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Joined: Jan 2006
Posts: 47
I am working under the assumption that DC circuits fall under the maximum 3% allowable voltage drop as stipulated by the NEC.

Is this correct?

It is for DC feeds to radio equipment located on the roof of a building being fed from rectifers in the basement.

Joined: Mar 2005
Posts: 1,213
NEC never actually requires 3%, just recommends it. I'd put this into the category of "it's always a good idea to oversize for voltage drop" though!

What's the acceptable DC voltage range of the equipment? If it can handle the drop, it's not a big deal.

Joined: Jan 2003
Posts: 4,391
I agree with Steve, great idea to maintain less than 3% but not NEC required.

Bob Badger
Construction & Maintenance Electrician
Joined: Apr 2002
Posts: 7,282
Likes: 3
This was a class 'discussion' item last nite.

Vd is a fpn (Fine Print Note), not 'Code' not mandatory.

Vd is a design issue, and could/may be debated via 110.3 (B)

'Good Practice' & 'Common Sense' are not in the NEC, but should be observed by all.

I would check the equipment for a voltage, and take a reading under load, if it's already installed. Or, I would do the calcs on paper beforehand, and 'lean' to a larger conductor.


Joined: Jan 2006
Posts: 47
Thanks for the input,

Now that everyone mentions it I remember this coming up in past.

The equipment runs 48vdc and the equip manufacturer states that the equip is fully operational down to 36vdc and they often crank up the output on the DC rectifiers to compensate.

I will just have to come up with allowable voltage drop calcs for these designs based on that criteria.

Thanks for the help

Joined: Jan 2006
Posts: 47
Just curious about others opinions,

Designing engineer for the clients wants us (the consulting engineers) to spec out pulling a Cat5e cable in the same 2" conduit with (6) #2AWG DC cables for the radio equipment.

This cable is simply an alarm cable that will tell them if the door on the Equipment cabinet is open or closed.

The cable pulls will be in the range of 200-400'.

Anyone have any tips for getting the Cat5 pulled in with the #2's without tearing it up

Joined: Jul 2004
Posts: 9,663
Likes: 4
Back in the olden days when we were pulling fiber with other fat conductors in ducts we pulled it in innerduct, a smurf tube like deal. That may not be an option in 2" tho. You need to do a fill calc.

Greg Fretwell
Joined: Mar 2005
Posts: 1,213
I see smurf tube all the time to protect the fiber optic cabling, but yeah, not an option. Mil-spec cat5 has a stronger jacket and would easily hold up, but it's expensive.

Why not just pull the cat5e in last?

Joined: Nov 2000
Posts: 2,148
If you have a couple of 90s, the Cat 5 is likely to be damaged or broken. We won't even install #14 control wires in with power wires larger than #4. This happens where there is a 90 near the end of the run. Some of the conductors will take the short path on the inside of the 90 and others will have to be a bit longer riding on the outside of the inner conductors. This extra length has to come from the feed end and if the Cat 5 is on the outside and if it can't slide by the other conductors it will stretch or break.

Last edited by resqcapt19; 03/26/07 07:16 AM.

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