25 ft tap rule, 3 phase subpanel and derating

E

EricNY

Member
Location
New York
Occupation
Electrical Contractor
I am installing a tap (25' or less). The existing feeder is 480V and protected by 800 Amp fuses. The new feeder tap is called to be 4-600kCMil + 1/0 gnd and feeds a 400 amp main breaker in a new sub panel. The sub panel only has 3 phase loads ( (2) 90 Amp Air Handlers, (2) 125 Amp Condensers, and (2) 15 Amp Gas Heat), but room for someone down the road to install more loads.

I think there is a problem: am I supposed use Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway? 3 phases and 1 neutral. So 80%. 600s are 475 Amp *80% = 380. So this is less than the 400 Amps.

I was going to suggest to use 2 sets of (3) 3/0 + 1/0 gnd.

But if they want the neutral, do I need to put 2 sets of (4) 4/0 + 1/0 gnd. 2 * 260 Amps * 80% = 416 Amps.

I believe each of these would also satisfy the 25' tap rules.
 
Dennis Alwon

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
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Retired Electrical Contractor
EricNY said:
I am installing a tap (25' or less). The existing feeder is 480V and protected by 800 Amp fuses. The new feeder tap is called to be 4-600kCMil + 1/0 gnd and feeds a 400 amp main breaker in a new sub panel. The sub panel only has 3 phase loads ( (2) 90 Amp Air Handlers, (2) 125 Amp Condensers, and (2) 15 Amp Gas Heat), but room for someone down the road to install more loads.

I think there is a problem: am I supposed use Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway? 3 phases and 1 neutral. So 80%. 600s are 475 Amp *80% = 380. So this is less than the 400 Amps.

I was going to suggest to use 2 sets of (3) 3/0 + 1/0 gnd.

But if they want the neutral, do I need to put 2 sets of (4) 4/0 + 1/0 gnd. 2 * 260 Amps * 80% = 416 Amps.

I believe each of these would also satisfy the 25' tap rules.
Click to expand...

In most cases the neutral for a 3 phases feeder would not count as a current carrying conductor.

Here is an explanation written up by Infinity

3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's

Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.

Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.

Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
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Journeyman Electrician
There is no non-linear neutral load so the tap conductors are only 3 CCC's therefore no 80% derating required. 600 kcmil has an ampacity of 420 amps so that would be the minimum size for a single set of conductors. Since the OCPD ahead of the tap is 800 amps from T250.122 #1/0 is the correct size for the EGC.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Usually the neutral in a 3 phase wye system is not  counted as a CCC even if it carries some current.

The rare special case is when the majority of the loads served by are nonlinear loads. Your system as described doesn't sound like it triggers the special case, so you will have 3 CCC. You can likely get away with a reduced neutral, though the engineers may want a full size neutral for future capability.

(Note: one issue that bugs me: if the majority of the loads served are nonlinear 3 phase loads (3 phase VFDs for example) , then I think the words of code require the neutral count as a CCC even though L-L nonlinear loads don't impact the neutral...)
 
E

EricNY

Member
Location
New York
Occupation
Electrical Contractor
Thank you all!

Follow up question: So I don't need to worry about someone adding a load with a neutral in the future?
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
EricNY said:
Thank you all!

Follow up question: So I don't need to worry about someone adding a load with a neutral in the future?
Click to expand...
No. For the neutral to considered a CCC the total load would need to be more than 50% non-linear which is not a real world scenario for this installation.
 
augie47

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
A side: As noted with no neutral load you do not need a neutral + if you decide to pull one for future use I would just pull a 1/0
 
S

sgtsparky

Member
Location
Dexter, Oregon, USA
Dennis Alwon said:
In most cases the neutral for a 3 phases feeder would not count as a current carrying conductor.

Here is an explanation written up by Infinity

3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's

Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.

Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.

Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.
Click to expand...
I like this answer but find rarely do true 3phase loads exceed 50% my current plant just over 100 drives I also have a bunch of large UPS’s 50-80 kva and as long as they are evenly loaded they play well also. My big issue is when the lighting loads are imbalanced this is when I see large neutral currents . This is where I think code should have requirements for balancing, not just well balanced under normal operating conditions.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Remember that while these rules describe counting the 'neutral' as a current carrying conductor for purpose of derating, the reality is that the neutral almost always carries some current.

IMHO it is better to think of effective number of heat generating conductors, not 'does the neutral carry current'.

Consider a single phase MWBC. Worst case heating happens when leg A and leg B are fully loaded to maximum current. But in this case the neutral carries no current, so you have '2 wires' worth of heating, hot A and hot B. Now consider the worst case unbalanced situation. Leg A is fully loaded, leg B carries no load. In this second case you again have '2 wires' worth of heating, but on hot A and neutral. All other combinations of loading result in less heating. In the single phase MWBC where code says the neutral doesn't count as a CCC, the set of three wires actually carrying current produces the same (or less) heating as 2 wires normally loaded.

-Jonathan
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
winnie said:
(Note: one issue that bugs me: if the majority of the loads served are nonlinear 3 phase loads (3 phase VFDs for example) , then I think the words of code require the neutral count as a CCC even though L-L nonlinear loads don't impact the neutral...)
Click to expand...
It also bugs me that it is black and white with no consideration for the degree of non-linearity. IMO that should just be deleted.
 
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