220.61(A) Exception Clarification

[paullmullen]

I am confused about the Exception. Here's what I read:

IF
3-wire/2-phase

or

5-wire/2phase

THEN multiply the max load by 140%.


But, I never see this 140% applied in any of the example calculations that I see. So this must imply something beyond your usual 120/240 system with L1, L2, N.

What am I missing?

Paul

 

[jaggedben]

Yes 120/240 is single phase, not two phase.

This is two phase. You'll probably never see it.

 

[paullmullen]

Thanks. I saw other explanations of this and I saw descriptions of phases being 90° apart and I thought they were typos until I saw them in multiple places. That's when I got the cue that I was thinking of generic apples and they were talking granny smiths. Appreciate the response.

 

[charlie b]

Two phase systems do have 90 degrees between phases. I have never encountered such a system.

 

[tortuga]

Hey its Charlie B have not seen you in a while, welcome back.

Two phase is still system in use dating back to the first power pants in Niagara Falls.
Its defiantly still used in center city Philadelphia, and rumored to also be in some parts of New England and a old part of Seattle.
NYC converted two phase to DC.

Two phase has some very interesting applications to the present day, such as supplying very large single phase loads in a balanced manner off a 3 phase substation or transformer bank called a Scott-T.
Examples are electric rail, smelting furnaces, and dredging/ electric traction equipment.
This is for loads so large that you need to balance x number of mw on a 3 phase system due to the capacity limits of your system.

Siemens is the only gear manufacturer I know of that supports 5-wire two phase.

 

[John A]

Many, many years back, I went to Philadelphia with one of the 'old timer' contractors out of Newark, NJ as a favor.

Building was some type of church facility, and there was an issue with a feeder to a 200 amp sub panel.

2 phase, 5 wire; I scratched my head. He told me what to do, I laid out 5 new 4/0 CU TW; ID 1 as neutral; 2 as blue; 2 as red.

Pulled it in; he did the terminations; I drank coffee & cleaned up.

Thats all I remember, and know about 2 phase, 5 wire. Never saw it anywhere else.

 

[LarryFine]

Just think of it as two otherwise independent 1ph sources that happen to share the same neutral conductor.

 

[tortuga]

Just think of it as two otherwise independent 1ph sources that happen to share the same neutral conductor.

Exactly, a interesting application is say you have a site with a higher incoming service voltage like 480/277 or even a primary customer.
Say for example the customer requires 100kVA of 120/240 single phase loads where 208 is unacceptable.
Additionally you need to balance this load evenly on your incoming service due to the loading of the 480 padmount.

Normally you would use 3 X 37.5 kVA single phase transformers and balance them on the 480V lines.
Thats 3 separate xformers + bonding conductors to install etc.
With a two phase bank one could set up a Scott-T with something like 2 X 50kVA's.
and have 2 larger panelboards instead of three single phase panels.
Only one 'transformer' to bond / locate etc.