Undersized neutral

[mcelweec@gmail.com]

I have a service going to an commercial office on the 9 th floor of a building consisting of 3# 2's and a #4 neutral,is it permissible to fuse the service at 80 amps or should grounded and ungrounded conductors be the same size.

 

[suemarkp]

Depends on the load calculation. If you have some equipment that is 208V only (assuming 120/208V Service or Feeder), it will have 0 for neutral current. If everything is unbalanced, then you'll probably have about the same values for grounded and ungrounded current. The neutral does not need to be derated for continuous duty because it doesn't land on an overcurrent device.

You can probably use the 75C column (for sure on the neutral, most likely on the breaker) if the wire is 75C or higher. However, you may not want to push the neutral if you have high harmonic loads.

Are these aluminum or copper conductors? You could probably fuse above 80A if the load calculates to less depending on if you're in the 60C or 75C column and AL -vs CU.

 

[mcelweec@gmail.com]

The calculated load is around 40 amps,the only 208v load is a single phase 30 amp a/c,the rest is lighting and outlets.I thought if it's under 200 amps I can't reduce the neutral(220.61),this is why I'm leaning towards changing the fuses if allowable.

 

[suemarkp]

As usual, the NEC is not easy to read and understand. I see:

220.61(a) saying to calculate the neutral as the maximum unbalanced load (so remove all your 208V loads with no neutral and the highest phase current would most likely be your calculated neutral current). This probably keeps the neutral load still around 40A since you only have one 208V single phase load.

220.61(b) says certain appliances get to be counted at 70% as can the load over 200A. Neither of those probably apply in your case.

220.61(c) says back in part (a) you can't exclude circuits that are 208/120 single phase (e.g. 2 hots and a neutral) or any of the feeder that is supplying non-linear loads.

This last one can be difficult, as you may have no idea what type of loads are connected (office computers would be non-linear, but coffee pots not). Regardless, if your calculated load comes to 40A, your #4 neutral can easily carry that even if everything is considered harmonic. The issue will affect someone adding more load in the future. If they add things that are harmonic, they'll have to watch their calculation. If they add purely 3 phase things, then it won't be an issue. Remember that the neutral doesn't need the 125% continuous factor applied, so that there buys you some headroom (you'll need to do the original calculation again with the 125% factor removed to see what the neutral load is).

Bottom line, if these are copper conductors, you have a lot of margin (non-continuous factor exempt on the neutral, 75C ampacity allows 85A on #4 CU). If they are aluminum, you'll want to ensure the neutral load is under 65A.

 

[mcelweec@gmail.com]

Thanks for the responses Mark,these are copper conductors ,now I have to convince an inspector of this.My supply house goofed and spooled a #4 onto the drum instead of a #2 which my guys should have at least caught when they were terminating.unfortunately the inspector caught it on final,which in NYC is anywhere up to 6 months after owner occupies which makes the oversight even more embarassing.

 

[kwired]

The difficult part can be determining what the actual imbalanced load is on the neutral.

Otherwise if the load is not there the neutral needs to be at a minimum, no smaller than the required equipment grounding conductor, it still needs to safely be able to handle the current required to open the overcurrent device if there is a line to neutral fault. see 215.2(A)(2).

 

[kwired]

As a worst case the max unbalanced current would be how much current can flow on the neutral if only the line with the most neutral connected would carry if it were the only one energized. Not very often will you run into that though where all connected and operating load is on one phase only, any line to neutral on a second phase starts to become offset and equal loading on all three phases completely offsets to no neutral load. (Harmonic currents opens a whole different game BTW)