ECN Forum Forums General Topics of the Trade General Discussion Area Neutral Current Carrrying?

Hmm. Which ampacity table is used for 'single-conductor cables, not installed in raceways' ?

-Jon

Jon

I do not know, the more I read 520 the more my head was spinning.

I imagine 400.5 for cords.

Bob


[This message has been edited by iwire (edited 09-17-2005).]

If I could ask a quick question here...

Why is it important to not connect both ungrounded conductors to the same phase on a multi-branch circuit at the panel?

I kind of understand that the neutral will 'heat-up' but I'd like an explanation of why it would heat-up.

Thanks

--Ron

When you are on opposing phases the neutral current is only the unbalance of both sides (balanced current returns on the phases). If you are on the same phase neutral current is the sum of the phase currents.

Ron, look at the drawing below, notice how current adds on the grounded conductor, as Greg points out, when only one leg is used.



Now notice how only the unbalanced current is carried on the neutral when opposing legs are used.



Roger

I could be wrong, and will probably get corrected if I am, but the two phases are 180 degrees apart. When one wave is passing along one particular point in the wire, it's hitting its maximum positive amplitude as the other phase is hitting its maximum negative amplitude. These two waves counter each other and cancel. IIRC it's superposition of waves. You can see this on a rope if two people try to send waves down the rope at the same time. In the middle of the rope, where the waves pass each other out of phase, the rope doesn't move.

let the corrections begin...

Sounds pretty good to me.

But what happens when the two phases are 120 degrees apart?


Dnk....

As in a using only 2 phases from a 3 phase supply?

Then they do not cancel each other out entirely.

I'd actually take this one step further: In a multi-wire branch circuit using two phases from a three phase wye supply, the current flowing into the shared neutral essentially doesn't cancel out.

Consider: a multi-wire branch circuit using all three phases could balance perfectly, if each is carrying the same current. This means that the current flow from any _single_ phase is somehow being balanced by the other two phases taken together. If you remove this third phase, then the current that flows into the neutral must be the same as the current that would flow in this third phase.

If you have a multi-wire branch circuit using two phases from a three phase wye supply, then you can calculate the current flow in each conductor, so here are a few examples:
HotA HotB Neutral
0 0 0
10 10 10
20 0 20
0 20 20
20 20 20
20 10 17.3 (this is the condition that minimized neutral current flow)

All of the above presume the same power factor on all loads; if this is not true, then you have different phase angles for the current flow, and different results. If you have a line to line load in the above example, then clearly you won't have any neutral current, but now the phase angles of the current flowing in HotA and HotB will be 180 degrees apart, even though the phase to neutral voltages are 120 degrees apart.

-Jon