I apologize ahead of time for this elementary, and quite possibly poorly posed, question, but I am a coward and want to make sure that I am understanding everything correctly, and not blow four motors...

We have a hydronics heating system, and we are adding a fourth heating zone. Each zone is controlled by a motorized zone valve (https://forwardthinking.honeywell.com/related_links/water/5000_series/install/95c_10932.pdf).

The (above) manual says that the motors have, as

Actuator Electrical Rating:
24Vac 60 Hz, 0.30 A Current Draw,
5 W, 7.2 VA maximum

[The label on the actual device says 0.32 A; in any case, I note that 0.3 X 24 = 7.2 above]

Now, these are on the 'secondary' side of a transformer; the relevant portion of the transformer label says

40 VA
PRI 120 V
SEC 24 V 

My practical question is:

  • Since $$ 40 > 4 \times 7.2, $$ am I reading things correctly, and it is safe to add the fourth motor?

I also have an 'enquiring minds want to know' question:

  • Are the numbers above enough to know what is the power draw on the 120 Vac side, when no motors are running, and when the 4 motors are running?

Thank you very much...


  • 1
    \$\begingroup\$ Provided there aren't any other loads, your 40VA transformer will safely drive 4 valves, each pulling about 7VA. \$\endgroup\$ Commented Oct 30, 2017 at 17:11

1 Answer 1


As pointed out in comments this will be fine.

The VA rating of the transformer is the maximum you can take from the secondary: This is literally just output current times output voltage so you want 28.8VA which is less than 40 so OK.

Note its VA not watts because power also depends on phase and shape of the current taken by the load. Transformers like this are typically better than 95% efficient so the input VA will be slightly higher than this but not much. We can estimate this \$ VA_{input} \approx \dfrac{28.8}{0.95} = 30.3 VA\$ About 0.28 amp from your 110V mains.

Without any load there will be some current in the primary. This is called the magnetising current as is a result of the inductance of the primary winding. The more inductance you have the less magnetising current there will be.

  • \$\begingroup\$ Thanks - so can one say what happens on the primary when nothing is running/connected on the secondary? Inductance, and a buzzing sound, the circuit breaker doesn't flip, and obviously I don't have a short circuit... \$\endgroup\$
    – peter a g
    Commented Oct 30, 2017 at 18:16
  • \$\begingroup\$ Right - but I guess one can't know the value of the current by 'pure thought' - correct? It's not related to your 95% efficiency? \$\endgroup\$
    – peter a g
    Commented Oct 30, 2017 at 18:24
  • \$\begingroup\$ A proper data sheet for the transformer should tell you but it is not usually written on the transformer label usually. \$\endgroup\$ Commented Oct 30, 2017 at 18:30

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