Sub Panel Help

I am using a 70amp main lug, which I am connecting to a 200amp main panel. The main panel has a ground bus bar the main lug did not come with one. My question is should the main lug have a ground bus bar? I am assuming it should especially since for each circuit I install, I will have a dangling ground. Are these grounding bus bars generic? Thanks for your help.

 

Yes, you do need an Equipment Grounding Buss. No, they are not generic. The vender that sold you the Main Lug Only (MLO) panel should be able to sell you a Ground Buss that fits your panel's cabinet. The other thing you will need to do is to remove and discard the bonding screw or "noodle." These are used to connect the Grounded Conductor Buss Bar to the cabinet of the panel. Having any such connection on the load side of the Service Disconnecting Means not only violates the NEC but it introduces the hazard of stray neutral currents traveling over various metallic parts and surfaces in the home. The voltage drop over these unintended pathways can raise the touch potential of exposed metallic surfaces to dangerous levels. In the event that anything goes wrong with the feeder neutral those elevated touch potential problems are almost inevitable.
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Tom

 

I decided to go with a 100amp main lug rather than the 70amp. The 100amp had 6 slots as opposed to 2 in the 70. I also purchased and installed the grounding bus bar kit, as suggested. The main lug is now mounted and I ran have run 10/3awg between the 2 boxes. I used 10/3 because I won’t need anything higher than 30amps. I plan on using a 30amp breaker as a feeder from the main. This is the part I need some guidance on. When I looked at connecting the 30amp breaker to the main panel using the 10/3awg there is a red wire left over. The setup looks like it would work fine with a double pole 30, but I’m not sure of the ramifications of using a double pole.

1. Can I make the single pole 30 work as a feeder in the main panel?

Also, I am not totally clear at the sub panel. I have wired in the 10/3awg according to the instructions that came with the box.

2. If I’m using a 30amps as a feeder can all 6 slots draw up to 30amps?
3. Would it be against code to put a 30 in the sub? (I only need one)

Thanks for any help.

 

The proper way is to connect the 10/3 to a 30-amp double-pole breaker. This will of course give you a place to connect that red wire. It will also provide you with 7200 watts of power available from your subpanel.

It is possible, although highly unusual, to use a 30-amp single-pole breaker to connect your feeder. In this case, you will have one wire in the 10/3 that you don't need. And you will also only have 3600 watts of power available from your subpanel. And none of these loads can be 240 volts loads -- they must all be 120 volt loads. All in all, this is a pretty bad idea.

The size of the feeder limits the total amperage used by all of the circuits fed from the subpanel. If you use a 30-amp single-pole breaker, and had six circuits in the subpanel, the average amps available to each of the six circuits would only be 5 amps before the 30-amp breaker tripped. But if you use a 30-amp double-pole breaker, you would have 30 amps to share among three circuits, and another 30 amps to share among the other three. In this case, you would want to think about how you balanced the loads for optimum usage.

I'm not sure what you want with a 30-amp breaker in the subpanel. Is this a 30-amp single-pole breaker? Keep in mind that you cannot connect any regular receptacles or lighting to a 30-amp circuit. You can only use a 30 amp receptacle and plug in appliances with a 30-amp plug (I can't think of any, but there must be some).

Or is this a 30-amp double-pole breaker you want to put in the subpanel, perhaps for a electric dryer or a water heater. If this is the case, then the dryer or water heater will consume virtually all of the power from the feeder (assuming you use the 30-amp double pole breaker to protect the feeder), leaving none for any receptacles and lighting.

I'm not sure you understand how a subpanel works. But I hope this helps. I'm guessing that you are about to decide that the 10/3 cable you ran between the two panels is not big enough to handle the load you plan.

 

John thanks for the reply. Here is what I was planning to do in the sub panel. One single pole 30 I could not find a single 25 and was told by an authorized service repairperson for my air compressor (Campbell Hausfeld 5hp portable upright), that I needed at least a 25amp circuit. I also planned to put in a 20amp circuit for gfci receptacles and a 15amp for some lights. This is for an attached garage by the way.

Will the 30amps double pole providing 7200 watts be enough power in the above scenario?

 

You probably do not want to hear what I and others will tell you but thirty amps is too small a feeder for this panel unless you will never need any more power in this garage. If you have already run the 10/3 w GND then you can carry the loads you outlined just fine but there is not much ampacity left for any future additions to the garage panel. You will have to balance the loads in this panel very carefully. If the panel holds the breakers in a single row then you would put your receptacle breaker and your lighting breaker on either side of the compressor breaker. After the panel is wired and energized you should check the voltage between the thirty ampere, single pole, compressor breaker and each of the others. It should be 240 volts. The voltage measured between the lighting and receptacle breakers should be zero. The voltage between any single pole breaker and the grounded conductor buss should be 120 volts. If the voltages do not come out this way you will have nuisance tripping of the thirty ampere double pole feeder breaker all the way back at the service equipment panel in the house.

If you anticipate needing to add loads in the garage in the future then you may want to install a heavier feeder now. A #4 American Wire Gauge (AWG) / 4 Conductor Aluminum type SE cable would allow you twice as much power in the garage and leave you ample room for future increase in load. You would have to make the terminations very carefully because the Aluminum connections will cause trouble if you do not. If you do not want to tackle the problems of making good aluminum conductor terminations then stay with the copper conductor cables. If you can get it #6 Copper type SE would do the same thing. If you cannot find #6AWG CU type SE then you can buy #6AWG CU type NMC at a home depot. Using type NMC cable reduces the ampacity of the conductors to 55 amperes but only fifty ampere, double pole, breakers are readily available.

A good practice with lighting and appliance panel board feeders is to protect the feeder at its theoretical maximum safe ampacity and then to provide a main breaker at the panel that feeder supplies sized for the ampacity of the feeder without excessive voltage drop. What that does for you is it provides for overload protection of the feeder at the location of the panel it feeds instead of all the way back at the service equipment panel in the house.
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Tom

 

Thanks Tom, I thought I would be cutting it close with available power, but figured if additional power was needed later I could upgrade the feeder and wire at that time. Since all I need at this time is to install the double pole 30amps to see if the setup meets my needs, I will try that first. If that does not work out, I will try a double pole 50amps, and rerun the feeder wire. I have only worked with cooper before, so I think I would stick with it and use the #6awg. Thanks to everyone who replied. This has been a good learning experience.

 

YOu originally said that you planned to use a single pole breaker to serve that air compressor. Did you catch that Tom was telling you that you most likely have a 220 volt rated air compressor if you have a 5 hp power motor. Read you motor name plate to confirm the voltage it needs. If you install a single pole breaker the 220 volt motor will not run but will stand a good chance of burning out and become useless to you. Be sure of you motor voltage rating on your motor name plate 240 volt requires a double pole breaker not a single pole braeker.

Wg

 

Actually I totally missed that very important point and I am sure glad you caught it. I took their word for the compressor being 120 volt and with a five horse power unit that is not likely.
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Tom

 

Wg,,, that 5 hp probably isnt 5 hp,,, thats just what it says on the box it came in. That was one of my early peaves and you quoted some code that let the manufacturers rate that way. How did they ever end up with that? I realize how thay do it,, but how did it ever become legit to advertise that way.

 

The air compressor in question does say it is a 5hp on the box and in the manual. I did look on the nameplate and it indicates it is a 120v, 15amps. The "authorized service repairman was the one who told me I needed at least a 25amps circuit. After all the information I have received here I am wondering if I even need anything higher than a 15 or 20amps breaker in the sub.

 

Ya,,, thats 1 1/2 HP comp. It will probably run ok on a 20 brkr and if it doesnt you can change it later. Aprox 12 amps per HP on 110. The law allows the add guys to rate that stuff on how much current it draws on start up,,, never buy on the info on the box,, always look at the plate and check amps.

 

Well that's interesting. Why do I keep forgetting that everything but the Laboratory listing plate lies!

That leaves you in relatively good shape with your thirty ampere feeder. Look at the compressor cord cap. If it is one round pin and two blade pins at a right angle to each other it is meant to plug into a 20 ampere receptacle only. Run a dedicated twenty ampere circuit to a single 20 ampere, NEMA 5-20R, receptacle.
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Tom