[Amps] Alpha 87A Power Supply Question

[Jeff DePolo]

> This is an interesting discussion of a topic that recurs frequently. I
> agree with Jim K9YC that house wiring should preferably bond the Neutral
> circuit to Ground at the service entrance, and nowhere else. 

The main bonding jumper is installed at the service disconnect.  It is not "perferrably" done there,  it is required that it be done there, and only there.  In residential services, it is fairly common to have a "mains" breaker in the panel as the service disconnect means, but that is not always the case.  For example, if you have a breaker or fused disconnect at the meter, then that would be where the main bonding jumper is installed, not in the panel, even if the panel has a "mains" breaker. 

> Our point of disagreement is whether
> we should connect the "third wire" in the amplifier power cord to N (my
> opinion) or to G (K9YC's opinion). 

Code is pretty clear - you can't bond neutral to the frame with a few exceptions associated with dedicated legacy circuits for certain appliances, none of which apply in this case.  I you have 120 volt loads within "240 volt" equipment, you need either a 4-wire circuit, or provide the necessary voltage conversion within the equipment via a transformer or otherwise.  In either case, you can't use the neutral as a substitute for the equipment grounding conductor (EGC), and likewise, youc can't bond it to the frame.

> If everything is working properly,
> the amplifier will work either way, and many users might not notice the
> difference. That said, there are significant safety issues in favor of
> connecting the amp to N and not to G.

> The 120/240V circuit feeding the amplifier is a multiwire branch circuit, with both neutral and ground required; you don't get to choose between the two, you need both.
 
> Now consider a typical 240VAC circuit. In the breaker box, there will be
> an L1 conductor (black), an identical L2 conductor (typically red), an
> identical insulated (white) wire, and a fourth bare ground wire that is
> often a lighter gauge. 

By "lighter gauge", I think you are speaking of the grounding electrode conductor, i.e. the wire that connects the bus bar to the electrode(s) such as  ground rods, water pipes, ufer, etc..  The sizing of that conductor is permitted to be smaller than the service conductors (table 250.66 in NEC if you're interested).  But for the branch circuit to the amplifier, the equipment grounding conductor is going the same as the current-carrying conductors, i.e. 14 AWG for a 15A circuit, 12 AWG for a 20A circuit, and 10 AWG for a 25A or 30A circuit.

> In our
> hypothetical amplifier, however, which has 120V components (e.g.,
> filament xfmrs, blowers) and 240 V components (plate xfmrs), there can
> be significant imbalance. In that case, the neutral wire carries the
> imbalance current, but no current ever flows through the ground wire.

That's how it should be.  You need the neutral for the 120V components, and you also separately need the equipment grounding conductor, so a total of 4 wires from the panel to a 4-wire receptacle, and a 4-wire cord from the receptacle to the amplifier.  Inside the amplifier, the EGC gets tied to the chassis, the neutral is the return for the 120V loads, and the 240V loads are across the two legs.
 
> The problem is that older appliances and amplifiers have only three
> wires in their power cords, so do we connect them to L1, L2, N or to L1,
> L2 G?  

Replace the power cord with one having all four required conductors.

If the problem is the receptacle on the wall is only 3-wire, but there exists both a neutral and an EGC in the box (quite common), replace the 3-wire receptacle with a 4-wire receptacle and use the 4-wire cord above.

If the problem is that the branch circuit wiring to the receptacle has two legs and a neutral but no EGC, then you can't use it.  You cannot use the neutral as a substitute for the equipment ground unless it is being re-used for a replacement clothes dryer, range, or oven, and even then, only under a specific set of conditions (NEC 250.140 if you're interested).

> It is /always/ better to use L1, L2, N, because we know then that
> any imbalance current will go back to the breaker box through
> appropriately sized insulated wire. 

That would only apply if it were permissible to let the chassis float.  Note that grounding the amplifier chassis by some means other than through the electrical system is not an acceptable alternative.   What IS permissible, however, is to connect the ground terminal on the receptacle (and the box as well, if it is a metallic box) directly to the grounding electrode system, and then use a 4-wire cord from the receptacle to the amp, with the green wire connected to the amp chassis.  But you can't "skip" grounding the receptacle by tying the amp chassis to ground alone - the receptacle gets grounded first, and then the amplifier cord plugs into the receptacle. 

> Note that if your house wiring uses a green insulated conductor for
> ground, instead of a bare wire, then then you're probably on safe ground
> (pun intended) using it instead of a white Neutral wire. 

Using the green wire as a return path for neutral current?   Aside from the obvious code violation, it doesn't solve the fundamental problem of having a chassis that is not grounded.

> The reason, of
> course, is that the green and white wires are basically in parallel, all
> the way from the electrical outlet to the breaker box, so it doesn't
> make any difference which one you connect to. 

It certainly does.  Neutral and ground are only equipotential at the point where the two are bonded together.  Anywhere beyond that point, there will be potential on the neutral as a function of conductor resistance and the current through it.  Also consider the case where a neutral goes open - you will have 120 volts on the neutral as soon as a 120V device is plugged in.

EGC should never carry current except under fault conditions.  Never ever ever.  And, with the exception of legacy 3-wire (L1, L2, N) fixed appliance circuits noted previously, neutral cannot be used as a replacement or substitute for EGC.

						--- Jeff WN3A




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