1200 amp service conductor derating

[wwhitney]

I will add that, a decade or three ago, I did a detailed analysis of the heat generated by all three phases and the neutral under random loading (balanced and not balanced) loading conditions. I proved that if the three phase loads are not equal (i.e., some neutral current), the total heat generated by the four wires will be lower than the heat generated if the three phases alone (i.e., no neutral current).

OK, I've been trying to understand exactly what you mean by "random" loading conditions, so I checked the following version of what the above might mean:

For a wye secondary, the currents I_A, I_B, I_C, and I_N will sum to zero as phasors for an appropriate sign convention. So take I_A and I_B to be of random magnitude uniformly distributed between 0 and some fixed maximum current, which can be taken to be 1 without loss of generality, and of zero phase shift (resistive loads). Then we have two cases:

(1) I_N = 0, so I_C = - I_A - I_B. Here I_C will have a non-zero phase shift unless | I_A | = | I_B |. Then the expected value of |I_A|² + |I_B|² + |I_C|² = 13/12. (Compared to the maximum possible value of 3 when I_A, I_B and I_C are all maximum).

(2) Instead I_C is also of random magnitude uniformly distributed between 0 and 1, and of zero phase shift. Then I_N = - I_A - I_B - I_C. The expected value of |I_A|² + |I_B|² + |I_C|² + |I_N|² = 23/16.

Now 23/16 > 13/12, so for this formulation of "random loading", the heating is less when there is no neutral current. Did you mean something else? I could try allowing random phase shifts for the line currents, between some chosen bounds, although then the integration for the expected values would be at least twice as complex

Cheers, Wayne

P.S. If the "no neutral current" case is supposed to also have a resistive load for I_C, i.e. there is only one degree of freedom as all the line currents are of equal magnitude, then the expectation value of the sum of square currents is just 1, so still less than case (2) above.

 

[rjn72]

After reaching out to the engineer this morning he says that the neutral does need to be counted as a current carrying conductor.

With that being said, is there any reason I cannot derate the wires through the gear or does the derate have to run from breaker to panel?

 

[electrofelon]

After reaching out to the engineer this morning he says that the neutral does need to be counted as a current carrying conductor.

With that being said, is there any reason I cannot derate the wires through the gear or does the derate have to run from breaker to panel?

The engineer is almost certainly mistaken. This is potentially a costly mistake and to help you and future clients avoid spending unneeded money, I suggest sending him a link to this thread.

As far as your question, I'm having a hard time understanding it.

 

[roger]

After reaching out to the engineer this morning he says that the neutral does need to be counted as a current carrying conductor.

With that being said, is there any reason I cannot derate the wires through the gear or does the derate have to run from breaker to panel?

I agree with electrofelon, it sounds as though the engineer could benefit from visiting the forums. Did the engineer give you his reasoning.

 

[Strathead]

There are nearly no HVAC loads as this is for a welding facility. I am unsure how welders and shears operate when it comes to harmonics and unbalanced loads. This is a design build, but I will reach out to the engineer offsite for clarification.

The welders would have virtually no harmonic load.

 

[infinity]

After reaching out to the engineer this morning he says that the neutral does need to be counted as a current carrying conductor.

With that being said, is there any reason I cannot derate the wires through the gear or does the derate have to run from breaker to panel?

Whoever is paying is getting hosed but that happens every day. In this installation the neutral is NOT a CCC.

 

[winnie]

The welders would have virtually no harmonic load.

If the facility is mostly L-N electronic/inverter welders?

 

[rjn72]

Whoever is paying is getting hosed but that happens every day. In this installation the neutral is NOT a CCC.

I agree, however one upping an engineer is kind of like one upping an inspector. You might win this time, but what about next time?

 

[infinity]

I agree, however one upping an engineer is kind of like one upping an inspector. You might win this time, but what about next time?

It all depends on who is supposed to be watching the money being spent by the guy paying the bills. Many of our jobs have a clause in the spec about value engineering where they are asking the electrical contractor to find ways to cut costs. Correcting this error by the engineer would be one way to do so.

 

[LarryFine]

I agree, however one upping an engineer is kind of like one upping an inspector. You might win this time, but what about next time?

I must disagree with this. If anything, my inspectors respect my code knowledge.

 

[roger]

I must disagree with this. If anything, my inspectors respect my code knowledge.

Same here.

 

[charlie b]

OK, I've been trying to understand exactly what you mean by "random" loading conditions, so I checked the following version of what the above might mean

Perhaps a poor word choice on my part. I simply meant that I left all current values as variables, rather than experimenting by assigning different values to see how things change.

 

[charlie b]

After reaching out to the engineer this morning he says that the neutral does need to be counted as a current carrying conductor.

With that being said, is there any reason I cannot derate the wires through the gear or does the derate have to run from breaker to panel?

First, speaking as an engineer, your engineer is wrong. Invite him to post his opinions on this forum. I would be interested in reading them.

Secondly, if a wire's ampacity has to be reduced for any reason, its updated ampacity applies for its entire length. I suppose there could be exceptions to that statement, for example when a wire is routed through several areas that have significant differences in ambient temperatures. But that does not apply to your installation.

 

[charlie b]

I agree, however one upping an engineer is kind of like one upping an inspector. You might win this time, but what about next time?

For my part, if I was wrong, I wanted someone (that included electrical contractors and inspectors) to tell me (politely) that I was wrong. Then, and only then, could we discuss the situation and arrive at the correct decision.

An engineer's primary responsibility is to protect the health and safety of the public. You can't fulfill that responsibility by placing your ego over correct design.

 

[texie]

While we don't know all the details here, when the OP said welding facility, and unless they are motor/generator types (unlikely), I think harmonic currents.
What am I missing here?

 

[electrofelon]

While we don't know all the details here, when the OP said welding facility, and unless they are motor/generator types (unlikely), I think harmonic currents.
What am I missing here?

The welders are unlikely to be powered line to neutral. Even if they were, it seems unlikely they would be the majority of the load.

 

[winnie]

We can keep pushing back on the neutral being a CCC or we can accept that this is one of those rare cases and help answer the original question: how best to land these conductors on the breaker given that they are larger than usual.

With that being said, is there any reason I cannot derate the wires through the gear or does the derate have to run from breaker to panel?

Derating applies in the conduit, not in the gear. So you could transition to smaller conductors inside the gear to make them easier to land.

In the original post, you asked about transitioning to smaller conductors in two ways. Both would be legal.

Look for the conbector style where conductors enter from either side, so your conductor path doesn't have a huge kink in it.

With PVC conduit you might be able to do an isolated phase installation. This makes conductor routing easier in the cans.

Double check the list of available lugs for the breaker. They might not have 4x600 lugs, but maybe they have multi layer lugs that take more and larger conductors. Eg they might skip 4x600, but have 5x700 further down the list.

Jon

 

[Strathead]

Carrying on Winnie, the main options are as he stated. If not enough room for polaris lugs in the gear, an aux gutter can be added to splice to smaller conductors , or you can get reducing lugs.https://www.hubbell.com/burndy/en/P...erminals/Pin-Terminals-and-Adapters/c/2146657

Take care with option 2. There are several styles, but may require physically checking them to find out whether you can fit them as the barrel is larger and often offset so they can interfere with each other and even phase to phase.

 

[electrofelon]

I would just step down to 500 in the gear or box with some inline splices, they don't take up mish space particularly of you use the uninsulated unlike mechanical ones.

There is another option, and that is land the 600's right on the breaker. Since the aluminum conductors will almost certainly be compact stranded, they will fit in a 500 hole. The compression lug series I use take one larger size up conductor of it's compact stranded. Most mechanical lugs don't make the differentiation so it's probably not technically approved.

 

[electrofelon]

These are are nice because they are the most inexpensive (at least for mechanical) and lowest profile.

https://www.gordonelectricsupply.com/p/Penn-Union-Sr-750-750Mcm-Alum-Splicer/6073701

Just make sure to have the owner make the engineer pay for them since he's the one causing this mess.