Our prints show that our AHU equipent about 150-160' away is scheduled per conduit schedule my 35a ckts to have #8, 45a ckts to have #4. and 60a ckts to have #3awg. in 1'' Conduit. Im sure not much VD is lost but the wire seems a bit to high. I was thinking about at minimum for 35a and 40a ckts to have #8 and my 60a #6. I know i need to follow drawing but for arguement sake i dont think i need that large a conductor. would changing it seem not professional or is common to make changes in field as needed as long as it follows NEC?
WIRE SIZE PER DRAWING
- Thread starter mannyb
- Start date
- Status
- Location
- Massachusetts
No you do not change the wire to the NEC minimum when the prints show sizes.
As an installer it is not up to me to change the EEs design. I have run 2500 amps worth of copper for a 1,600 amp feeder that was only 200'.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Sounds like the customer paid for what's on the drawing even if it's overkill. We might occasionally use smaller conductors if the spec seems ridiculous and more importantly can we get away with it. 
For VD you can check using this calculator:
http://www.southwire.com/support/voltage-drop-calculator.htm
For VD you can check using this calculator:
http://www.southwire.com/support/voltage-drop-calculator.htm
Dr Duke
Member
- Location
- North Dakota USA
Changing things to what you think or know they should be is also kinda like slapping the engineer in the face. I wouldn't advise against straying from the print unless you have it in writing. I just had an engineered print that had me running 4/0 bare to the ground rods, cadweleded. This was grounding two panels. One 225a 208v and one 225a 480v panel. He also had 4/0 to the bldg steel, water, etc. I found it to be ridiculous, but it won't help by changing without asking.
Duke
Duke
Sounds like the customer paid for what's on the drawing even if it's overkill. We might occasionally use smaller conductors if the spec seems ridiculous and more importantly can we get away with it.
For VD you can check using this calculator:
http://www.southwire.com/support/voltage-drop-calculator.htm
NO! I agree. Thanks
Or if what's being asked for physically can't be done. In either case, a written variation order/instruction should be issued before the change is made.Sounds like the customer paid for what's on the drawing even if it's overkill. We might occasionally use smaller conductors if the spec seems ridiculous and more importantly can we get away with it.
JFletcher
Senior Member
- Location
- Williamsburg, VA
The wire may be upsized that much for possible future expansion, or lots of starts by the air handler with high inrush currents. Also, is the AHU 150-160' "away as the crow flies" (straight line distance from the panel)? If so you may have closer to 200-250' of conduit/wire.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
I second that. If you change the design in the field from what is in the drawings without getting authorization to do so you have shifted the responsibility onto yourself if anything should go wrong or if the customer is dissatisfied. How's your E&O insurance?
- Location
- Massachusetts
I would add that in my opinion if you intentionally do not provide what is on the print you bid and won job the with it is theft.
kwired
Electron manager
- Location
- NE Nebraska
Those values don't seem too overkill for 208 volts, probably overkill if it is 480 volts though.
Still need to talk to the designer if you question it.
Are you sure it is Cu and not Al?
send an rfi
seems way over sized imo
35 A #8 seems ok
45 A 4 is overkill, 8 is good, 6 would be more than enough
65 A 6 is good, 4 more than enough, 3 waaaaay too much
assume no derating
continuous load
75 deg C
send an rfi
seems way over sized imo
35 A #8 seems ok
45 A 4 is overkill, 8 is good, 6 would be more than enough
65 A 6 is good, 4 more than enough, 3 waaaaay too much
assume no derating
continuous load
75 deg C
Last edited:
kwired
Electron manager
- Location
- NE Nebraska
Keep in mind he mentioned these are for AHU's. If the MOCP is 35 the MCA is likely only around 20 amps, if MOCP is 45 the MCA is maybe around 30 amps, and a 60 amp MOCP maybe has MCA of around 40 amps. But he also had 150-160 foot length to each circuit. He did not mention if this is single or three phase and what the voltage is.
Carultch
Senior Member
- Location
- Massachusetts
On some occasions, equipment manufacturer's instructions may call for wire sizes that are larger than what the NEC requires. The engineer might have anticipated this, or as others have said, voltage drop my be a factor.
If you really believe it is an oversight on the engineer's part, document all your facts with a sizing calculation, and send an RFI to confirm that there isn't a factor you are missing.
often when I was designing a bid package the specific AHU equipment was not known or specified
I would do a rough estimate on the size required and upsize so I would not have change order, but possibly a 'back pocket' credit
cheaper to downsize once contracts are let than to upsize
are the 35, 45 and 60 the OCPD sizes?
I would do a rough estimate on the size required and upsize so I would not have change order, but possibly a 'back pocket' credit
cheaper to downsize once contracts are let than to upsize
are the 35, 45 and 60 the OCPD sizes?
- Location
- Massachusetts
While you guys have been talking I have installed the conductors per print, cashed my check and have moved on.
FWIW in my experience not many EEs take well to questions about their choice of wire sizes and I have learned to stop asking questions about it. YMMV.
Seriously?
kwired
Electron manager
- Location
- NE Nebraska
I think you misunderstood the question.
I think he is asking if those figures are the MCA or the MOCP.
Important question because if the units have across the line started motors/compressors, the MCA very well is only 40-60% of MOCP.
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