Transformer VA

Jamie_5

This is topical for me because I am a DIYer about to install thermostats and a transformer to run a set of thermal actuators with isolated end switches. I am *not* knowledgeable about this, just asking, but can't you undersize a little? In my case for instance, I have 11 actuators to run. They take 200mA to open and 70mA to maintain, according to their instruction sheets. Assuming that all 11 open at the same time, which I believe is unlikely since there will be seven zones, a 50VA transformer would still be supplying 22.72 volts. Is this a significant voltage drop requiring upsizing? I realize that the upfront cost difference between a 50VA and 60VA transformer is negligible, but I assume, perhaps incorrectly, that the larger transformer also costs more to operate. What is the correct approach?

[Deleted User]

Transformer VA

What is the proper size fuse to use to protect a 50 VA 24 volt transformer. Also, what is the electrical formula used to convert VA to amps. Lastly, what are the maximum amount of Taco zone valves or Honeywell zone valves to be installed on a 50 VA transformer.

Firedragon_4

Right here:

VA Formulas

A 15 amp fuse or breaker should be heavily oversized for the job.

[Deleted User]

V A is the same as

watts it is a power rating.

An example for application:

A Taco zone valve runs at about 1 amp of current. If you multiply 1 amp x 24 volts = 24 VA on this basis you would need 24 VA for each Taco ZV. There is however a NEMA Standard DC20-92 which allows a class 2 transformer to run at 200% of capacity. This means a 40 VA could actually handle up to a 80 VA draw and still hold voltage at 20 to 24 volts.

If you want to follow the fact that bigger is better then you need 80 VA for three Taco ZV's.

Your question on a 50 VA would theoretically handle two Taco ZV's. If the standard is used however you could go up to 100 VA which means 4 Taco ZV''s.

Honeywell valves draw about .2 amps so you can run many more of them on a 40 or 50 VA transformer. If my math is correct about 10 could be run on a 50 VA.

Eugene Silberstein 3

VA Rating

The formula sheet that George posted is correct. However, in the field, calculating the power factor is not a very easy task. Since you are referring to a low voltage control circuit, and power factors are in the range from about 0.70 to 1.0, we often ignore the PF for general troubleshooting processes.

As far as the VA rating goes, VA stands for volt amps, which, from Ohm's Law is a rating of power. If you have a 50 VA control transformer that is supplying 24 volts to the control circuit, the maximum amperage for that circuit would be 50VA/24V = 2.1 amps. If I wer installing a fuse in the control circuit to protect the transformer secondary, I would use a 2 amps fuse, as any current over the maximum of 2.1 amps can potentially damage the device.

As far as the maximum number of devices that can be controlled by your 50 VA transformer, you would need to determine the actual amperage draw of one of the devices and then divide that amperage into the maximum amperage of 2 amps. Be sure to round down.

To be on the safe side, round down and then subtract 1.

Hope this helps.

johnnyd

Try the TACO ZVC?

I was confused about this issue and decided to save time and spend a little $ on this item:

http://www.taco-hvac.com/products.html?current_category=141&PHPSESSID=15a673689faf8fe078159ed60b63a889

Works great!

greg_28

transformer

Save your money. Very unlikely they all open at once and for the few seconds, no damage will occur anyway. A larger transformer does not cost more money to operate, just to buy. A transformer mearly adjusts the voltage, so 50 watts (or 50 VA) IN almost exactly equals 50 watts (or 50VA) out. Difference between in and out is a very small amount lost to heat in the transformer. don't sweat it.

Now to save a lot of time wiring the actuators, a Taco or Argo Zone Valve Controller will make things very neat and easy.

BUT as a wise man once said, "How do you want it, fast, good, and cheap? Can't do it, you have to pick 2 choices only."

Greggy

P.S. a ciruit breaker only limits amperage (an that is current to the primary side of the coil), so a 10 amp house breaker used only for a transformer, could handle a transformer output of 1200 Watts (VA). <- a huge transformer! The house breaker will not trip until it's too late, like a 50 amp secondary draw, by then your electronics are toast.
You will need to use an automotive type fuse.

Jamie_5

Thank you

Thanks to Greggy and Johnnyd for the helpful responses.

jerry scharf_3

VAs are not watts

(small rant) It looks from units that VA (VoltAmps) = watts, but in practice, they are not the same. With pure resistive loads, the power factor is 1 and they are the same. In anything with capacitors or coils, it's less than one. Almost everything (motors, power supplies, transformers) have coils in it, and so they have a power factor < 1. Power systems are rated in VA (ever look at a UPS) as that's what counts in terms of load for things like transformers. This is based on the size of the wires wound in the transformer and their heat dissipation capacity. (I feel better now.)

To figure out the current on a 50VA 24V transformer, you divide 50/24 to get 2+A. I never overload a transformer (more VA of load than transformer rated capacity.) It would probably work, but how long does it take to figure it out if it doesn't. Transformers can burn out (there are really skinny wires in there) which is a pain. Worse is if the voltage sags below 24V at full load. This can be really hard on controllers and actuators, and can lead to early failure of those. Other times things won't happen when you expect them to for mysterious reasons... That's a real pain.

jerry

greg_28

I guess Jerry has the more technical answer, of course do you calculations, but to size for all your actuators to be activated at once seems like way overkill. Jerry, thanks for the clarification.
Greggy

[Deleted User]

Thanks Eugune for your response. A 2 amp fuse might do it and I may install a "FAST-BLO" type fuse to protect the thermostats and Tekmar control from taking a hit if a valve motor shorts or any other unforeseen event occurs in the low voltage circuit.

Jamie_5

additional thanks

Additonal thanks to Jerry Scharf for his input.

I have heard of but never read an explanation of the difference between capactive/inductive loads and resistive loads and I probably don't have the background to fully grasp it. If Greg is right (and I have no reason to doubt him) that a transformer only draws a little more current than the load it is supplying (with the little more being the loss due to the less than perfect efficicency of the step-down process), then I think I might as well be safe and spend a couple of extra bucks on the 60VA transformer. I am a little curious as to why the heat motors or the transformer would be damaged by the attempted excessive draw, however. If someone is willing to put the time in to an explanation, I can promise I'll read it carefully.

As for zone valve panels, it seems like overkill to me for this particular purpose. The actuators are on two remote manifolds which are themselves fed water by separate pairs of (Honeywell) gear-motor-zone-valve-controlled pipes. There is a zone valve relay panel controlling those two zones and a third priority indirect zone. If this seems a bit inelegant, well, perhaps it is, but it is the result of a change of plans in the heat source. It's a long and tedious story to anyone but me.

Again, thanks to all who offered their knowledge.

jerry scharf_3

a quick follow up

There is a thing denoted Q which indicated the coupling ability of the primary to the secondary in a transformer. The loss is due to resistance and eddy currents in the transformer. So a superconducting transformer with an air core has a Q of 1, and it's down hill from there. Most transformers have a Q fairly close to 1, so the statement that input is the same as ouput is close enough for boiler work. No harm in a 60VA transformer rather than a 50.

A heat coil shouldn't care if it sags a bit. Motors driving full load heat up quite a lot when the voltage sags. depending on how often and how far the voltage sags, you can burn out the actuator motors. Controls can reset on a voltage sag as well (very dependent on the power supply design.)

I have one kind of actuator that auto-zeros each time they get initial power, then take a control voltage to set the level. It drives the motor to full travel and back, taking about 60s. So when I feed initial power, all the motors run to full load at just about the same time. You can bet I count the VA on those closely.

jerry