Parallel Conductors

[Impac]

I have a 400amp 3ph service that I am doing and it calls for 700al now I have paralleled it own to 4/0al @132ft. The inspector at first said that could not run all of the phases in one 4inch conduit. I quickly won that argument. But then he came back to me with my derating factor. Now after I did my calculations i came up with .06 and the inspector came up with .083. Can someone tell me if this is correct because I have been thrown for a loop here and I am either going to have to pull all this wire out and redo the entire service or leave it put. Someone out there please respond thank you in advance.

 

[charlie b]

Sorry, but I do not understand your question. For starters, a 700 AL would only be good for 375 amps. So that design appears to have been inadequate from the beginning. Secondly, a 4/0 AL is good for 180 amps, so two in parallel would only be good for 360 amps. That falls short of the (apparently) required 400. Have you done a load calculation, or did the engineer do one? If the load is above the original 375 or your 360, then you can’t use that set of conductors anyway.

But my real confusion is with your derating factors. I don’t know what you mean by .06 or .083. Where do those numbers come from, and how would they be used?

My take on the situation is that you plan to put 8 conductors in the same conduit. 6 of those would be current-carrying ungrounded conductors, and the other 2 would be (possibly) non-current-carrying (by NEC rules) neutral conductors. That would give you a derating factor of 80%. A 4/0 that has a 90C insulation system could start with an ampacity of 205. When you derate that to 80% and put two of them in parallel, your net ampacity is 328 amps. You would still be well short of the required 400.

Welcome to the forum.

 

[Dennis Alwon]

250 kcm copper or 400 kcm alum will work, of course in parallel
70% is what you must use to derate but you can use the 90C rating

 

[augie47]

250 kcm copper or 400 kcm alum will work, of course in parallel
70% is what you must use to derate but you can use the 90C rating

why 70% ?

 

[petersonra]

why 70% ?

perhaps because the "7" is next to the "8" on the keyboard and he fat fingered it???

 

[augie47]

perhaps because the "7" is next to the "8" on the keyboard and he fat fingered it???

 

[charlie b]

why 70% ?

He is treating the neutral wires as being current-carrying. I don't think we know enough to say that.

 

[petersonra]

Sorry, but I do not understand your question. For starters, a 700 AL would only be good for 375 amps. So that design appears to have been inadequate from the beginning. Secondly, a 4/0 AL is good for 180 amps, so two in parallel would only be good for 360 amps. That falls short of the (apparently) required 400. Have you done a load calculation, or did the engineer do one? If the load is above the original 375 or your 360, then you can’t use that set of conductors anyway.

But my real confusion is with your derating factors. I don’t know what you mean by .06 or .083. Where do those numbers come from, and how would they be used?

My take on the situation is that you plan to put 8 conductors in the same conduit. 6 of those would be current-carrying ungrounded conductors, and the other 2 would be (possibly) non-current-carrying (by NEC rules) neutral conductors. That would give you a derating factor of 80%. A 4/0 that has a 90C insulation system could start with an ampacity of 205. When you derate that to 80% and put two of them in parallel, your net ampacity is 328 amps. You would still be well short of the required 400.

Welcome to the forum.

as it is a service wouldn't he have to only have as much ampacity as his calculated load?

 

[charlie b]

as it is a service wouldn't he have to only have as much ampacity as his calculated load?

That's why I asked whether he, or the engineer, did a load calculation.

 

[infinity]

as it is a service wouldn't he have to only have as much ampacity as his calculated load?

What if he has a 400 amp main?

To the OP:
83% (or .083) is for a dwelling service so I'm guessing that's where the number came from. As stated you would have an 80% derating factor with 6 CCC's in a single raceway. As Dennis stated parallel 250 kcmil Al gets you to 368 amps, next standard size up is 400 amps.

 

[kwired]

What if he has a 400 amp main?

To the OP:
83% (or .083) is for a dwelling service so I'm guessing that's where the number came from. As stated you would have an 80% derating factor with 6 CCC's in a single raceway. As Dennis stated parallel 250 kcmil Al gets you to 368 amps, next standard size up is 400 amps.

But when you are allowed to use that method that 83% isn't the same thing as an adjustment factor on conductor ampacity.

You adjust conductors same way you always do but after applying adjustments in the usual way you are permitted to have 83% of the conductor ampacity you would normally be required to have. Basically if conductor ampacity were determined to be 83 amps - you can treat it as a 100 amp conductor when this provision is allowed.

 

[infinity]

But when you are allowed to use that method that 83% isn't the same thing as an adjustment factor on conductor ampacity.

You adjust conductors same way you always do but after applying adjustments in the usual way you are permitted to have 83% of the conductor ampacity you would normally be required to have. Basically if conductor ampacity were determined to be 83 amps - you can treat it as a 100 amp conductor when this provision is allowed.

That's not what I meant, he said the inspector told him to use .083 :? I was guessing (probably incorrectly) that he was thinking of the 83% rule which does not apply to his installation.

 

[Impac]

Sorry, but I do not understand your question. For starters, a 700 AL would only be good for 375 amps. So that design appears to have been inadequate from the beginning. Secondly, a 4/0 AL is good for 180 amps, so two in parallel would only be good for 360 amps. That falls short of the (apparently) required 400. Have you done a load calculation, or did the engineer do one? If the load is above the original 375 or your 360, then you can’t use that set of conductors anyway.

But my real confusion is with your derating factors. I don’t know what you mean by .06 or .083. Where do those numbers come from, and how would they be used?

My take on the situation is that you plan to put 8 conductors in the same conduit. 6 of those would be current-carrying ungrounded conductors, and the other 2 would be (possibly) non-current-carrying (by NEC rules) neutral conductors. That would give you a derating factor of 80%. A 4/0 that has a 90C insulation system could start with an ampacity of 205. When you derate that to 80% and put two of them in parallel, your net ampacity is 328 amps. You would still be well short of the required 400.

Welcome to the forum.

Good Morning fellas,
First let me say this the blueprints were vague from the start. I engineered this service to use 900 XHHW rated @ 75deg but the utility came back to me, since they are the AHJ in this area, and stated that the two building historically use on 71 and 77kw respectively. They informed me not to run that size conductor for a 400 3phase service at a distance of 132ft from the transformer. I argued this point for 3 weeks and finally did what I was told to do then came the Inspector who agreed to what I had originally designed for these buildings! So now I am stuck with over 2000ft of #4/0 XHHW. But there is light somewhere if you guys agree I can run another (2) sets of #4/0Al to give me the amperage that I need at the mains and switch over to 4 position lugs for each phase and neutrals. Do you guys agree??

 

[tw1156]

Good Morning fellas,
First let me say this the blueprints were vague from the start. I engineered this service to use 900 XHHW rated @ 75deg but the utility came back to me, since they are the AHJ in this area, and stated that the two building historically use on 71 and 77kw respectively. They informed me not to run that size conductor for a 400 3phase service at a distance of 132ft from the transformer. I argued this point for 3 weeks and finally did what I was told to do then came the Inspector who agreed to what I had originally designed for these buildings! So now I am stuck with over 2000ft of #4/0 XHHW. But there is light somewhere if you guys agree I can run another (2) sets of #4/0Al to give me the amperage that I need at the mains and switch over to 4 position lugs for each phase and neutrals. Do you guys agree??

In Texas the utility owns everything from the meter back to the transformer, but the installation is typically done by the developer/contractor to the POCO standards; if the POCO accepted and approved the design on the line side of their equipment, is it typical for the AHJ there to comment and approve/disallow certain installations?

 

[charlie b]

. . . is it typical for the AHJ there to comment and approve/disallow certain installations?

Let me take this one step further. If the utility is the AHJ, then who is the "inspector," and what organization does that person represent?

 

[JFletcher]

There is a potentially very easy fix to your problem... use Polaris connectors about 1 foot away from the lugs on both ends of the cables, then you can use the 90-degree table all the way. just upsize those one foot stubs to 250 MCM aluminum. 4/0 xhhw al is rated 260a @ 90*c, x .8 for 6 ccc, gets you 184a/set or 368 total, as long as calculated load is 368 or under you should be good to go... provided the neutrals are not current-carrying conductors, and you have physical room to do this on both ends

 

[JFletcher]

. Disregard All the Above, was looking at the ampacity for copper wire not aluminum

 

[infinity]

What type of raceway? You could probably fit three sets (12-#4/0's) in a 4". That would give you 430.5 amps.

205 amps * 3 = 615 * 70% = 430.5 amps

 

[tw1156]

Let me take this one step further. If the utility is the AHJ, then who is the "inspector," and what organization does that person represent?

I'm thinking this is similar to municipalities that own and maintain their own electrical distribution; this sounds similar to how the City of Austin operates and a few other cities (San Marcos, etc). I have worked on only a handful of projects there, but my experience with that is similar to if working with different departments within the City, where everything on the line side of the service entrance disconnect was required to meet Austin Energy's requirements, but everything else was inspected and required to meet the typical building permit requirements. All of this to say that typically the Utility can be an AHJ up to a certain point of the project, but I know we're in a bubble down here in Texas.

 

[ElectricMatt]

I'm thinking this is similar to municipalities that own and maintain their own electrical distribution; this sounds similar to how the City of Austin operates and a few other cities (San Marcos, etc). I have worked on only a handful of projects there, but my experience with that is similar to if working with different departments within the City, where everything on the line side of the service entrance disconnect was required to meet Austin Energy's requirements, but everything else was inspected and required to meet the typical building permit requirements. All of this to say that typically the Utility can be an AHJ up to a certain point of the project, but I know we're in a bubble down here in Texas.

Not a bubble in Texas a bubble in the greater Austin area. With your purple phase conductors and screwed up color code different from the rest of the world!!


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