1

I'm familiar with MWBCs and how they're used, especially as a cost saving mechanism when running home-runs for nearby circuits (e.g. dishwasher + food disposal).

What I don't understand is how these are considered safe from the perspective of NEC, since the shared neutral between both circuits can be carrying 2x the current it's rated for under regular operating conditions.

For example, consider 14/3 romex used to power two 15A circuits with a 2-pole 15A breaker. The breaker is only looking at current being pulled by each "hot" wire, but if each of the hot wires pulls 15A (or close to that), that will result in almost 30A returning on the neutral without the breaker tripping. I believe 30A is more than 14-gauge copper wire is rated for, so this feels like a fire risk.

Given all other safety precautions in the NEC, why is this allowed? Is it actually a fire risk despite being permitted?

peter
  • 1,662
  • 12
  • 30

3 Answers3

9

A properly installed MWBC does not work that way!

In any 240V/120V 2-hot + neutral circuit, from your entire house to a subpanel on down to a 15A MWBC, the neutral carries the difference between the hots.

On a pure 240V circuit (e.g., water heater), the hots are equal and the neutral carries no current at all, which is why in that type of circuit you don't even have to have the neutral wire.

On a typical 240V/120V appliance, such as a clothes dryer or oven, the hots are almost equal and the neutral carries very little current. For example, a dryer might use 20A on both hots for the heater and 3A on one hot + neutral for the motor and controls. In this case, one hot would have 20A, the other hot 23A and the neutral 3A.

In fact, at the very large level (building size) there is an expectation that loads will be balanced and the neutral wire will carry a lot less current than the hot wires, so that in some cases on very large circuits the neutral can actually be smaller wire than the the hot wires. That doesn't apply with circuits inside your home (including subpanels, etc.) but it does apply in some larger situations.

So that brings us to MWBC. An MWBC uses two hot wires that must be on different legs/poles of your panel and a neutral wire. If it is a 20A circuit then you could have many different loads running. For example, let's say it is a kitchen countertop circuit. You could have:

  • Appliance 1 (e.g., toaster) 1500W on hot A/neutral: Hot A 12.5A, hot B 0A, neutral 12.5A.
  • Appliance 2 1200W on hot A/neutral: Hot A 0A, hot B 10A, neutral 10A.
  • Appliance 1 and 2 at the same time: Hot A 12.5A, hot B 10A, neutral 2.5A.

How does this work? The hots are out of phase with each other. They really return on the same neutral at the same time, but they effectively cancel each other out.

Related to this, the power used by your home (whether by the utility or an energy management system) is measured only on the hot wires, not the neutral wire.

Of course, if an MWBC is not installed properly then indeed you can have double current on the neutral and some serious problems.

The old problem with that was allowing MWBCs to have the hots connected far away from each other. That allowed for a good MWBC to suddenly become a bad MWBC because one of the breakers was moved for some reason. That was fixed by requiring handle ties or double-breakers for MWBCs.

Then along came tandem/double-stuff/half-size breakers. These brought the problem back again because if you replaced a pair of regular 15A breakers with 4 half-size breakers (for example) in order to add two more 15A circuits, if the original pair was an MWBC then you need to make sure they become either an inner pair or outer pair (and handle-tied) to keep the hots on opposite legs.

Problems can also occur with old fuse panels. I found one when doing a heavy-up. The problem had existed for at least 30 years, but fortunately never resulted in an overload and fire.

If you are not sure if an MWBC is OK, the easiest thing to do is to check it with a multimeter. If the voltage between the two hots is 240V (or 208V) then you are fine. If the voltage is 0V then you have a problem and need to get it fixed.

manassehkatz-Moving 2 Codidact
  • 139,495
  • 14
  • 149
  • 386
6

You fundamentally misunderstand MWBCs. The Neutral has no current if both sides are "fully loaded." It only carries the difference between the load on the Hot sides.

So, 15A & 15A = 0A in Neutral. 12A & 15A = 3A in Neutral. 15A & 0A = 15A in Neutral.

This is, of course, for properly wired MWBCs with 240V between Hots. If there's 0V between Hots, it's not a MWBC, it's an error. In that case you can get overloading the Neutral, but, that's an error, not a MWBC.

"Then where does the current go?" - back through the other "Hot" side. Which is equal and opposite. The pair of 120V loads becomes a 240V load to the extent that they match, and the difference, but only the difference, returns via Neutral.

Ecnerwal
  • 235,314
  • 11
  • 293
  • 637
3

The key is split-phase and 3-phase AC power. The two hot legs are opposite phases/polarity.

This video here helps a lot because it illustrates simpler DC. It's not quite about how much current neutral uses in a MWBC, but it even in the first 30 seconds, it beautifully illustrates that on a balanced MWBC, neutral carries nothing at all actually. Disconnecting it does not affect the circuit.

enter image description here

The white lights get 6 volts. The red and green lights get 12 volts.

This only works on North American split-phase systems, as well as 3-phase "plain wye" systems (not 240V wild-leg delta).

With 3-phase wye, the math is a little different - when all 3 phases are loaded equally, neutral current is 0. The worst-case scenario for neutral is 1 phase with zero but the other 2 phases maxed out. In that case, neutral carries the same current as the 2 active phases, owing to flux vectors, phasors and all that stuff that gives me a headache. But it works.

Harper - Reinstate Monica
  • 313,471
  • 28
  • 298
  • 772