Branch circuit conductors for more than one motor

[maxpaxr]

When sizing the branch circuits for more than one motor, using THHN. Do I use the 75 degree column as stated in 110.14 (C) for my ampacitance, or use the 90 degree column.

 

[jumper]

You use 90C for derating and 75C for final ampacity.

 

[ritelec]

You use 90C for derating and 75C for final ampacity.

Would I be correct to add ........ or 60C for final ampacity if the circuit is 100 amp or less fed from a breaker?


(if this is correct, if the 100 amp or less was fed from fusses, I'm thinking you could work from the 75C for final ampacity....yes? no?)

 

[ritelec]

Could I also add.............I would have to look for the post which brought this to my attention..........but there where different ampacities for thhn in wet or dry locations?

 

[augie47]

I believe you actually use both.

You didn't state "use" for what. If you are uning THHN you can use the 90? column for derating (conduity fill, ambient, etc), but you must keep 110.14 in mind and for terminations.
For example 6 #10 THHNs would dearte to 32 amps (40 x 80%), but if you had 60? terminations you could only use the 30 amp (60? ) rating

 

[jumper]

Could I also add.............I would have to look for the post which brought this to my attention..........but there where different ampacities for thhn in wet or dry locations?

I hope you are referring to dual rated THHN/THWN.

90C dry, 75C wet.

THHW, XHHW, - 90C dry, 75C wet.

Any insulation marked -2, 90C wet or dry.

110.14 still applies for final ampacity due to terminations.

 

[ritelec]

Thanks Derek................that was it.

 

[maxpaxr]

If all 3 phase motors have an ampacity less than 100A but using THHN, do I size the branch circuit conductors from tbl. 310.15(b) 16, using 60 degrees or 90 degrees.
Example: motors have a FLA of 22A x 125%=27.5A. But I am using THHN. Would this be sized as a 10AWG from 60 degree or a 12AWG from 90 degree.

 

[ritelec]

Are you going 125% of the largest motor..then adding 100% of each additional............................


You start in the 90 degree column so you can derate the conductors , THEN when you get that number, the actual conductor rating must be sized from the 60 degree column for heat dissipation from the equipment.

 

[ritelec]

Wow, I just got stymeed!

Please read on, as to my answer which made me question.




From what I see of it...............

Lets say you did 125% of the largest then added the additional motors at 100%.......and came up with 27.5 amps. (and the actual continuous load wasn't over 24 amps for 80 % to protect the breaker......but just for #'s sake).

The 90 degree column shows the 10's rated for 40 amps.......however........ because of the equipment, you could only size those conductors to carry 30 amps.


If you had those 10's in a conduit with other conductors...if your total number of conductors in that conduit was 7-9 then you would derate from the 90 degree column at 70% which would now rate the 90 degree's (40) to be 28 amps.

(now my question)

Would you be allowed to put the 28 amp rated wire on a 30 amp breaker because of the next size higher over current protection.........or because of the 60 degree showing 30 would you have to go for the 8's at 40 amp????

Thank you...hope my lack of an answer and my proposed question made sense.

 

[ritelec]

May I also add to my question............

Same numbers as the last post.........but lets say there where 4-6 conductors in the conduit at 80% (of 40) which would be 32 amps.........


There I would think that works..........

So I'm thinking the 10's would work at either calc.


However if there would be 10-20 conductors in the conduit.......... you could use the 8's which would get rated at 50% of 55.......which would be 27.5 amps .............????

Next higher breaker should make the install ok at 30 amps correct.

Sorry for confusing the issue maxpaxr...........
jezzzzz..........I'm confusing myself...

 

[maxpaxr]

thats ok ritelec, but I'm still cornfuzzzed. This is individual branches running to each motor from one main feeder.

 

[ritelec]

Individual branches (or taps), you will also need to look at tap rules.

Give a bit more info........ Feeder (circuit) size.......breaker or fused. Size and how many motors.... distances of tap to loads. OC if any on the loads (for tapped circuits). HP as to overloads and starters.......

Tap rules are helpful, but sometimes it's best to run separate circuits......

 

[david luchini]

From what I see of it...............

Lets say you did 125% of the largest then added the additional motors at 100%.......and came up with 27.5 amps. (and the actual continuous load wasn't over 24 amps for 80 % to protect the breaker......but just for #'s sake).

The 90 degree column shows the 10's rated for 40 amps.......however........ because of the equipment, you could only size those conductors to carry 30 amps.


If you had those 10's in a conduit with other conductors...if your total number of conductors in that conduit was 7-9 then you would derate from the 90 degree column at 70% which would now rate the 90 degree's (40) to be 28 amps.

So for the sake of the discussion, a feeder with 4 motors, three have an FLA of 5 and one has an FLA of 10. Your feeder would need an ampacity of (10*1.25)+5+5+5 = 27.5A, per 430.24.

The adjusted ampacity of your #10 THHN (90deg) conductors in the conduit with 6 other ccc's would be 28, as you point out.

(now my question)

Would you be allowed to put the 28 amp rated wire on a 30 amp breaker because of the next size higher over current protection.........or because of the 60 degree showing 30 would you have to go for the 8's at 40 amp????

Thank you...hope my lack of an answer and my proposed question made sense.

You could protect the #10 motor feeder with an ampacity of 28 with a 40A c/b in this case, per 430.62(A).

However if there would be 10-20 conductors in the conduit.......... you could use the 8's which would get rated at 50% of 55.......which would be 27.5 amps .............????

Next higher breaker should make the install ok at 30 amps correct.

You could use #8 THHN with an adjusted ampacity of 27.5 (for 10-20 ccc's in the raceway,) fed from the same 40A c/b for the feeder for the same motors with an MCA of 27.5.

 

[ritelec]

Thank you David.

You could protect the #10 motor feeder with an ampacity of 28 with a 40A c/b in this case, per 430.62(A).

Sure ....ha ha ha..lets really confuse me (and maxpaxr) and throw the ole ac specific load calculation in there.


maxpaxr...... About the 90 degree column...The wire in that column with that insulation has ampacities as listed. However because of the heat generated around the wire and the connections to the equipment, 100 amp and under have to apply for that wire size in the 60 degree column and 800 amp and under have to apply for the wire size in the 75 degree column.

So in the end, it's basically like wiring something with TW (60 degree column) or THWN (75 degree column) but your initial calculations (if the insulation type is) done from the 90 degree column to help with derating (ampacity adjustment)......

 

[david luchini]

About the 90 degree column...The wire in that column with that insulation has ampacities as listed. However because of the heat generated around the wire and the connections to the equipment, 100 amp and under have to apply for that wire size in the 60 degree column

Unless the terminations are listed for use with 75 deg conductors. In that case you can use the ampacity from the 75 degree column for equipment rated 100Amps or less.

and 800 amp and under have to apply for the wire size in the 75 degree column.

Just curious, where did the 800Amp come from?

 

[ritelec]

Unless the terminations are listed for use with 75 deg conductors. In that case you can use the ampacity from the 75 degree column for equipment rated 100Amps or less.

Yes......:thumbsup:

Just curious, where did the 800Amp come from?

:thumbsdown:

My bust.....................I was thinking next OC higher...................... Thanks for the catch........
OK..... maxpaxr......Everything over 100 amp rated for 75 degree column....