Sizing 3 phase service conductors

[david luchini]

Look at example D3 in the Annex...
Particularity :"Minimum size service conductor"

Minimum Size Feeder (or Service) Overcurrent Protection (see 215.3 or 230.90) Subtotal noncontinuous loads 12,200 VA Subtotal continuous load at 125% (16,200 VA × 1.25) 20,250 VA Total 32,450 VA

There are no adjustments or correction factors in that example. Look at D3(a) where they have a correction factor for ambient temperature.

 

[augie47]

There are no adjustments or correction factors in that example. Look at D3(a) where they have a correction factor for ambient temperature.

True... it does not have any adjustment or corection for such things as fill or ambient but it doers include 25% load for continuous.

You will notice on D3a when they size the conductors the add 25% continuous.

It appears to me the load calculation is one number but when you size the conductor you must address the continuous aspect..


Subtotal, actual load [actual load in amperes] [99,000 VA ÷ (480V × √3) = 119 A] 95,500 VA (25% of 56,600 VA) (See 215.3 ) 14,200 VA Total VA 109,700 VA

 

[wwhitney]

Look at example D3 in the Annex and see Minimum Size Feeder IService)

As David said, look instead at D3(a), particularly this section:

Ungrounded Feeder Conductors
The conductors must independently meet requirements for (1) terminations, and (2) conditions of use throughout the raceway run.

Minimum size conductor at the overcurrrent device termination [see 110.14(C) and 215.2(A)(1), using 75°C ampacity column in Table 310.15(B)(16)]: 1/0 AWG.

Minimum size conductors in the raceway based on actual load [see Article 100, Ampacity, and 310.15(B)(3)(a) and correction factors to Table 310.15(B)(16)]:
95,500 VA / 0.7 / 0.96 = 142,000 VA [70% = 310.15(B)(3)(a)] & [0.96 = Correction factors to Table 310.15(B)(16)]
Conversion to amperes: 142,000 VA / (480V × √3) = 171 A

Note that the first check directly uses the 75C ampacity column, no correction or adjustment factors.

While the second check uses the adjustment and correction factors, but it uses the load without any 125% continuous use factor. [See the preceding section for the computation that the load is 95,500 VA, and 109,700 VA or 132A with a 125% continuous use factor.]

Cheers, Wayne

 

[david luchini]

True... it does not have any adjustment or corection for such things as fill or ambient but it doers include 25% load for continuous.

Yes. That's part (1).

Figure part (2) and you use the calculated load (without 125% for continuous loads) as shown in example D3(a).

 

[david luchini]

You will notice on D3a when they size the conductors the add 25% continuous.

Not when they size the conductors with the correction and adjustment factors.

They calculate the load as 95,500VA. Adding 25% of the continuous load gets them to 109,700VA.

When they calculate the required ampacity for the adjustment and correction factors (for number of current carrying conductors and ambient temperature), they use 95,500VA...The actual calculated load WITHOUT the 25% of continuous load added in.

 

[augie47]

Yes. That's part (1).

Figure part (2) and you use the calculated load (without 125% for continuous loads) as shown in example D3(a).

I'll agree,,, then when you go back to 230.42 General it states use the LARGEST of (1) or (2)....

 

[david luchini]

I'll agree,,, then when you go back to 230.42 General it states use the LARGEST of (1) or (2)....

Indeed. In the OPs example, both (1) and (2) would give you a 300kcmil conductor, so the largest would be 300kcmil.

If the OP's question had stated to use a THWN (75deg) conductor, then part (2) would calculate to a 350kcmil conductor, and that would be the largest.

 

[augie47]

I think my brain has melted .... I'm going to regroup tomorrow but may never fully understand ..you guys are smarter than I am.....

 

[wwhitney]

I have various PIs in for the 2026 NEC to correct this issue.

See https://forums.mikeholt.com/threads/pi-210-19-a-number-one.2573389/ for the PI on this issue.

Cheers, Wayne

 

[Tulsa Electrician]

See https://forums.mikeholt.com/threads/pi-210-19-a-number-one.2573389/ for the PI on this issue.

Cheers, Wayne

Thanks for sharing.
Hope they fix this one.