240v systems

[Jmoney69]

Is there such a thing as 120/240v 3 phase? Trying to expand my knowledge here. 1st year Apprentice. Please educate me and not degrade. Honest answers only.

 

[ActionDave]

Yes. It's called a Delta
.

 

[jim dungar]

Yes. It's called a Delta
. View attachment 2566237

There are several types of delta connections which do not provide this neutral point.

It is less confusing to included adjective in the description. Common ones are 'high leg', 'wild leg', and 'center tapped'. I like to include 3-phase 4-wire in my descriptions. The preferred ANSI standard voltage description has the high voltage first for three phase systems, however I don't know of any POCO that follows this practice.

So I would call this a closed delta 240/120 3P4W system.

 

[don_resqcapt19]

Is there such a thing as 120/240v 3 phase? Trying to expand my knowledge here. 1st year Apprentice. Please educate me and not degrade. Honest answers only.

The important thing with that setup is that you have 120 from A and C to the neutral, but 208 from B to netural.

 

[LarryFine]

A high-leg delta is a standard 1ph 120/240v source AND a standard 3ph delta source, superimposed on each other. In fact, the high-leg, open-delta service originated as a modification to existing 1ph services for new 3ph equipment.

 

[Carultch]

Is there such a thing as 120/240v 3 phase? Trying to expand my knowledge here. 1st year Apprentice. Please educate me and not degrade. Honest answers only.

Since 240V panels/switches/breakers are automatically backwards compatible with 208V, manufacturers like Square D & Eaton don't build 208V-specific inventories of equipment. So if the factory label on a panelboard or disconnect is your only data, you'll need to more information to conclude whether you have a standard 120/208V wye system, or a high leg system.

As others have mentioned, the high leg delta system is what a 120/240V system would be, which are a lot less common on new services made today, but are something you'll find on older buildings. The advantage of 120/240V high leg systems is that you have 15% more voltage available to power major loads that are either 3-phase or connected phase-to-phase. You only have the A and C phases available for 1-pole to neutral circuits to loads like receptacles, which ideally would be a minority of the total load. If it's my choice, I'd recommend 1-phase subpanels to host all 120V loads, to avoid accidental use of the high leg. The B-phase is only allowed for 3-phase loads and phase-to-phase loads, since most 1-pole breakers are slash rated (instead of straight-rated for 240V) and require 120V to ground.

One word of caution is that the high leg standards have changed over the years. The current standard is a B-phase high leg marked orange, but you may see older buildings or service meters with C-phase high legs. So always confirm your voltages with a meter, to know what system you have, before connecting anything.

 

[jmellc]

A red flag is if you look at a panel and see every 3rd breaker skipped.

 

[Electromatic]

One word of caution is that the high leg standards have changed over the years. The current standard is a B-phase high leg marked orange, but you may see older buildings or service meters with C-phase high legs. So always confirm your voltages with a meter, to know what system you have, before connecting anything.

A caveat to the word of caution:
ANSI and many utility standards call for the high leg to be on the "C" phase. I believe the NEC used to require it be on the "B" phase in switchboards and panelboards, but the requirement now is simply that it be marked "Caution ___ Phase Has ___ Volts to Ground"

 

[don_resqcapt19]

A caveat to the word of caution:
ANSI and many utility standards call for the high leg to be on the "C" phase. I believe the NEC used to require it be on the "B" phase in switchboards and panelboards, but the requirement now is simply that it be marked "Caution ___ Phase Has ___ Volts to Ground"

The NEC still requires it to be B phase.

408.3(E) Bus Arrangement.
(1) AC Phase Arrangement.
Alternating-current phase arrangement on 3-phase buses shall be A, B, C from front to back, top to bottom, or left to right, as viewed from the front of the switchboard, switchgear, or panelboard. The B phase shall be that phase having the higher voltage to ground on 3-phase, 4-wire, delta-connected systems. Other busbar arrangements shall be permitted for additions to existing installations and shall be marked.

Exception:
Equipment within the same single section or multisection switchboard, switchgear, or panelboard as the meter on 3-phase, 4-wire, delta-connected systems shall be permitted to have the same phase configuration as the metering equipment.

 

[hillbilly1]

Since 240V panels/switches/breakers are automatically backwards compatible with 208V, manufacturers like Square D & Eaton don't build 208V-specific inventories of equipment. So if the factory label on a panelboard or disconnect is your only data, you'll need to more information to conclude whether you have a standard 120/208V wye system, or a high leg system.

As others have mentioned, the high leg delta system is what a 120/240V system would be, which are a lot less common on new services made today, but are something you'll find on older buildings. The advantage of 120/240V high leg systems is that you have 15% more voltage available to power major loads that are either 3-phase or connected phase-to-phase. You only have the A and C phases available for 1-pole to neutral circuits to loads like receptacles, which ideally would be a minority of the total load. If it's my choice, I'd recommend 1-phase subpanels to host all 120V loads, to avoid accidental use of the high leg. The B-phase is only allowed for 3-phase loads and phase-to-phase loads, since most 1-pole breakers are slash rated (instead of straight-rated for 240V) and require 120V to ground.

One word of caution is that the high leg standards have changed over the years. The current standard is a B-phase high leg marked orange, but you may see older buildings or service meters with C-phase high legs. So always confirm your voltages with a meter, to know what system you have, before connecting anything.

I think the standard has been “B” phase as far back as I can remember, but always has been “C” phase on the utility side due to proper metering, it just a lot of electricians don’t know the requirement of moving it to “B”, and it gets left on “C”.

 

[Carultch]

always has been “C” phase on the utility side due to proper metering, it just a lot of electricians don’t know the requirement of moving it to “B”, and it gets left on “C”.

The legacy metering that the utilities used to use, did something different with the high leg, than it did with the remaining two phases. Modern smart meters don't "care" which phase is the high leg, as they can take in all three voltages and all three currents, and perform vector math as needed to determine the power consumption.

 

[jaggedben]

The NEC still requires it to be B phase.

But I've seen a few that didn't follow the rule. I advise against making assumptions in the field.

 

[don_resqcapt19]

But I've seen a few that didn't follow the rule. I advise against making assumptions in the field.

sure...the same applies to assuming the wire color of any conductor actually means something.

 

[hillbilly1]

There’s an old WalMart in Georgia that everything is black red blue, 120/208 and 277/480! For some reason that contractor went bankrupt!

 

[LarryFine]

I advise against making assumptions in the field.

Same here. I teach like this:
"What is that wire?"
"It's a neutral."
"No, it's just a wire with white insulation. It depends on what the other end is attached to."

 

[Joethemechanic]

I get to work on stuff that has been touched by numerous boneheads over decades. Color, tags, placement mean absolutely nothing. What's scary is in one plant that I know of, they let these guys have access to medium voltage areas. If you saw their electrical test scores it would give you nightmares. My buddy had to stop one guy from testing medium voltage with his multimeter. He was pretty freaked out.

 

[garbo]

In my 50 years only came across the 120/240 volt three phase a few times. They get the 120 from a center tap on one of the three windings. Was told over 50 years ago that you have to be careful not to pull too much ampere off the center tap believe doing so might cause too much voltage imbalance that might cause problems with three phase motors. Think they recommend no more then a 3% voltage on three phase. Hope somebody can provide an accurate % that you should not exceed on this 120 volt Delta center tap. Some how 10% rings a bell in my old brain.

 

[jim dungar]

In my 50 years only came across the 120/240 volt three phase a few times. They get the 120 from a center tap on one of the three windings. Was told over 50 years ago that you have to be careful not to pull too much ampere off the center tap believe doing so might cause too much voltage imbalance that might cause problems with three phase motors. Think they recommend no more then a 3% voltage on three phase. Hope somebody can provide an accurate % that you should not exceed on this 120 volt Delta center tap. Some how 10% rings a bell in my old brain.

The neutral loading is only an issue in a closed delta configuration particularly when the windings are on a common core, so it is usually only a problem for customer owned and small dry type transformers. The loading limit is typically less than 5% unbalance on one L-N. Over the past 10 years or so some small transformers are being built such that they have no unbalance limit.

 

[hillbilly1]

I get to work on stuff that has been touched by numerous boneheads over decades. Color, tags, placement mean absolutely nothing. What's scary is in one plant that I know of, they let these guys have access to medium voltage areas. If you saw their electrical test scores it would give you nightmares. My buddy had to stop one guy from testing medium voltage with his multimeter. He was pretty freaked out.

He least he got stopped, I was talking to one of the safety guys at my previous company, and one of his guys opened a medium voltage cabinet to test for voltage right after he told him not too. Guy blew up his meter, and miraculously got out without injury. He is looking for a new job right now!

 

[Joethemechanic]

He least he got stopped, I was talking to one of the safety guys at my previous company, and one of his guys opened a medium voltage cabinet to test for voltage right after he told him not too. Guy blew up his meter, and miraculously got out without injury. He is looking for a new job right now!

This guy is a supervisor now, and I'm not joking