# 120 volt circuit to VAV boxes

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#### johnj

##### Member
We have many VAV boxes on a floor of a new building. Each box has controls that add up to 20 VA. Each box has a 40VA transformer, 120v to 24v. How do I calculate how many VAV boxes can go on a 20 amp 120 volt circuit?

#### mcclary's electrical

##### Senior Member
We have many VAV boxes on a floor of a new building. Each box has controls that add up to 20 VA. Each box has a 40VA transformer, 120v to 24v. How do I calculate how many VAV boxes can go on a 20 amp 120 volt circuit?

I would put 1900 VA on each 20 amp circuit.

#### stevenj76

##### Senior Member
Try running 24VAC to each vav box instead, from the nearest data closet or elec rm. You can free-air the cable.

#### johnj

##### Member
120 volt circuit to VAV boxes

That's how we normally do it with 24 volt open power runs. In order to keep it class 2 we can only use 100 va transformers for each run. Your limited to the amount of VAV's that can go on each power trunk. Since this building has about 1700 VAV's it would be better to run 120 volt circuits with individual transformers in each box. I can get a lot of boxes on a 120 volt circuit. With 24 volt circuits I would need a lot of transformers, with a lot of wall space and a lot of connections.

#### stevenj76

##### Senior Member
You might be okay, esp. if you feed the next five boxes with the one from 120v.

#### benaround

##### Senior Member
We have many VAV boxes on a floor of a new building. Each box has controls that add up to 20 VA. Each box has a 40VA transformer, 120v to 24v. How do I calculate how many VAV boxes can go on a 20 amp 120 volt circuit?

You could put 47, 40va trans., on a 20a / 120vac circuit.

1900/120v=15.833333amps

1900/40=47.5

#### johnj

##### Member
120 volt circuit to VAV boxes

Thanks for your help. I understand it now.

#### iwire

##### Moderator
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Variable air volume box

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