120/240V 3PH 4WIRE (DELTA)

[kwired]

Guys,

Ungrounded equipment incase there's a short is where I was confused... not sure what would happen and wouldn't want to touch the unit.
The load is small (12A 208V on a 100A 3PH disconnect and it's only for testing not permanent) but still dont know what would happen if there is a heating element failure short to ground.

Thanks again

Even though you have been told it is not a great idea to use the high leg to supply this load, if you would do so proper method would to be run a "hot" (the high leg), a white (neutral) and an equipment grounding conductor to the load. Is in just about every way no different than supplying any 120 volt load from an ungrounded conductor to a grounded conductor. You have 2 conductors at ground potential but one is carrying load current the other is a non current carrying bonding conductor and only is supposed to carry current in abnormal conditions.

Not a great idea to do so because of source operation issues and not really a code violation to connect a 208 volt load to this high leg. Some the code limitations have been mentioned though. You basically won't find a single pole breaker rated for the voltage unless you get into 277/480 series breakers/panels, you can get 240 straight rated breakers in 2 pole units, but they are not popular sellers and because of that tend to be expensive and probably not stocked in many places either. 3 pole breaker will be 240 volt straight rated but also has higher cost compared to a single poled and it burns two additional spaces in the panel.

If you keep this high leg to neutral load fairly minimal you probably get away with it with little or no issues, but if you had a lot of such load, it can be harder to predict how the loading though the transformer(s) comes out, other 120 volt loading will impact how things ultimately balance out

 

[JSK]

 

[winnie]

Just to summarize on the topic of using the 208V high leg to neutral voltage of a high leg delta system.

1) Don't do this.

2) Don't do this!

but if you are still considering doing this

3) High leg to neutral loading makes poor use of the supply transformers. Large 'out of phase' current flows in the coils even if the load itself has perfect power factor.

4) High leg to neutral loading suffers from poorer voltage regulation because of increased effective transformer impedance. High leg to neutral loading is similar to line-line loading across the 'open jaw' of an open delta.

5) You would need to use a breaker rated for 208V (or greater) to ground. Breakers in 240V panels are often 'slash rated' for 240V L-L but only 120V L-G. General experience is that single pole breakers are invariably 240/120V rated, that two pole breakers are often 240/120V rated, and that three pole breakers are usually full 240V rated.

Not previously mentioned in this thread:
6) 'High leg' delta systems come in different flavors. Some of these are only designed for relatively small 3 phase loading superimposed on 120/240V single phase loading. When someone says that a system would likely tolerate small 208V loading, this is relative to the high leg capacity.

7) If you are considering using 'multi wire branch circuits' for line to neutral loads, you must not do this including the high leg. The neutral is not balanced relative to this leg, so current in the neutral wire won't cancel properly.

But if you have the appropriate rated breaker, and are connecting a load that is small compared to the three phase capacity of the service, and can tolerate poorer voltage regulation....then connecting a 208V high leg to neutral load is a code compliant bad practise.

-Jon

 

[tortuga]

and it's only for testing not permanent

since its only for testing, if you have a test area or test bench one other option you have is set a small 3 phase fused disconnect and just tap off the b leg for a quick test.
If your testing these all the time get a buck/boost.

 

[electrofelon]

You would need to buy a 480/277V panel if you wanted to use single pole breakers for the 208V loads.

If the 240 panel is a bolt on type, I believe most brands' 277/480 breakers are the same geometry and a single pole unit could be used. I.e. for Siemens, a BQD would be their standard 277/480 breaker and will fit in a 208/240 panel board.

 

[ptonsparky]

There is nothing more permanent than a temporary that is used only on occasion.