Mike Holt's Code Quandries April 2008 Page 46

[CaliforniaElectricCo.]

In Mike's latest EC&M article he suggested that for a 320A load he should use 600kcmil conductors which would be adjusted to 380A, due to the number of current carrying conductors. These feeders can be installed on a 400A OCP device per 240.4(B).

1) If we have more than 3 current carrying conductors as shown per the *picture aren't we safe to assume you are running this 400A feed in parallel? If so why wouldn't you cut the wire size to two sets of 4/0 in parallel?
Or even 3/0 if run in 2 separate raceways which is often the case.

2)If this was a 480V 3phase service and a step down transformer is installed on the load side then we would not necessarily need a neutral. With no more than 3 current carrying conductors couldn't we use 500kcmil which is good for 380A acceptable per 240.4(B)?

3) What is the cut off for 240.4(B)? meaning breakers and fuses all jump from 350A to 400A. So if my conductors after adjustments come out to 360A could they still be protected by a 400A breaker? I have always, by safe practice increased the wire size or decreased the breaker size.

In existing situations can I tell a customer that their installation is up to code if their conductors are rated for 351 Amps after adjustments but installed on a 400A breaker? I do not know if this adjusted calculation is even possible using listed materials but just look at it as an extreme hypothetical installation. Am I misinterpreting 240.4(B)?


Please note that I do not advocate or suggest we use the minimum requirements. It is good to know what the minimum requirement is in the case of existing installations.

*California just adopted the 2005 NEC in January, so this is my base of reference. I recognize the article pertained to 2008 NEC.

*The picture I reference shows a cross section of a single conduit with three current carrying conductors on the left and a single conduit with 8 current carrying conductors (incl. 2 neutrals "grounded conductor") on the right.

 

[Dennis Alwon]

Read art. 240.6 for standard size breakers. I am having trouble following your thought since 600KCmil is good for 420 at 75C. What is the insulation on the wire?

It does not sound like parallel because the neutral could be a current carrying conductor. 3 phases and a neutral is 4 conductors. Table 310.15(B)(2)(a) makes us adjust 4 CCC at 80%.

 

[Smart $]

It should not be assumed automatically that terminations are rated 75?C. Conductors terminated with high-pressure lugs are commonly rated 90?C.

 

[don_resqcapt19]

It should not be assumed automatically that terminations are rated 75?C. Conductors terminated with high-pressure lugs are commonly rated 90?C.

But the equipment that the lugs are landed on is not noramally rated at 90?C so you are still limited by the rating of the equipment.

 

[CaliforniaElectricCo.]

Sorry I should have stated I am referencing Mike's latest article in EC&M on code quandries page 46. The background for the conductor ampacity adjustment is given in this text.

http://www.ecmweb-digital.com/ecmweb/200804/

Mike suggests using 600kcmil but the picture looks as though the circuit is fed in parallel. Why use parallel sets of 600kcmil if only feeding a 320A commercial office load.

 

[petersonra]

But the equipment that the lugs are landed on is not noramally rated at 90?C so you are still limited by the rating of the equipment.

what would prevent one from running wire using the 90deg chart in conduit, and splicing it to a larger size wire to make the connection to a device with terminations only rated at 75deg?

 

[CaliforniaElectricCo.]

The breaker terminals feeding these hypress lugs seldom are rated 90deg. I would love to see the panelboard/ breaker manufactures change all of their lugs and termination points to a 90deg rating. think of th ultimate conservation of our resources not to mention it would save all of us a bundel on wire costs.

 

[bob]

Sorry I should have stated I am referencing Mike's latest article in EC&M on code quandries page 46. The background for the conductor ampacity adjustment is given in this text.

http://www.ecmweb-digital.com/ecmweb/200804/

Mike suggests using 600kcmil but the picture looks as though the circuit is fed in parallel. Why use parallel sets of 600kcmil if only feeding a 320A commercial office load.

The pictures shown have nothing to do with the question. The question asks
"what conductors should be used for a 320 amps 3 phase feeder for a commercial building." The drawing on the left is an example of a 3 conductor
circuit that needs no derating. The drawing on the right is an example of 4 or more conductors that need derating. Since the circuit is 3 phase you are going to have 4 conductors. The assumption that "more that 50% of the load is non-linear" makes the neutral a current carrying conductor. Now you have 4 current carrying conductors and the circuit must be derated.
Your other question was

If we have more than 3 current carrying conductors as shown per the picture aren't we safe to assume you are running this 400A feed in parallel? If so why wouldn't you cut the wire size to two sets of 4/0 in parallel?
Or even 3/0 if run in 2 separate raceways which is often the case.

If you make the same assumption that Mike made regarding the circuit
"more that 50% of the load is non-linear" then you will have 4 conductors per circuit(2 ckts) and 3/0 or 4/0 must be derated.
3/0 = 225 amps x 0.80 = 180 amps 4/0 = 230 amps x 0.80 = 184 amps.
2 ckts of 4/0 = 184 amps x 2 = 368 amps. If Mike had not made the assumption that "more that 50% of the load is non-linear" then
say 3-500 kcm + 1- 4/0 neutral may have been adequate depending on the load calculations.

 

[CaliforniaElectricCo.]

The pictures shown have nothing to do with the question. The question asks
"what conductors should be used for a 320 amps 3 phase feeder for a commercial building." The drawing on the left is an example of a 3 conductor
circuit that needs no derating. The drawing on the right is an example of 4 or more conductors that need derating. Since the circuit is 3 phase you are going to have 4 conductors. The assumption that "more that 50% of the load is non-linear" makes the neutral a current carrying conductor. Now you have 4 current carrying conductors and the circuit must be derated.
Your other question was
If you make the same assumption that Mike made regarding the circuit
"more that 50% of the load is non-linear" then you will have 4 conductors per circuit(2 ckts) and 3/0 or 4/0 must be derated.
3/0 = 225 amps x 0.80 = 180 amps 4/0 = 230 amps x 0.80 = 184 amps.
2 ckts of 4/0 = 184 amps x 2 = 368 amps. If Mike had not made the assumption that "more that 50% of the load is non-linear" then
say 3-500 kcm + 1- 4/0 neutral may have been adequate depending on the load calculations.

"what conductors should be used for a 320 amps 3 phase feeder for a commercial building." The drawing on the left is an example of a 3 conductor
circuit that needs no derating. The drawing on the right is an example of 4 or more conductors that need derating. Since the circuit is 3 phase you are going to have 4 conductors. The assumption that "more that 50% of the load is non-linear" makes the neutral a current carrying conductor. Now you have 4 current carrying conductors and the circuit must be derated.

If we go with the picture in the left then we have a 480V 3 phase primary system no neutral. on the feeder line side of our xfmr we would only have 3 current carring conductors no neutral and so no derating. Why cant we use 500kcmil instead of the 600kcmil as suggested. We would derive our neutral on the secondary side 120/208 three phase.

The picture on the right shows 8 current carrying conductors run in parallel. derating would be required unless we run this parallel circit in two sepparate raceways as is commonly done. if this was for the same 480V 3W feed, In which case why couldn't we use 3/0 in parallel.

 

[CaliforniaElectricCo.]

The pictures shown have nothing to do with the question. The question asks
"what conductors should be used for a 320 amps 3 phase feeder for a commercial building." The drawing on the left is an example of a 3 conductor
circuit that needs no derating. The drawing on the right is an example of 4 or more conductors that need derating. Since the circuit is 3 phase you are going to have 4 conductors. The assumption that "more that 50% of the load is non-linear" makes the neutral a current carrying conductor. Now you have 4 current carrying conductors and the circuit must be derated.
Your other question was
If you make the same assumption that Mike made regarding the circuit
"more that 50% of the load is non-linear" then you will have 4 conductors per circuit(2 ckts) and 3/0 or 4/0 must be derated.
3/0 = 225 amps x 0.80 = 180 amps 4/0 = 230 amps x 0.80 = 184 amps.
2 ckts of 4/0 = 184 amps x 2 = 368 amps. If Mike had not made the assumption that "more that 50% of the load is non-linear" then
say 3-500 kcm + 1- 4/0 neutral may have been adequate depending on the load calculations.

say 3-500 kcm + 1- 4/0 neutral may have been adequate depending on the load calculations.

I wonder at what point a conductor would need to be placed on the next breaker size down. per 240.4(b)
eg. If my breaker is rated 400A and I used two sets of three 3/0 and a 3/0 neutral in parallel (sepparate raceway) then at 80% we have a 360A capacity. I err to the side of safety by increasing the wire size or dropping down to a 350A breaker since it is available, but would code allow this installation with a 400A breaker and 360A rated conductors? assuming the load is adequate.

 

[bob]

If we go with the picture in the left then we have a 480V 3 phase primary system no neutral. on the feeder line side of our xfmr we would only have 3 current carring conductors no neutral and so no derating. Why cant we use 500kcmil instead of the 600kcmil as suggested. We would derive our neutral on the secondary side 120/208 three phase.

You can as long as the calculated load does not exceed 380 amps.

The picture on the right shows 8 current carrying conductors run in parallel. derating would be required unless we run this parallel circit in two sepparate raceways as is commonly done. if this was for the same 480V 3W feed, In which case why couldn't we use 3/0 in parallel.

2 ckts of 3/0 is rated at 400 amps and you could use a 400 amps breaker.
Don't forget the EGC.

 

[bob]

I wonder at what point a conductor would need to be placed on the next breaker size down. per 240.4(b)
eg. If my breaker is rated 400A and I used two sets of three 3/0 and a 3/0 neutral in parallel (sepparate raceway) then at 80% we have a 360A capacity. I err to the side of safety by increasing the wire size or dropping down to a 350A breaker since it is available, but would code allow this installation with a 400A breaker and 360A rated conductors? assuming the load is adequate.

The code allows the installation as long as the calculated load does not exceed the conductor ampacity. This is allowable as long as the breaker ampacity does not exceed 800 amps. Once the breaker exceeds 800 amps
the conductor ampacity must match or exceed the breaker ampacity.

 

[Smart $]

But the equipment that the lugs are landed on is not noramally rated at 90?C so you are still limited by the rating of the equipment.

So why do you say not normally?

I say it is the same reason I say "not automatically". :grin: