On 26 Feb 2007 01:28:12 GMT, firstname.lastname@example.org (Curt Welch) wrote:
Right. The normal convention is to mark the High Leg with Orange
tape or paint everywhere it shows up, and put it on the B phase -
middle busbar. And put a big warning on the face of the panel, to try
to rein in the clueless before they do something very stupid.
I've hit it a few times where a "Handyman" fixes the lights at a
condo or commercial building, or adds a new circuit - and then the
Management Company calls and says 'the bulbs don't last long...'
Yup - new breaker on the High Leg for the new circuit he added.
"Well, there was a vacant spot, so I used it..."
Yup. Home Depot is even stocking a few variants of 3-phase 240V
panels and breakers, if you want to put your hands on one to see. But
most commercial gear is rated as Panelboards which gives you bolt-on
breakers and a lot heavier construction. If it's mostly motor and
equipment loads, you can even go larger than 42 breakers in the panel,
and 400A main busses in a bolt-on panel are not too uncommon.
Sometimes you even see four-pole breakers - but the last one is a
shunt trip coil for electric fryers and griddles at a restaurant, so
the fire suppression system can kill the equipment power.
If it's a 3Ph Open Delta panel and you don't use up the B phase
stabs as A-B or B-C for all the straight 240V 1Ph loads, you start
running out of 120V (Ph-to-N) A and C phase spaces real fast.
You only have to worry about that on ranges and clothes dryers,
because they DO have 120V loads on them. Or if it goes to a 4-wire
120/240V rated receptacle for portable items like welders and
industrial floor buffers, because you never know what portable
equipment they're liable to connect. If the receptacle has a neutral
prong in it they're going to assume that it's 120V on each leg.
On new construction the clue for 120V connected loads would be there
is a neutral wire and it is connected, but on existing structures
(more than about 5 years old) that is NOT a reliable method - you used
to run the oven light and the clock/timer from one phase to ground.
Delta service is more for industrial parks and small commercial
spaces where they will need some 3-phase, but they are all smaller
occupancies where 200A of 120/240V 3Ph is plenty, separate 120/240 1Ph
and 277/480 3Ph services would be overkill. (And another monthly
meter charge to pay.)
Getting supplied at 277/480V and running a local transformer in each
unit to run the 120/240V loads is a PITA for small stores and shops,
and it's more expensive on several fronts.
First, 480V switchgear is a LOT more expensive than 240V, and then
you have to buy a step-down transformer and another breaker panel for
each unit, and when you want to work on the 480V you have to turn off
their lights and computers too.
And you have to turn it off to work on it - 240V isn't so bad (a few
sparks and the main pops) but working on 480V hot is insanity...
Open Delta is very common on Condos or Apartments - they put up a
big transformer (50 or 100 KVA) for all the single-phase loads in the
building, and they hang a small second transformer (10 or 25 KVA) to
get the third leg for one 3-phase service for the Elevator(s).
They can have one 200A 3-Phase service and panel for house loads, or
on larger buildings they use a 400A or 600A 3Ph Standing Section, send
a 3Ph breaker straight to the elevator machine, and send A and C
phases past that to regular single-phase panels for house circuits.
More common on Condos where there's one service transformer for the
whole building, and not much 240V load where people would complain
about slow electric ovens and wimpy cooktops... (That's fine, most
people prefer Natural Gas for that, the bills are a WHOLE lot lower.)
They put in one three-phase House Meter for the elevator, three-phase
disconnects, and then the different Modular Metering stacks for the
units are rotated through the phases to balance the loads. First
stack of meters is connected A-B, second B-C, third C-A.
Welcome to R.C.Metalworking - where if you aren't really careful you
might learn something. ;-P
--<< Bruce >>--
So how does that work? The fire system just shorts that 4th leg to ground
or puts a load on it to cause the system to trip off the power?
Thanks! Lots of good new stuff to know! I never thought about why an
apartment building would need 3 phase power but the fact they have
elevators and that elevators would use it is something that seems obvious.
I guess the air handling systems in larger buildings (like a hotel or
office building) are probably all 3 phase as well.
On 26 Feb 2007 21:00:47 GMT, email@example.com (Curt Welch) wrote:
No, the shunt trip is electrically isolated from the rest of the
breaker. They are usually 24V or 120V coils, even when the breaker is
The fire system has one or two Form C (SPDT or DPDT) microswitches
on the initiator arm for triggering alarms and external equipment.
When the fire system discharges the switch closes and sends 120V to
the shunt-trip coil - the breaker has two wires sticking out like a
GFCI to connect to the trip circuit. The solenoid coil of the shunt
trips the breaker the same way an overload would.
If the equipment in question has a motor starter, it's far easier to
wire the Normally Closed circuit of that fire-system trigger switch to
drop out the starter coil. Then you don't have to go buy a special
shunt trip breaker.
(Just did a paint spray booth that way - the fire system kills the
starter for the exhaust fan, and the exhaust fan starter dropping out
opens the auxiliary contacts and the power to the air supply cutoff
solenoid, which cuts air to the spray guns. The spray air is
interlocked so they can't spray without turning the booth exhaust fan
Oh! There's another oddball breaker with wires sticking out, but
you won't ever see it except at gasoline service stations, to power
the turbine pumps in the storage tanks - Switched Neutral Breakers.
Large motors are inherently more efficient running on 3 Phase,
because it's kick-kick-kick instead of kick-coast-kick-coast.
And with the constant starting and stopping of elevator pump motors,
they live far longer on 3-phase - KISS, no starting capacitors or
It would make sense for Condos to send 3-Ph 208V up to the units, if
only for the AC Condensing Unit compressor motors - since every little
bit of added energy efficiency helps reduce overall demand. They even
used to make special sub-panels just for this purpose that had one or
two 3-Ph breaker slots at the top of the buss, and the rest were
But some power utilities <coughL.A.DWPcough> have this absolute
neurotic phobia against supplying any sort of 3-Ph power for end-use
by a (Gasp!) residential customer.
--<< Bruce >>--
Ah, so the fire control system has to simply send the correct voltage to
cause it to trip. Simple enough.
It's even better than that. I just learned about this in the past year. I
never would have guessed it.
With single phase, the torque goes up and down with the AC cycle as you
say. But with 3 phase, the back and forth forces sum and the result is a
constant torque. Not just smoother, but actually constant (at least in
theory - in practice I think the motors are not perfectly linear so there
is some small variation in the torque).
Here's a simple animated graphic from wikipedia that shows how the three
vector forces sum to produce a constant rotating field - the end of the
three arrows produces a constant rotating vector of fixed length (aka fixed
This also works with only 2 phases 90 degrees apart. It produces the same
constant torque either way. But to deliver 2 phase power would require 4
wires (2 for each phase). Or, if it was cut down to 3 wires by making one
side of each phase a common, it produces a higher amount of current in the
common wire. It will have to carry 1.414 times more current than the two
phase wires. This means for power delivery, you would have to use either 4
wires of the same size, or 3 wires with a larger wire for the common. But
with 3 phase, where each wire acts as a common between two phases, only 3
wires are needed and all 3 wires carry the exact same current load which
gives the advantage of constant torque motors with only 3 equal sized
Home shop types are known to buy surface grinders that are designed around
three phase motors, then change them to single phase because they lack
three phase service. Big mistake in many cases. The uneven power pulses
translate into surface finish problems that are virtually impossible to
Three phase motors are not only smoother, they are also much cheaper to
build because of their simplicity.
Dunno, never having experienced either, but if I was to venture a guess,
yeah, it's possible, although at a diminished rate due to small
discrepancies in phase angle. Properly balanced, it's possible that each of
them could perform equally to properly generated 3 phase. Bruce would be
the best person to address this issue, at least from the electrical
standpoint. I think that each grinder would have its own characteristics.
No reason to not-----they already do have. Some are just created better
Thanks Wes and all.
I don't have a current project - just something I wondered about and
thought would be good to know for reference.
I'd been aware that in theory this was possible, but thought there might
be a "gotcha" in practice.
As I thought, the answer was right here. Long live RCM.
Jordan as you seem to be posting from Australia I wonder if your mains
is the same as that in the UK. My understanding of UK mains is the
single phase is one 3 phase leg and neutral giving 240V between phase
and neutral but the voltage between any 2 3 phase legs is 415V due to
the 120 degree phase difference. As was mentioned by another check the
voltage before connecting. AFAIK the UK setup is the same across Europe
but the US seems to have many 3 phase supply arrangements from what
others have said at various times on RCM.
Depends on if you are using a three phase generator or not. My 50kw three
phase generator can do single phase by using two of the three legs but on
heavy draw it creates an imbalance that shuts the generator off.
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