Secondary conductor

[hhsting]

I have 75kVA transformer primary 480V to secondary 208/120V three phase. The secondary conductors have 200% neutral so I have 5 secondary conductors. The secondary panelboard is within 10 feet of transformer and rated 225A with 225A main breaker.

The secondary conductors are THHN 90 degree and terminals are 75 degree.

So now the secondary conductor size more than three current carrying ampacity correction factors would apply but not sure how to check secondary conductor ampacity.

Right now plans show 5-400kcmil Al (3 phase and 2 neutrals) secondary conductor.

Questions:


1. How would this work do I first need to take a look at 75 degree column 400 kcmil AL ampacity is 270A and then take a look at 90 degree column and 400 kcmil Al ampacity is 305A and then apply more than 3 current carrying conductor factor which 305x0.8 = 244A. Since 244A is less than 270A 400 kcmil is ok?

2. If not then exactly what is the correct method of checking?

3. 5-400 kcmil correct size with more than 3 current carrying conductor factor and 90 degree THHN cable with 75 degree termination?

 

[d0nut]

Since 244A is greater than 225A, the secondary conductors are ok.

You are allowed to derate from the 90 degree column since your conductor insulation is 90 degree insulation. Since this derated value is less than the conductors in the 75 degree column (to match your terminations), this is the value you must use for the capacity of your conductors. If the 90 degree value was greater than the value in the 75 degree column, you would have to use the ampacity of the 75 degree column because of your terminals. When derating you must use the smaller of either the derated value using the column corresponding to the insulation or the value in the column corresponding to the ratings of your terminals.

Also, I didn't check your math or any of the values in the table. I am assuming those are correct.

 

[hhsting]

Since 244A is greater than 225A, the secondary conductors are ok.

You are allowed to derate from the 90 degree column since your conductor insulation is 90 degree insulation. Since this derated value is less than the conductors in the 75 degree column (to match your terminations), this is the value you must use for the capacity of your conductors. If the 90 degree value was greater than the value in the 75 degree column, you would have to use the ampacity of the 75 degree column because of your terminals. When derating you must use the smaller of either the derated value using the column corresponding to the insulation or the value in the column corresponding to the ratings of your terminals.

Also, I didn't check your math or any of the values in the table. I am assuming those are correct.

I only derated 90 degree since this is my cable insulation. Then do I also derate 75 degree termination as well then compare or compare without derating just look at 75 degree column? Don’t follow.

 

[d0nut]

You are allowed to derate the conductors based on their insulation. In your case, you would derate from the 90 degree column and compare that to the 75 degree column and use the smaller value. If your insulation was only rated for 75 degrees, you would have to derate from that column.

The terminations don't have anything to do with what column to use when derating, only the conductor insulation. The only time the terminals come in to play is when that value is smaller than the derated conductor value. For example, look at a similar derating exercise with #12s in THHN. If you look at the 60 degree column, the ampacity is 20A. If you look at the 90 degree column, the ampacity is 30A. If you derate this 80% like in your question, you end up with 30A*0.8=24A. Since this value is greater than the 60 degree column, you are still limited to 20A by that column. You don't have to derate that 20A from the 60 degree column, but you can't use the derating and conductor insulation to increase the ampacity of your circuit past what the terminals will allow.

 

[kwired]

In all cases your EGC is not a current carrying conductor so doesn't count toward the adjustments.

Your neutral with all three phase conductors is not normally considered a current carrying conductor either unless a majority of the load is non linear loads. If that does not apply you only have three current carrying conductors and no adjustment for number of current carrying conductors is necessary.

 

[infinity]

I have 75kVA transformer primary 480V to secondary 208/120V three phase. The secondary conductors have 200% neutral so I have 5 secondary conductors. The secondary panelboard is within 10 feet of transformer and rated 225A with 225A main breaker.

The secondary conductors are THHN 90 degree and terminals are 75 degree.

So now the secondary conductor size more than three current carrying ampacity correction factors would apply but not sure how to check secondary conductor ampacity.

Right now plans show 5-400kcmil Al (3 phase and 2 neutrals) secondary conductor.

Questions:


1. How would this work do I first need to take a look at 75 degree column 400 kcmil AL ampacity is 270A and then take a look at 90 degree column and 400 kcmil Al ampacity is 305A and then apply more than 3 current carrying conductor factor which 305x0.8 = 244A. Since 244A is less than 270A 400 kcmil is ok?

2. If not then exactly what is the correct method of checking?

3. 5-400 kcmil correct size with more than 3 current carrying conductor factor and 90 degree THHN cable with 75 degree termination?

Since there is a 200% neutral I would assume that you have a majority of the load that is non-linear therefore the neutral counts as a CCC.

1) You need 225 amps after derating which you have for the secondary conductors

2) See #1

3) 400 kcmil Al is the minimum size for 225 amps after apply the 80% derating factor. {400 kcmil: 305*80%=244 amps} {350 kcmil: 280*80%=224 amps} The 350's are 1 amp too small.

 

[kwired]

I guess I missed that it is a 200% neutral, so probably is a 4-6 current carrying conductor situation, sorry about that.

 

[hhsting]

It’s 200% neutral and more than 3 current carrying conductor. Also cable insulation is 90 degree C and termination is 75 degree C.

 

[infinity]

It’s 200% neutral and more than 3 current carrying conductor. Also cable insulation is 90 degree C and termination is 75 degree C.

That's why 400 kcmil is the minimum size required.

 

[electrofelon]

It’s 200% neutral and more than 3 current carrying conductor. Also cable insulation is 90 degree C and termination is 75 degree C.

If I may nitpick a little: 200% neutral doesnt necessarily mean it must count as a CCC. I dont recall the term "200% neutral" or "oversized neutral" appearing in the NEC. IT could be just a blanket spec without there being any significant nonlinear loads. There are all sorts of ridiculous un-necessary job specs. These days, it is highly unlikely a 200% neutral is needed. I understand that you may not know what the loads are and may find it prudent to assume the load is non linear due to the 200% neutral. Just wanted to clarify that.

 

[kwired]

If I may nitpick a little: 200% neutral doesnt necessarily mean it must count as a CCC. I dont recall the term "200% neutral" or "oversized neutral" appearing in the NEC. IT could be just a blanket spec without there being any significant nonlinear loads. There are all sorts of ridiculous un-necessary job specs. These days, it is highly unlikely a 200% neutral is needed. I understand that you may not know what the loads are and may find it prudent to assume the load is non linear due to the 200% neutral. Just wanted to clarify that.

Yes, some designers like to go overkill and could specify 200% when in reality they might even be able to get away with a neutral that is smaller than ungrounded conductors. Most the discussion here has been assuming neutral has an actual load that is 200% of the ungrounded conductors.

 

[infinity]

If I may nitpick a little: 200% neutral doesnt necessarily mean it must count as a CCC. I dont recall the term "200% neutral" or "oversized neutral" appearing in the NEC. IT could be just a blanket spec without there being any significant nonlinear loads. There are all sorts of ridiculous un-necessary job specs. These days, it is highly unlikely a 200% neutral is needed. I understand that you may not know what the loads are and may find it prudent to assume the load is non linear due to the 200% neutral. Just wanted to clarify that.

True, but the engineer specified a 200% neutral and a set of conductors (400 kcmil) that would appear to be assuming that the neutral is a CCC because when derated to 80% the 400 kcmil is the smallest conductor permitted. I do see your point if we make no assumptions.

Right now plans show 5-400kcmil Al (3 phase and 2 neutrals) secondary conductor.