Wire Size/Load Question

[Ron Reaugh]

In the standard 60cycle 240V 3 wire feed from the pole to a house or small
business I've noticed that the neutral wire is sometimes smaller than the
two hot wires. Why is that? Has the assumption been made that 1/2 or more
of the load will be from 240V devices that don't use the neutral wire? Is
that always a reasonable assumption?

 

[Ron Reaugh]

Neutral load is part of the load calculation but it should also be noted that
the neutral in a standard triplex cable is not insulated so it will always look
smaller from the ground. The utility side is usually smaller than the customer
side too. They use the "free air" column of the ampacity chart and they are on
the NESC, not the NEC.

So it's a power dissipation issue and not just circular mils/voltage drop
per foot issue? The neutral wire should be twice the circular mils(area)
from a voltage drop standpoint if the load is all 120 gadgets, right?

 

[danny burstein]

In the standard 60cycle 240V 3 wire feed from the pole to a house or small
business I've noticed that the neutral wire is sometimes smaller than the
two hot wires. Why is that? Has the assumption been made that 1/2 or more
of the load will be from 240V devices that don't use the neutral wire? Is
that always a reasonable assumption?

The neutral only has to carry the unbalanced difference between the other
phases. So if each hot wire carries 40 amps, the neutral has nothing.

In a more realistic situation of 50 on one and 30 on the other, the
neutral only (if you're talking a single phase 120/240 set of circuits)
carries 20 amps.

(If you're talking about a neutral as part of a three phase circuit, the
math is a bit trickier, but the concept is the same).

Note that this use of smaller neutrals is frowned upon by many current
local codes, and may have been dumped in the latest National ones. (I
don't have a copy at hand to double check).

 

[Bob Weiss]

So it's a power dissipation issue and not just circular mils/voltage drop
per foot issue? The neutral wire should be twice the circular mils(area)
from a voltage drop standpoint if the load is all 120 gadgets, right?

Nope. The neutral only has to carry the DIFFERENCE between the currents
drawn off the 2 hot wires. When the branch circuits are connected to the
panel, the electrician SHOULD distribute the loads equally between the 2
sides of the panel, minimizing neutral current.

Bob Weiss N2IXK

 

[Ron Reaugh]

Nope. The neutral only has to carry the DIFFERENCE between the currents
drawn off the 2 hot wires. When the branch circuits are connected to the
panel, the electrician SHOULD distribute the loads equally between the 2
sides of the panel, minimizing neutral current.

DUH, thanks.

 

[Mark or Sue]

So you maintain that, as the load drawn from the computer power supply
DECREASES, the neutral load INCREASES.

Lets go with that. I'm open minded.

What is magic about the 50% current pulses? Why would a <50% pulse result in
200% neutral current?

Not arguing your point, just looking for an understanding of the theory
behind it.

The increased harmonic current can only happen in 3 phase line to neutral loads and not split phase
120/240 line to neutral loads. What happens is that the current is increasing as the voltage is
decreasing along the waveform. At some point, the current really drops. If they all drew current
proportional to their voltage, then the neutral currents would cancel. But the definition of
non-linear load is that the current drawn is not proportional to the voltage at any given time.
Therefore, you can get two harmonic waveforms adding current and the third waveform that normally
subtracts current happens to be 0 at that time.

From what I read, the worst case possible current overdraw was 1.717 times the rated load current of
one phase.

Search for "3 phase harmonic current" and you'll get a better explanation than the one I just gave.

 

[John Woodgate]

I read in sci.engr.electrical.compliance that [email protected]

I see a lot of equipment (transfomers, switch panels) rated for 200%
neutral current. But I can see scenarios where it could reach as high
as 300%. Take, for example, an AC to DC power supply that uses three
phases and a full wave rectification. Wouldn't these rectifiers only be
conducting when the respective phase is the highest voltage? If this
were happening and it were the only (very large) load, you'd have to
make sure everything is well overrated for it (3x on neutral for wye,
and 2x on each leg for delta). Does anyone even use stuff like that?

Yes. You have done well to work out these scenarios independently. But
the situation isn't quite as bad a pure arithmetic addition of the
current pulses in the neutral. The upper limit is about 200 %, not 300%.

I understand the common switching power supply for computers switches on
the current when the voltage is high enough to replenish the tank it
keeps. So it would see the smaller the computer load relative to the
power supply capacity, the narrower the current pulses would be. Once
they are smaller than 50%, you could be going past the 200% mark on the
neutral,

But there is a balancing effect; these short pulses are associated with
less than maximum loads on the power supplies. In other words, a 300 W
supply would do this if loaded to say 100 W, but the currents involved
would be related to 100 W, not 300 W.

if all your load is these power supplies. For a few computers
in an office, maybe that would never happen. But for building a large
data center, I suppose it could get serious.

Yes, it does. Smoke from the neutral cable.

 

[John Woodgate]

I read in sci.engr.electrical.compliance that Paul Hovnanian P.E.

Look at this problem the other way around. Assume that the 100 W per
system is fixed. The situation becomes worse as the capacities of the
typical system's power supplies are increasingly overrated.

The same effect occurs to some extent when increased demands are made
for 'hold-up' when the supply voltage decreases or is interrupted, even
though there is no increase in system capacity (as opposed to
'capacitance'!); the conduction angle of the rectifiers is still
decreased.

The question is: If one rates the circuit neutral based upon the
connected power supply ratings rather than the estimated system loads,
will this overrating compensate adequately for the proportional increase
(relative to real load) of the neutral harmonics?

Yes. AFAIK, there is no condition in which the ratio of the r.m.s.
triplen components to the **full load** fundamental current increases
with decreasing conduction angle, at least down to 36 degrees, below
which the inrush current becomes a big problem and it is, in any case,
difficult to get a low enough total resistance in the rectifier circuit
economically.

 

[John Woodgate]

I read in sci.engr.electrical.compliance that [email protected]

If the current approached linear as the DC load approached power supply
capacity, that might work. But I doubt that happens. Maybe there is a
virtual point where it could. What someone would need to do is measure
the characteristics of a wide sampling of power supplies to see if such
a thing could be done. I suspect there is a lot of variation in quality
and design that could affect this.

The matter has been thoroughly investigated many times, largely in
connection with the provisions of IEC/EN 61000-3-2 on limitation of
harmonic current emissions into the public low-voltage supply. Practical
ranges of rectifier conduction angle are approximately 36 degrees (below
which inrush current becomes a problem) and 72 degrees (above which
efficiency is much too poor).

Fortunately, only the radical cases (like a data center with hundreds or
more computers) would need to worry about it. There is a "safe harbor"
that can be used if these measurements cannot be made, and that is to
assume the 300% worst case, and derate the stuff with 200% neutral to
66% of rated capacity, and the rest to 33% (rather expensive).

Instead of derating, new neutral cable can be installed. Still not
cheap, but at least the outcome of the expense is that the system has
been uprated rather than downrated.

 

[[email protected]]

Fortunately, only the radical cases (like a data center with hundreds or
more computers) would need to worry about it.

Actually, you don't need that kind of load (hundreds
of computers) before it becomes a concern. They
manufacture cable (called super neutral?) for branch
circuit wiring in offices with 12 gauge phase and 10
gauge neutral conductors. So the excess neutral
current is a concern even on 20 amp circuits.
Certainly the problem would be magnified at the
service, which is what you may have in mind.

By the way, your approach to showing the problem
with a diagram is great. You can see where the
pulses on each phase would create neutral currents
that would not cancel each other out.

 

[danny burstein]

[ snip ]

| By the way, your approach to showing the problem
| with a diagram is great. You can see where the
| pulses on each phase would create neutral currents
| that would not cancel each other out.

Thanks. Picture do often say so much more.

I'm finding this fascinating as I have just enough background in this
stuff to be dangerous without being helpful...

so please forgive what may be a dumb question to you both:

In the case of the 300 watt supply provviding 100 watts
of power, the diagram, as I read it, demonstrated some
hefty power peaks. BUT they only seemed to be for
one third the time, with the other 2/3rds showing nothing.

Did I interpret it correctly? If so, shouldn't the wiring
capacity be rated for the averaged out load rather than
the short term (fractions of seconds) peaks?

Or am I completely off here... (wouldn't be the first time)

Thanks

 

[Robert Casey]

Fortunately, only the radical cases (like a data center with hundreds or
more computers) would need to worry about it.

Also offices full of electronic florescent ballasts also do this.

As for the computers, most are configurable to operate on 208-250VAC and
100-125V. So all the computer power supplies could be set for 208-250V
and connected to 208v sources. Thus the neutral never sees any of these
loads. But this would require new power cords on all the computers to be
fitted with 6-15 plugs. And don't forget to switch all those switches
on the
supplies (some are hidden behind plastic bezels on the back). Probably
cheaper to
just install a super neutral.