Sizing 100 Amp 3 Phase 480 Service

Status
Not open for further replies.
T

tganz96

Member
Location
Dallas , Texas
I am trying to size wire and conduit for a new 480 Volt 3 phase 100 amp service. The calculator I used gave me 3 AWG copper wire in 1 inch rigid but on our electrical contractors proposal he has 2 AWG copper wire in 1 1/2 conduit.

The run will be about a 100 feet from a MCC bucket.

Which is correct?
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
#3 copper is good for 100 amps at 75C however the contractor may feel better with #2 because of the distance. Depending on the load #2 may be a better choice.
 
Pharon

Pharon

Senior Member
Location
MA
How old is the MCC? If you're not certain that the terminations are rated for 75 degrees C, then #2 AWG is what you should use.
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Pharon said:
How old is the MCC? If you're not certain that the terminations are rated for 75 degrees C, then #2 AWG is what you should use.
Click to expand...


I agree and I also think that I would go with the electricians choice here. I would not want to be told to use conduit or conductors that I thought were not sufficient
 
T

tganz96

Member
Location
Dallas , Texas
I got my info from a sizing calculator so the 1" RMC is for 3 # 3 conductors. If I size the service for 125% in the same tool I get qty 3 #2 Awg but still in 1" RMC. I assume the 1"RMC on these calculations are code compliant but the 1"1/2 would likely be an easier pull.

According to the tool I have qty 3 # 3 awg conductors in 1" RMC is 32.9% fill and qty 3 # 2 awg conductors in 1" RMC is 39.1% fill.

If going with the # 2 wire what size would the ground conductor need to be? Using # 2 the service would be 100 amp 125% rated.
 
Pharon

Pharon

Senior Member
Location
MA
tganz96 said:
I got my info from a sizing calculator so the 1" RMC is for 3 # 3 conductors. If I size the service for 125% in the same tool I get qty 3 #2 Awg but still in 1" RMC. I assume the 1"RMC on these calculations are code compliant but the 1"1/2 would likely be an easier pull.

According to the tool I have qty 3 # 3 awg conductors in 1" RMC is 32.9% fill and qty 3 # 2 awg conductors in 1" RMC is 39.1% fill.

If going with the # 2 wire what size would the ground conductor need to be? Using # 2 the service would be 100 amp 125% rated.
Click to expand...
Are you taking into consideration the fill for the #8 AWG equipment grounding conductor? If you include that, it may put you above the 40% fill threshold.

That said, you probably want to use at least 1-1/4" RMC.
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Since you are using RMC, it is possible that there is a raceway rather than a wire EGC.
Or were you thinking of a GEC or bonding jumper?


Tapatalk!
 
Last edited:
O

Optimal1

Member
Location
Sanford, Florida
tganz96 said:
I am trying to size wire and conduit for a new 480 Volt 3 phase 100 amp service. The calculator I used gave me 3 AWG copper wire in 1 inch rigid but on our electrical contractors proposal he has 2 AWG copper wire in 1 1/2 conduit.

The run will be about a 100 feet from a MCC bucket.

Which is correct?
Click to expand...

You have a proposal for this job? if so you must have an approved set of plans? look at the plans 1st, your contractor should follow the plans 1st and if he has the ability to make changes to save money it should be noted on the proposal.
also the (3) #3 wire would work but pulling that with a ground wire in 1" will be rough. also there is not much money difference between the #2 and #3 it is much better to stick with the #2 in the larger size conduit.
 
kwired

kwired

Electron manager
Location
NE Nebraska
cowboyjwc said:
I'm always told that #2 is easier to find than #3, but then I don't buy wire any more so I couldn't say.
Click to expand...
That likely depends on the supplier, manufacturers do make it.

Optimal1 said:
You have a proposal for this job? if so you must have an approved set of plans? look at the plans 1st, your contractor should follow the plans 1st and if he has the ability to make changes to save money it should be noted on the proposal.
also the (3) #3 wire would work but pulling that with a ground wire in 1" will be rough. also there is not much money difference between the #2 and #3 it is much better to stick with the #2 in the larger size conduit.
Click to expand...
Why is it much better?

tganz96 said:
I am trying to size wire and conduit for a new 480 Volt 3 phase 100 amp service. The calculator I used gave me 3 AWG copper wire in 1 inch rigid but on our electrical contractors proposal he has 2 AWG copper wire in 1 1/2 conduit.

The run will be about a 100 feet from a MCC bucket.

Which is correct?
Click to expand...

Some have assumed no EGC because you said this is a service. You then said it is from a MCC bucket which indicates is is likely a branch circuit and not service conductors.
 
Status
Not open for further replies.
Top