Two circuits feeding one load

[roger]

Ken, you are still confused, let me give you a simpler illustration and you can take it from there. I would suggest you go to the test bench if the following illustration doesn't help you understand the theory.

Both sides of the circuit are identical as I said before in post #28, you must have missed it. :roll:

Both sides have a grounded and an ungrounded conductor.

Hope this helps.

Anyways, if it doesn't help, don't worry about it, parallel OCPD's are only allowed in factory assemblies so you probably won't be affected by it in field installations. :wink:

Roger

 

[roger]

I know roger,i'm laughing at the whole sinerio.

Why? It can be a useful wiring method and is used in certain factory assembled applications.

Roger

 

[480sparky]

Ken, you are still confused, let me give you a simpler illustration and you can take it from there. I would suggest you go to the test bench if the following illustration doesn't help you understand the theory.

Both sides of the circuit are identical as I said before in post #28, you must have missed it. :roll:

Both sides have a grounded and an ungrounded conductor.

Hope this helps.

Anyways, if it doesn't help, don't worry about it, parallel OCPD's are only allowed in factory assemblies so you probably won't be affected by it in field installations. :wink:

Roger

I guess I've never seen a load like that, hence my confusion.

But I think I can wrap my brain around it now.

Communication helps!

 

[Lxnxjxhx]

I thought schematics would clear this up

I thought schematics would clear this up

. . .but,no.

If two 120v 30A sources in phase are supplying a 120v 40A load, the current won't share equally unless the voltages and circuit impedances are exactly equal.

So, assume a 120.0v source in series with 0.01 milliohms, this in parallel with a 120.0v source, same phase, is series with 0.02 milliohms, these two series circuits in parallel with a 3 ohm, 40 A, 120 v load.

Trying three times to correctly solve simultaneously for the two loop currents, I finally come up with the load voltage being 119.9v and the load current being 39.96 A. One source supplies 29.63 A, the other supplies 10.33 A. The numbers all seem to work and I selected the direction of the loop currents so that they both go into the load as you would expect.

Here's the weird part; to get the right answer I had to carry at least four decimal places in my calcs because of the strong effect of low circuit impedances and high voltages on the final currents; using these values the sources can deliver 6000 A and 12000 A, but we only need 40 A.

Which tells me you shouldn't do this, because wire heating will nullify these calculations and then you don't know what you have and neither does anyone else.

I can't believe the problems that land in and on this forum! No wonder you guys take home $500/hr!

 

[480sparky]

Ken, you are still confused, let me give you a simpler illustration and you can take it from there. I would suggest you go to the test bench if the following illustration doesn't help you understand the theory.

Both sides of the circuit are identical as I said before in post #28, you must have missed it. :roll:

Both sides have a grounded and an ungrounded conductor.

Hope this helps.

Anyways, if it doesn't help, don't worry about it, parallel OCPD's are only allowed in factory assemblies so you probably won't be affected by it in field installations. :wink:

Roger

I think I've got it now. The original drawing had me confused. What you're drawing with the bulb, Roger, is in reality this:

So to draw it out as a ladder diagram, I get this:

No wonder I couldn't sleep last night! My brain hurt just thinking about this.

 

[roger]

Where's the service conductors and the utlity transformer? :wink:

Roger

 

[K8MHZ]

But in order for current to flow, it would need the grounded conductor, so in effect you would have this:

Has any one taken a volt meter to their panel and checked the voltage between two adjacent breakers as the drawing illustrates? (say between 3 and 4). You will get 240 volts as the legs zig zag as they travel down the panel. 1 and 4 are on the same leg, 2 and 3 are on the opposite leg, etc.

The current capability of two 20 amp breakers on adjacent positions, no matter if they are next to each other vertically or horizontally, would be 20 amps at 240 volts, *no neutral required*. If a MWBC is created by bringing a neutral into the circuit, the system would now be 2 20 amp 120 volt circuits as the voltage would be measured from each ungrounded conductor to the neutral, not to each other. At no time can a 20 amp breaker carry 40 amps without tripping. It is either Thevenin or Norton's law that the current entering a node must equal the current leaving the node. If only 20 amps at either voltage enters, only 20 can return.

If 40 amps is needed and 20 amp breakers are used, two conductors from the same leg (such as positions 2 and 3) need to be connected in parallel to a load also connected to a neutral. The current would be divided between the two ungrounded conductors and added together at the load. The neutral would then return 40 amps at 120 volts. As I understand, such a parallel connection may violate the NEC as (just as one observation) the breakers needed could not be placed in such a manner that they could be tied together and trip simultaneously.

Also, any connection in a 120/240 residential installation is on the same phase. A 'leg' is just a tap from a different point on the transformer, it is not a different phase.

 

[480sparky]

.....1 and 4 are on the same leg, 2 and 3 are on the opposite leg, etc....

No panel I have ever worked with does that.

Single phase panels, 1&2 are on the same leg, 3&4 are on the opposite.

Three-phase, 1&2 are A, 3&4 are B, 5&6 are C.

 

[K8MHZ]

No panel I have ever worked with does that.

Single phase panels, 1&2 are on the same leg, 3&4 are on the opposite.

Three-phase, 1&2 are A, 3&4 are B, 5&6 are C.

I stand corrected!

Just for curiosity I went and checked the QO panel here and you are correct about that panel. However, I have seen them laid out as I describe but I can't remember what brand or type they were. I just don't remember where it was. My mistake for assuming all panels were built like the one I checked with my volt meter.

I work in some places that have some very old and strange panels.

Thanks for the correction. Now I am going to have to check each one out of sheer curiosity!

 

[480sparky]

I stand corrected!

Just for curiosity I went and checked the QO panel here and you are correct about that panel. However, I have seen them laid out as I describe but I can't remember what brand or type they were. I just don't remember where it was. My mistake for assuming all panels were built like the one I checked with my volt meter.

I work in some places that have some very old and strange panels.

Thanks for the correction. Now I am going to have to check each one out of sheer curiosity!

There are some old panels (Bulldog / Pushmatic comes to mind) that have two vertical, parallel busbers. So a 240v breaker will have one wire on each side of the panel.

 

[K8MHZ]

Now that you mention it...

Now that you mention it...

There are some old panels (Bulldog / Pushmatic comes to mind) that have two vertical, parallel busbers. So a 240v breaker will have one wire on each side of the panel.

I think you are on to something!

Last year for a few months I was doing electrical maintenance at an old shop that was full of Pushmatic panels. It was one of those places where you checked the voltage on *everything* you worked on. That may be where I came up with my conclusion.

 

[LarryFine]

Does the NEC permit feeding a single load from two half-sized circuits?
Ex.: A 40 A range fed by two #10 AWG circuits each protected by a 30 A breaker.
Note that this is not the same as parallel circuits where the cables are joined together at each end and protected by one breaker.

Has it dawned on anyone that the OP is asking about using two separate 30a 2p breakers, via two separate 10-2 or 10-3 cables, and supplying a single 40a 240v load? He believes, incorrectly, that this does not qualify as a paralleled installation because of the use of two separate breakers.

A couple of years ago, I did a service change in a house that had two separate 10-3 cables supplying a cooktop and a wall oven, the cables paralleled, combined at both ends, and supplied via a single 50a pull-out. I ended up separating the load ends and using two separate 30a breakers.

Stuff like this is one of many reasons why I open panels before making my materials list and pricing the job.

 

[iwire]

Has it dawned on anyone that the OP is asking about using two separate 30a 2p breakers, via two separate 10-2 or 10-3 cables, and supplying a single 40a 240v load?

No, only you are that sharp. :wink: :grin:

 

[James McCusker]

Thanks for the replies. My question pertains to an original installation of two separate 30 A circuits to an oven and a stovetop each fed by a 30 A 2p breaker from a 120/240 V panel. If I replace the two appliances with one range rated 40 A, 240 V, does the NEC permit the two circuits to feed a single load terminal block even if they are separately protected?

 

[roger]

Yes, the NEC prohibits your scenario, see 240.8 (the breakers are in parallel) and 310.4 (your conductors are not big enough to be wired in parallel).

Roger

 

[LarryFine]

Aha, I was right.

The only option without rewiring is to use a single 30a circuit. Odds are the 30a breaker will hold.