Bonding two MDP's

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PCN

PCN

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I have two 800 amp 480v MDP's fed from the same transformer with separate feeders going to each MDP (which are installed next to each other).
Each MDP has it's own main breaker.

My question is do I need to bond the MDP's neutral bars? We have one common grounding electrode system per 250.58.
 
Bonding two MDP's

Good question, section 450.3(B) dose permit up to six overcurrent protection devices to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device to protect the transformer. So you may have many problems, is the transformer capable of supplying the two 800 ampere breakers.
As for the grounding of the transformer section requires a system bonding jumper to connect the equipment grounding conductors of the separately derived system to the grounded conductor.
?This connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices.?
It is my opinion, the system bonding jumper be located that one single point, ether at the transformer or at first overcurrent device.
 
OOps!

OOps!

Wow, how embarassing, I thought you meant bond the two neutral bars to each other, but you are talking about bonding the neutral bars to GEC...oops!
 
wirenut1980 said:
Wow, how embarassing, I thought you meant bond the two neutral bars to each other, but you are talking about bonding the neutral bars to GEC...oops!
Click to expand...

Don't be embarassed, you read it correctly. It was suggested that I bond the the neutral bar of MDP1 to the neutral bar of MDP2.
.
 
I'd like to resurrect this thread. I came across the OP's question at a hospital. In this situation, there is a tie breaker between two mains. Should the neutral bars between the two mains be permanently bonded or should the neutrals be broken when the tie is open? My gut says the neutrals should be bonded, but then I am thinking about the secondary neutral current of one main flowing through a different transformer.

Thanks!:)
 
Being in a hospital, it might be good to check 517.14 in regard to panelboard bonding to see if it applies to your situation.
Since an SDS is involved, it sounds like your neutral may also be your grounding buss and there are situations where the grounding buss on panelboads are required to be bonded together.
 
augie47 said:
Being in a hospital, it might be good to check 517.14 in regard to panelboard bonding to see if it applies to your situation.
Since an SDS is involved, it sounds like your neutral may also be your grounding buss and there are situations where the grounding buss on panelboads are required to be bonded together.
Click to expand...

Thanks, but it looks like that section is talking about a panelboard serving patient care branch circuits.

I am talking about at the 480 V wye main service disconnect. There are two utility padmount transformers side by side. Each feed their own main disconnect. On the load side of each main disconnect there is a tie breaker. It is at this location that I am questioning the neutral bus of one main tied to the other neutral bus. The hospital wants to be able to power both services from one transformer. I was curious how the neutral buses are treated in this situation.
 
If this is a typical main tie main line up, the neutral is a common bus between the two sources. Since this is service equipment, the GEC can be common and there should be a neutral disconnect link in each main breaker section.

Rick
 
Thanks! So the neutral is broken at each main breaker when that breaker is tripped. Is breaking the neutral something that is separately done, or is there something tied into the main breaker handle that also opens the neutral at the same time?
 
PCN,

Both of the services come from the same transformer, both neutrals are landed on the XO

terminal in the transformer, so they are already tied together.
 
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