Continuous Loads

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RB1

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Two three-phase, four-wire multi-wire circuits are installed in a single raceway in an industrial building with an ambient temperature of 115? F. Each circuit supplies 120 volt fluorescent lighting with a connected load of 1920VA. The conductors are THWN-2. What is the minimum size wire required?
 

RB1

Senior Member
David,

Does this look correct?

Continuous load before the application of any adjustment of correction factors = 1920 X 125% = 2400VA/120V = 20A

Ampacity for load served: No. 12 THWN-2 = 30A X .70 X .82 = 17.22A > 16A :: OK
 

david luchini

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David,

Does this look correct?

Continuous load before the application of any adjustment of correction factors = 1920 X 125% = 2400VA/120V = 20A

Ampacity for load served: No. 12 THWN-2 = 30A X .70 X .82 = 17.22A > 16A :: OK

That's the way I did it.
 

RB1

Senior Member
SPierce,

The conductor has an ampacity of 20 amps before the application of the adjustment or correction factors.
 

david luchini

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I'm confused, why wouldn't the final ampacity of the conductor have to be 20 amps since this is a continuous load?

See the first sentence of 210.19(A)(1):
Branch circuit conductors shall have an ampacity not less than the maximum load to be served.

The actual load on each circuit is 16Amps. So a conductor with an ampacity of 17.2 is large enough to carry the load.

The next sentence of 210.19(A)(1) says
Where a branch circuit supplies continuous loads...the minimum branch circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity of not less than...125% of the continuous load

In this case 16*1.25=20. The conductor that has an allowable ampacity of at least 20 is #14 AWG. (But of course 240.4(D) does not allow #14 on a 20A c/b, so #12 would be the smallest allowable conductor size.)

So my 8 CCC's which are #12 THWN-2, in a 115 degree ambient have an ampacity 17.2, which is large enough to carry the load, and is not smaller than the minimum required conductor size. I meet both conditions of 210.19(A)(1)
 

infinity

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See the first sentence of 210.19(A)(1):

The actual load on each circuit is 16Amps. So a conductor with an ampacity of 17.2 is large enough to carry the load.

The next sentence of 210.19(A)(1) says

In this case 16*1.25=20. The conductor that has an allowable ampacity of at least 20 is #14 AWG. (But of course 240.4(D) does not allow #14 on a 20A c/b, so #12 would be the smallest allowable conductor size.)

So my 8 CCC's which are #12 THWN-2, in a 115 degree ambient have an ampacity 17.2, which is large enough to carry the load, and is not smaller than the minimum required conductor size. I meet both conditions of 210.19(A)(1)

You would also need to apply 240.4(B) to use the 20 amp OCPD on a conductor with an adjusted amapcity of 17.2 amps.
 
And of course assuming the length of the multi-wire branch circuits is less than ~ 116' to accommodate voltage drop. (Using information from Ch 9 Table 8 and not adjusting for lower ambient temp of rated resistance at 75 C.)
 

cppoly

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See the first sentence of 210.19(A)(1):

So my 8 CCC's which are #12 THWN-2, in a 115 degree ambient have an ampacity 17.2, which is large enough to carry the load, and is not smaller than the minimum required conductor size. I meet both conditions of 210.19(A)(1)

Wouldn't it be 6 current carrying conductors? For (6) single phase 120V loads on phases A-B-C, the neutrals shouldn't carry current, only unbalanced current.
 
Correction

Correction

Too quick on the calcs....Voltage drop at < 3% dictates the circuits need to be less than 58'....


Wouldn't it be 6 current carrying conductors? For (6) single phase 120V loads on phases A-B-C, the neutrals shouldn't carry current, only unbalanced current.

The fluorescent load dictates that the circuit is non-linear, thus, the neutral must be counted a current carrying conductor (see NEC 310.15-(B)(4)).
 

curt swartz

Electrical Contractor - San Jose, CA
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As far as I know the smallest THWN-2 conductor available is #8 so if these conductors are actually installed in a wet location they would need to be sized using the 75C column.
 

jim dungar

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I don't think #12 THWN-2 is available. The smallest size THWN-2 I have seen is #8. Anything smaller is only THWN. Southwire and Cerro only make THWN-2 in #8 and larger. Maybe some other manufactures do?
Lack of a suitable product would be a "design issue". The NEC only requires a #12.
 

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
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Electrical Contractor
Lack of a suitable product would be a "design issue". The NEC only requires a #12.
If this is just a hypothetical question ok but if this is a real application you have to work with the products available. If the wire being use is only THHN/THWN (no -2) and being installed in a wet location you need to use the 75C column
 
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