vav boxes 2

[Reesh14]

I am circuiting several VAV fan powered boxes to a 120/208V electrical panel. I have selected the VAV box to be 208V. Each unit comes with a factory installed disconnect switch. If the fan motors in the VAV boxes range from 0.17 to 0.75 HP, I want to know how many I can combine to one 20A/1P breaker ( one feeder). Also, when wiring multiple circuits, do I have to provide another disconnect switch between the motor feeder and the provided disconnects? I have been told a couple different things, so I thought I'd give it a shot on this forum. Thanks!

 

[davidbeach]

First off, at those horsepowers, the HP rating is useless; the motors are so inefficient that you can't calculate load from HP. Add up full load currents, add 125% of the largest motor (total is 225% of largest plus sum of remainder) and don't exceed 16A (continuous load).

Whether or not you need another disconnect depends on what the supplied disconnect might or might not be. If it is listed as a disconnecting means you don't need anything more, if not you might well.

 

[jraef]

First off, a 1 pole breaker off of a 120/208V panel is going to give you 120V, not 208. You would need a 2 pole circuit to use 208V.

If you do it with 120V, your current draw for each motor is going to be a lot higher. For instance, the .75HP motor is going to draw 7.6A instead of around 4.1A at 208V, so that will affect how many units you can put on each circuit. You can theoretically load the circuit breaker to 80% of it's rating (i.e. 16A), but with motors you may have a problem with starting current. I would not load each circuit beyond 10A if it were me.

If you already have a disconnect at each unit, and that disconnect can be locked off, and is within a direct line-of-site to the motor, you do not need another one. If that disconnect is on one side of the VAV box but the motor s on the other side and obstructed from view of the disconnect, you will need another disconnect.

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[CJCPE]

jraef

Doesn't NEC allow the disconnect to be not in sight from the motor location if it can be locked open?

 

[jraef]

Conditionally, yes, but be sure to read all of the fine print. OSHA may be different, although I heard that the 2005 NEC is bringing OSHA and the NEC inline with each other.

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