Bath Exhaust Fan

2227 Views 6 Replies 5 Participants Last post by jontar, Jun 5, 2012

cmic

Discussion Starter · May 30, 2012

Hi all,

I am looking at a blue print that has a panel schedule with a demand calculation for commercial office space. I have two questions:

1. They have treated the bathroom exhaust fan as a motor load, applying the 125% rule. I come from an industrial background and we always treated devices with motors under .5 Hp as regular loads but I can't find anything in the code to support that. It may have been that most of the time there were larger motors involved. Does anyone know if there is a min size motor?

2. The panel schedule has the demand VA for each single phase circuit listed. They calculated the phase imbalance using the VA numbers which is fine, but I need to add a 3 phase device (steam humidifier) to the panel. Do I just divide the listed VA in 3 for each phase or do I use Pp=Vp*Ip? They come to almost the same number but I want to know the right way. In the past I always used current to calculate the imbalance.

Thanks,
Chris

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ElectricJoeNJ

· #2 · May 30, 2012

Funny. An engineer asking us lowly sparkies how to do his job. Lol. You must be one of those EE's that actually likes electricians.

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cmic

Discussion Starter · #3 · May 30, 2012

I had two mentors for my first few years as an engineer, an engineer and an electrician turned designer. The engineer talked much and did little while the electrician did much and talked little. I think you get the picture.

I'm hoping to fall somewhere in the middle.

I do like to joke with the electricians though ... if I can put it on paper why can't you build it? Or my favorite ... what do you mean there is no panel there, I can see it clearly ... in my drawing!

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xlink

· #4 · May 30, 2012

The 125% rule is for wire size. What am I missing?

I would use total va divided by root 3 to get va per phase.

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Shockdoc

· #5 · May 30, 2012

Are those bath fans strong enough to pull your hair up as you walk under them?

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cmic

Discussion Starter · #6 · Jun 1, 2012

Sorry for the slow response.

As far as I can tell, the previous engineer is treating the whole installation like a branch circuit and the fan motors are the largest (only) motors, hence the 125%.

For the 3 phase power, dividing by root 3 ends up giving a higher power requirement than listed for the equipment and when I add up the loads for the total demand the result will be higher. At least this way is conservative so I may settle for that for this project and hope I never see any prints from that engineer again.

And by the way, these 150 W fans can lift the rug off any respectable electrician!

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jontar

· #7 · Jun 5, 2012

usually if its under 1 amp don't worry about it, it counts as a single device on the circuit so 15 amp breaker could have up to 12 (1amp) devices, 80%, such as resi, in comm wiring usually its only 8-10 devices, yes I have seen prints were the bathroom fan was called to be on it's own circuit the as builts showed it was put on with the lighting provided it wasn't 347v lighting of course.

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