due to a low voltage situation or a change in amps due to electrical problems or say a plugged filter. Amps in this case will go down and that changes the overall formula, but the said above is still the same. I hope that makes sense. If you need it I can scan the pages for you.

Mike T.
• Member Posts: 2,144
???????

Mike T. I think your computor needs a new "spellcheck". The proper spelling is NASCAR Sunday!
• Member Posts: 2,343
;-) I like That to.

I switch back and forth.

Mike T.
• Member Posts: 47

Just a safety/legal reminder - split loads greater than the
capacity of one 40 VA transformer among two or more
transformers rather than use a higher capacity transformer.
40 VA transformers are designed especially to limit their
output current so as not to overheat the small wire used on
thermostats and controls in case of short circuts.

Rollie Peck
• Member Posts: 397

Here's another reason to stick with smaller transformers, besides. If you have a number of zones and you're using multiple transformers, if one burns out, you have the rest left to keep part of the building going.
• Member Posts: 2,144
Burning T- form

In a R.T. ( Lennox? or York?),on the 75va Tform there was a warning sign cautioning against shorting the secondary. Sure enough,the wind blew my jumper wire (hot)against the cabinet.Then 30-45 seconds later,poof, smoke came out, goo came out ,a couple of swear words also came out ,then down to the truck and out came a new T-former. Enjoy your day!
• Member Posts: 397

Ah, yes,letting the smoke out kills them every time, doesn't it.
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