A Question for the Electrical Gurus

[Larry Ewens]

Just for claification, on a 200 amp main disconnect, is the potential 200 amps per hot conductor or 100 amps per side?
Larry

[Paul W. Abernathy]

Larry,

Ok......this is a tricky question........why because of diversified loads and what have you....BUT.......it is also important to NOT take this to the extreme...

Yes, each Line has a potential of 200A ( taking in consideration it is sized correcrtly )......the real issue is on the Neutral which has to take the unbalanced load...and in a 200A panel....their is ALOT of cancellation going on....but technically..yes.......each conductor in the SE is rated for 200A...again provided it is sized correctly ie: 2/0 CU or 4/0 AL so to speak...

Now the OCPD will limit this to protect the conductors and well if you ever wondered why the grounded ( neutral ) can be sized down is because of the diversified loads and balance of loads accross the lines......interesting enough I explain this in my seminar in more detail...just as a learning thing...

FYI.....with that said...it does not MEAN a panel that is 200A has a capacity of 400A, remember the panel itself is only rated at 200A...and the main OCPD is rated at 200A...and will usually trip at 80% of that capacity......this can get VERY technical and this is not the place to explain that...because their always SOMEONE who will not understand it and take it the wrong direction.

Here is a nice article on Service Panels and Ratings Them - http://www.inspect-ny.com/electric/ElecAmps.htm

[Larry Ewens]

Thank you Paul, I knew you would come through for me.
I have seen a diagram explaining that the two busses shared the load and were only allowing 100 amps per side. That just did not seem right to me as I was under the impression that both legs were rated at 200 amps. I do reaiize that you cannot add up the two legs and get 400 amps.

[Richard L. Bennett]

Another way of looking at it is 200 amp in and thus 200 amps out and if a little more or less is being used it goes up and down the neutral.

(lets not get into power factor)

What say Paul? Does this sound like it helps one to understand?

rlb

[Paul W. Abernathy]

Hey Richo,

Sounds good to me.....basically think of it as true 200A capacity per LINE...if 50A DRAW is on one LINE...and 30A DRAW is on another line......quess what the unbalanced load is on the Neutral....???...Any Takers....

P.S. Fellow Electricians...DO NOT speak on Cont. Amps.......this is WAY to excessive for this example....

[Larry Ewens]

I am going to guess the difference between the two loads or 20 amps
Larry

[Patrick Bolliger]

So Paul,

Are you suggesting that a "perfectly balanced" load would have ZERO amps on the neutral...

[Richard L. Bennett]

Go for it Paul

[Richard L. Bennett]

Where do you want to measure the neutral current? It makes a difference

[Paul W. Abernathy]

lol....oh dear...not THIS discussion again...we are talking calculations for GENERAL knowledge in computing...lol......I dont believe in a true balanced system.......so regardless of OTHER examples...their is always some load on a Neutral....lets NOT go their....brains are smoking already.

lol...I am talking about the example loads...lol...50A on one line and 30A on the other.....the unbalanced load of the example....lol

I should have used smaller loads...lol.. ( Thanks Joe- I got the example from an old PDF..it is a Sallcup Reference...Thanks ! )

The Code considers the neutral conductor a current-carrying conductor only when it carries the unbalanced current from other ungrounded phase conductors. When circuits are properly balanced, the neutral carries very little current. When sizing the load for a 2-wire circuit, the grounded neutral conductor carries the same amount of current as the ungrounded phase conductor. This type of installation has no unbalanced load; therefore, the neutral conductor carries full current.

Example: What is the neutral load for a single-phase, 120V, 2-wire circuit supplying a load of 14A?

Step 1: Find amperage per Sec. 220-22 and Sec. 310-15(b)(4)(a).

Ungrounded conductor = 14A

Grounded neutral conductor = 14A

Solution: Size the neutral conductor to carry a load of 14A.

When sizing the load for a 3-wire circuit, the grounded neutral conductor must carry the unbalanced load of the two ungrounded phase conductors. This type of installation has an unbalanced load - unless both ungrounded conductors pull the same amount of current on each ungrounded phase conductor.

Example: What is the unbalanced neutral load for a 3-wire circuit carrying 64A and 52A on the ungrounded phase conductors?

Step 1: Find amperage per Sec. 220-22 and Sec. 310-15(b)(4)(a).

Ungrounded phase conductor: Phase A = 64A

Ungrounded phase conductor: Phase B = 52A

Unbalanced load = 12A

Solution: The grounded neutral conductor load is 12A for the unbalanced condition.

[Larry Ewens]

I kind of knew that I would be opening a large can of worms here but for my own peace of mind I had to ask. So now I am assuming that the answer to your first question re the unbalanced load was correct and the unbalanced load is the difference between the two loads, in the example given, 20 amps.
Larry

[Larry Ewens]

By the way Paul, thanks for the reference, it has proved to be very informative.
Larry

[jtedesco1]

Originally Posted by pbolliger

So Paul,

Are you suggesting that a "perfectly balanced" load would have ZERO amps on the neutral...

If that was ever possible, here is the link for an article: "Characteristics of the Neutral Conductor, written by James Stallcup.

Courtesy: EC&M

Code Basics

[Paul W. Abernathy]

SNAP.....thanks JOE.......I totally FORGOT where that PDF post came from.....do you have a link to it....I can't find the original..

[jtedesco1]

Here you go