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Discussion Starter · #1 ·
I have 4-10 ton, 460v 3ph RTU's in a row.
am wanting to hit 2- RTU's with one raceway.
There would be 7 current carrying cunductors.( 2- 25 amp MCA, circuits plus a 120v service recept in each conduit) the longest run is 275'. the other is 235'
I know how to calculate for VD but im sketchy on applying derating to the VD calculation.
so using the 70% derate for 7 conductors
do I take the 25 amps load and divide by .7? and this would give me 35.7 amps.
then I would use 35.7 in my VD calc?
When applying derating cant we use the 90 deg column?
 

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Yes use the 90C for VD.

10 awg is good for 40 amps. If you derate to 70% you will have an ampacity of 28amps. Of course you need top check if the 10 is good enough for the VD.

Probably best to do the VD first and I suspect that the conductor will be large enough for the load with the de-rating
 

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I did a VD calc and came up with a #8. Obviously the #8 will be fine for the 25 amp load since I showed above that 10 awg was sufficient without the VD
 

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Discussion Starter · #4 ·
I have two calculators for doing voltage drop calc's. one is by Electricalc Pro.. it says to use #8
the other is an app on my Iphone by southwire. it says to use #10?
75deg, 25a at 360v, 275'
So,
25/ .7 = 35.7amps. this represents the MCA after derating is factored in. #10 is good for 40 amps in the 90 deg column.
 

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25 amps is what you need your conductor to be capable of carrying. 40 amps time 70%= 28 amps. Not sure why you divided 25/.7

28 amps is enough for the load served but the VD seems to want a #8
 

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Discussion Starter · #6 ·
because the load (25 amps) upsized 30 % is 35.7.
conversely 30% of 35.7 =25 or 35.7 x .7 = 25

I like the way you did it though, makes sense.
 

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You don't upsize the load. The mca of the unit has the 125% figured in it. So work with the 25 amps.

Your VD calc will be based on 25 amps, 3 phase , copper, 480 (460) volts. Using the furthest distance I get 8 awg so the de-rating really is not an issue
 

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Discussion Starter · #8 ·
i have 3 VD calculator apps on my phone.
Cerrowire says #8
General Cable says #10
Southwire says #10
 

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i have 3 VD calculator apps on my phone.
Cerrowire says #8
General Cable says #10
Southwire says #10
What percentage drop are they using. I usually work with 3%.
 

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I have 4-10 ton, 460v 3ph RTU's in a row.
am wanting to hit 2- RTU's with one raceway.
There would be 7 current carrying cunductors.( 2- 25 amp MCA, circuits plus a 120v service recept in each conduit) the longest run is 275'. the other is 235'
I know how to calculate for VD but im sketchy on applying derating to the VD calculation.
so using the 70% derate for 7 conductors
do I take the 25 amps load and divide by .7? and this would give me 35.7 amps.
then I would use 35.7 in my VD calc?
When applying derating cant we use the 90 deg column?
Is this ok in the US? I don't think we could pull that off north of the 49th, or at least I've never seen a 120 volt lighting circuit tied into a higher voltage (MCC or CDP) feeder conduit...
 

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Is this ok in the US? I don't think we could pull that off north of the 49th, or at least I've never seen a 120 volt lighting circuit tied into a higher voltage (MCC or CDP) feeder conduit...

Yes it's allowed. Many times it will be avoided due to job specs and lots of people think it's forbidden but in the industrial world it's done every day. I don't go out of my way to do it but if it makes the project or run easier then I don't even think twice


Sent from my iPad using Tapatalk
 

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Discussion Starter · #12 ·
Thanks for the help on this project. appreciate the feedback. This message board thing is new to me. cant figure out how to like a comment or quote a comment.
 

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Is this ok in the US? I don't think we could pull that off north of the 49th, or at least I've never seen a 120 volt lighting circuit tied into a higher voltage (MCC or CDP) feeder conduit...
The way I read it , I think he means to pull the 125 VAC circuit in the same conduit as the 480 VAC conduit ?

Would not do that on a bet . We run a separate conduit for the 120 VAC circuit .

God bless
Wyr
 

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The way I read it , I think he means to pull the 125 VAC circuit in the same conduit as the 480 VAC conduit ?

Would not do that on a bet . We run a separate conduit for the 120 VAC circuit .

God bless
Wyr

Yes, but the install is compliant
 

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Is this ok in the US? I don't think we could pull that off north of the 49th, or at least I've never seen a 120 volt lighting circuit tied into a higher voltage (MCC or CDP) feeder conduit...
Compliant in the US, violation in Canada.

12-904 (2) No raceway or compartment of a multiple-channel raceway shall contain conductors that are connected to different power or distribution transformers or other different sources of voltage, except where the conductors
(a) are separated by the metal armour or metal sheath of cable assemblies of the types listed in Table 19;
(b) are separated by a barrier of sheet steel not less than 1.34 mm thick or a flame-******ant non-metallic insulating material not less than 1.5 mm in thickness; or
(c) are used for the supply and/or control of remote devices, are insulated for at least the same voltage as that of the circuit having the highest voltage, and none of the conductors of the circuits of lower voltages is directly connected to a lighting branch circuit.
 

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The way I read it , I think he means to pull the 125 VAC circuit in the same conduit as the 480 VAC conduit ?

Would not do that on a bet . We run a separate conduit for the 120 VAC circuit .

God bless
Wyr
So you would also run separate conduits for the 208 and 277v circuits?
 

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In the u.s. as long as the insulation is rated the same most is rated 600v or less.
Seperate conduit for low voltage t stat, fa shutdown etc.
Class 2 wiring
 

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Around here most would pull all of the circuits in a one inch conduit and not derate anything. They would worry to all hell about a little dog in an offset but wouldn't think twice about the fill or compliance.
 
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