MEP 805B wired to split 400amp panels

[Guyfang]

I can’t comment on connectors.

Yes, the bonding strap is the one that you describe. Bond means to connect.

You both need to ground your trailers. There’s a bolt on it somewhere for this purpose. Also, there should be a wire that runs from the bolt to the GND terminal. I believe it’s behind the terminal board. The notion here is that the electrical grounding connection from the generator cabinet/skid to trailer frame can’t be guaranteed. Probably because of the paint. Who knows? It’s a length of wire, right? Cheap money traded for safety. Inspect it to make sure it’s not broken.

You two are doing a good job. Very happy to see those diagrams. Thinking on paper is way cheaper than an awe crap. Oops! Forum rules????

Yes, Forum rules!!

🫢

Open to see comment.

 

[Evvy Fesler]

Cool conversation. I was wondering though. Since your supplying power to (2) different 200-amp service panels with this MEP-804A, you have the "potential" to overload one or more of the L1, L2 or L3 legs of the generator. Sure, the generator is "supposed" to protect itself by shutting down for over-current, but do we really want to test this? Assuming here that you'll be relying on the "manual" selection of the circuits on each of the (2) 200-amp service panels that you'll be feeding by turning off the circuits not being used. I'm not an electrician, but I believe that they look very closely at the "potential" for something to go wrong when they inspect.

As I understand it, the issue isn’t so much heavy load as it is imbalance. Any load creates heat. The case on the main generator has to dissipate it. Loading one pair of legs and not the other creates hot sections on the main generator. This can lead to failure. Running a three phase generator as single phase causes the imbalance problem. I had similar concerns because of my use case. I incorporated a three phase transformer to deal with it. This way, I’m transferring the unbalanced load to the transformer, which is a chunkier piece of steel. It can handle heat much better and arguably it’s cheaper to replace, a very unlikely occurrence.

 

[Guyfang]

I'll have to verify the trailer ground. I figured this was accomplished through the trailer/skid bolts. But I guess the thick CARC paint will inhibit that since both were painted and then assembled. I'll have to check the TM diagrams on where the wire should be located.

The thoughts from CECOM Guru's is that the gen set mounting bolts are often loose. There is often dirt, sand or fuel in between the gen set and trailer. Poor grounding . The Military kills about 1-2 people a year with grounding problems. So, there is now a bonding wire between the ground stud on the gen set and the ground stud on the trailer. When it first hit the streets, that was NOT so.

 

[Guyfang]

I'll have to verify the trailer ground. I figured this was accomplished through the trailer/skid bolts. But I guess the thick CARC paint will inhibit that since both were painted and then assembled. I'll have to check the TM diagrams on where the wire should be located.

This will NOT be found on the TM schematics. This was a directive that came out in, and I can not remember the exact date, but 2009-2010?

 

[Evvy Fesler]

On breaking out the 3ph into two 1ph I was planning on using a 100 amp 3ph panel with a main 100 amp disconnect and 1ph breaker to feed each panel. Placed side by side in the panel that should pull L1 and L2 for one breaker and L3 and L1 for the second.

Does anyone know if any leg on these generators is “weaker”. I seem to recall someone saying when you increase power the voltage you watch the hottest leg but I could be remembering that wrong.

Not weaker. They are all rated the same, both on nameplate and in the TM.

 

[Tommytone]

Not weaker. They are all rated the same, both on nameplate and in the TM.

To verify the 805B is rated at what 104amps and should be able to exceed that In a similar manner to those overloading an 802 or 803 during say the start up of a condensing unit (hp or ac).

Is the 104amps correct on these?

What is the view on balancing the legs what I mean is what is an acceptable deviation. Further is there a time component to that. Routine power outages here are shorter in nature so under 8 hrs most of the time.

 

[MDdeuce]

This will NOT be found on the TM schematics. This was a directive that came out in, and I can not remember the exact date, but 2009-2010?

I quickly looked at my gen set, I have no wires going between the set and trailer. Strange it's not there, I was under the impression that I had a later model unit. It's a 2009, and based on paperwork that was stuffed in a cubby it went through a depot reset in 2017. It does have the 30kW switch box on the front, so not sure if that changes anything (but its just bolted to the trailer like the set). It does have a male service post randomly installed on the front decking that I keep tripping over, perhaps that was used for grounding. I seems that the grounding wire is something I need to add.

 

[Evvy Fesler]

I quickly looked at my gen set, I have no wires going between the set and trailer. Strange it's not there, I was under the impression that I had a later model unit. It's a 2009, and based on paperwork that was stuffed in a cubby it went through a depot reset in 2017. It does have the 30kW switch box on the front, so not sure if that changes anything (but its just bolted to the trailer like the set). It does have a male service post randomly installed on the front decking that I keep tripping over, perhaps that was used for grounding. I seems that the grounding wire is something I need to add.

Can't hurt.

 

[MDdeuce]

We are on the same page. Mine is trailer mounted and keeping it mobile was my thought. I have ordered 5100p9w for the cord and 5100r9w for the generator and house plus one for the barn to do monthly’s and run 3ph equipment at times. Is that in line with your approach?

I'm using mine only as house backup power. I don't have any 3P equipment (but maybe this is a good excuse to get some). I went with the 804 to have some extra KWs so I really wouldn't need to worry about load shedding during an outage. Plus since we have 400A service with 2x 200A panels it was fairly simple to adapt the 3P set for residential use. My gen set will almost always be seeing unbalanced loads, but everything I've read seems to indicate that for these sets in a limited time backup power role it should not be an issue. I've read some just 'throw away' the 3rd leg and run all the loads on just 2 legs with no apparent ill effects. If I was using it for 24/7 prime power role I'd be concerned about the imbalances.

 

[MDdeuce]

As I understand it, the issue isn’t so much heavy load as it is imbalance. Any load creates heat. The case on the main generator has to dissipate it. Loading one pair of legs and not the other creates hot sections on the main generator. This can lead to failure. Running a three phase generator as single phase causes the imbalance problem. I had similar concerns because of my use case. I incorporated a three phase transformer to deal with it. This way, I’m transferring the unbalanced load to the transformer, which is a chunkier piece of steel. It can handle heat much better and arguably it’s cheaper to replace, a very unlikely occurrence.

How do you have the 3P transformer wired in? Are you running the gen set at 416V and using the transformer to get down to 208V? I guess I don't fully understand show this will completely eliminate the imbalance in the gen set. The currents in the stator head would be reduced compared to running at 208V and the heating from the load imbalances would be less?

 

[Scoobyshep]

To verify the 805B is rated at what 104amps and should be able to exceed that In a similar manner to those overloading an 802 or 803 during say the start up of a condensing unit (hp or ac).

Is the 104amps correct on these?

What is the view on balancing the legs what I mean is what is an acceptable deviation. Further is there a time component to that. Routine power outages here are shorter in nature so under 8 hrs most of the time.

balanced is more efficient, you can cause head damage by running 2 legs at their maximum and nothing on the 3rd leg. The short and skinny, Balance as close as you can but dont go nuts over a few amps.

 

[Evvy Fesler]

How do you have the 3P transformer wired in? Are you running the gen set at 416V and using the transformer to get down to 208V? I guess I don't fully understand show this will completely eliminate the imbalance in the gen set. The currents in the stator head would be reduced compared to running at 208V and the heating from the load imbalances would be less?

To understand this we have to back up a bit. There's a lot of talk here in the forum about the legs being unbalanced and this is a valid concern. What's happening when you try to make three phase (3P) into single phase (1P) is a little different. In the idea 3P world, the current that runs through each leg is out of phase and cancels the current in the other legs. The three work together for a net sum zero. Similarly with 1P what comes in via the one leg, leaves via the other leg. When we want 120v relative to neutral we introduce a circulating current into the neutral because it can't reach the other leg to return. With the transformer, the primary's L1, L2 and L3 is connected to the generator's L1, L2 and L3, but the neutral (L0) is not connected. Now we have a 3P system without circulating current, at least as far as the main generator is concerned. The secondary, connected to the load panel(s) is tapped with a neutral for the 120v circuits and it will return a circulating current. Do we care? Yes and no. While it's possible to burn up a transformer, the idea is to over spec it by a very large degree so that the laminations are capable of dissipating the heat caused by circulating current. This protects the main generator and the residential application has 120v (and 208v ) circuits. Everyone is happy!

 

[MDdeuce]

To understand this we have to back up a bit. There's a lot of talk here in the forum about the legs being unbalanced and this is a valid concern. What's happening when you try to make three phase (3P) into single phase (1P) is a little different. In the idea 3P world, the current that runs through each leg is out of phase and cancels the current in the other legs. The three work together for a net sum zero. Similarly with 1P what comes in via the one leg, leaves via the other leg. When we want 120v relative to neutral we introduce a circulating current into the neutral because it can't reach the other leg to return. With the transformer, the primary's L1, L2 and L3 is connected to the generator's L1, L2 and L3, but the neutral (L0) is not connected. Now we have a 3P system without circulating current, at least as far as the main generator is concerned. The secondary, connected to the load panel(s) is tapped with a neutral for the 120v circuits and it will return a circulating current. Do we care? Yes and no. While it's possible to burn up a transformer, the idea is to over spec it by a very large degree so that the laminations are capable of dissipating the heat caused by circulating current. This protects the main generator and the residential application has 120v (and 208v ) circuits. Everyone is happy!

Um, very interesting. I hadn't thought of using a transformer to eliminate the neutral currents on the gen set side and have it be honest 3P. I like this idea. A transformer with a 416 delta primary and a 208/120 wye secondary, or an isolation one with a 208 delta primary and a 208/120 wye secondary would do this job nicely. What level of 'over spec-ing' is generally required to allow sufficient heat dissipation in the transformer? The MEP-804B is "15kW", but load testing it will actually produce close to 20kW. Would a 30kVA transformer be expected have enough extra for this use case?

Though, quickly internet-ing, it seems a 30kVA 3P transformer costs about the same as I paid for the gen set and trailer, so its becoming fairly expensive protection for the gen set.

 

[Scoobyshep]

split between 2 panels, the risk of imbalance damage is minimal, the shared leg has a risk of overload. If fuel efficiency isn't a huge concern you can always use a heater to help balance. For example, Panel 1 is wired to AB panel 2 is wired to BC and the heater dummy load is wired to CA

 

[87cr250r]

There are 3 phase 208Y/120 panels for this application. No need to split.

 

[Evvy Fesler]

Um, very interesting. I hadn't thought of using a transformer to eliminate the neutral currents on the gen set side and have it be honest 3P. I like this idea. A transformer with a 416 delta primary and a 208/120 wye secondary, or an isolation one with a 208 delta primary and a 208/120 wye secondary would do this job nicely. What level of 'over spec-ing' is generally required to allow sufficient heat dissipation in the transformer? The MEP-804B is "15kW", but load testing it will actually produce close to 20kW. Would a 30kVA transformer be expected have enough extra for this use case?

Though, quickly internet-ing, it seems a 30kVA 3P transformer costs about the same as I paid for the gen set and trailer, so its becoming fairly expensive protection for the gen set.

Peter, a regular in the forum, gave me this guideline:

MEP 15KW - Transformer minimum 30 KVA (or 45KVA more availability) MEP 30KW - Transformer minimum 75+ KVA
MEP 60KW - Transformer minimum 125+ KVA
Input 3 Phase 208 Delta
Output 3Phase 208Y/120

There's lots of used electrical equipment out there. Ask your local electrical supplies store who's got a building salvage or used electrical business. I got mine for $175.

 

[MDdeuce]

Peter, a regular in the forum, gave me this guideline:

MEP 15KW - Transformer minimum 30 KVA (or 45KVA more availability) MEP 30KW - Transformer minimum 75+ KVA
MEP 60KW - Transformer minimum 125+ KVA
Input 3 Phase 208 Delta
Output 3Phase 208Y/120

There's lots of used electrical equipment out there. Ask your local electrical supplies store who's got a building salvage or used electrical business. I got mine for $175.

I'll have to call around. New 30kVA isolation transformers seem to be in the $2,700-$3,600 range, which is adding significant cost.

I also like that this approach eliminates the need to run 5 conductor feeder to the gen set (only need 4 now) snd eliminates the need to bond/unbond the N-GND connection on the set when being used as a non-separately derived source for house backup and a separately derived source for load testing or prime power, which is a safety concern.

 

[MDdeuce]

There are 3 phase 208Y/120 panels for this application. No need to split.

Yes, but my house is already built with standard 1P 240V panels. As the MEP will be backup power a 3P panel is not practical for me.

 

[Evvy Fesler]

I'll have to call around. New 30kVA isolation transformers seem to be in the $2,700-$3,600 range, which is adding significant cost.

I also like that this approach eliminates the need to run 5 conductor feeder to the gen set (only need 4 now) snd eliminates the need to bond/unbond the N-GND connection on the set when being used as a non-separately derived source for house backup and a separately derived source for load testing or prime power, which is a safety concern.

There should only be 3-conductors to the genset as the neutral (L0) doesn't get connected. Is your 4th conductor the safety ground (GND)?

 

[Ray70]

TommyTone, have you already sourced ( or located a source for ) the pin and sleeve connectors? Curious what you found as last time I was looking for them for an unrelated project I almost fell off my chair when the price popped up on the screen! Think the cheapest I found was over $600 per half, with some being well over $1000 per half. Needless to say that project went down an alternate path, veering away from the pin and sleeve connectors.