Feeders from an Emergency Generator

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mshields

mshields

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Boston, MA
I've designed a system consisting of a 125kW, 208Y/120V diesel generator in skin tight enclosure, belly tank and with a single OCPD (sized at 125% of FLA). I go underground and up into the main electrical room (RI Code allows for my Emergency System to be in the same room because it's less than 150V to ground). Directly above where I come from underground into this room I have a wireway off of which are:
1. My emergency OCPD
2. My Standby power OCPD

I utilize MI cable from the Emergency OCPD to the Emergency panel which is in the same room.

An inspector reviewing my drawings, had this to say:

"The OCD at the Generator (450 A) is in front of the two OCD's for the Emergency and Standby circuits are not in conformance with NEC article 700.10, ( if this OCD is eliminated the conductors to the building would be subject to NEC article 445.13 )"

He seems to be saying that if I eliminate the first OCPD at the generator, I'd be ok so long as my conductors are 115% in keeping with 445.13. Bearing in mind that this OCPD (at the generator) coordinates to .01 with the Emergency OCPD at the wireway as well as with Emergency panelboard breakers, I think my design is NEC 2014 code compliant. What do you think?
 
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mshields said:
I've designed a system consisting of a 125kW, 208Y/120V diesel generator in skin tight enclosure, belly tank and with a single OCPD (sized at 125% of FLA). I go underground and up into the main electrical room (RI Code allows for my Emergency System to be in the same room because it's less than 150V to ground). Directly above where I come from underground into this room I have a wireway off of which are:
1. My emergency OCPD
2. My Standby power OCPD

I utilize MI cable from the Emergency OCPD to the Emergency panel which is in the same room.

An inspector reviewing my drawings, had this to say:

"The OCD at the Generator (450 A) is in front of the two OCD's for the Emergency and Standby circuits are not in conformance with NEC article 700.10, ( if this OCD is eliminated the conductors to the building would be subject to NEC article 445.13 )"

He seems to be saying that if I eliminate the first OCPD at the generator, I'd be ok so long as my conductors are 115% in keeping with 445.13. Bearing in mind that this OCPD (at the generator) coordinates to .01 with the Emergency OCPD at the wireway as well as with Emergency panelboard breakers, I think my design is NEC 2014 code compliant. What do you think?
Click to expand...

At first glance, I think the reviewer is correct. The way I see this normally done is having 2 breakers on the genset. Then you have 2 feeders, 1 for optional loads and the other for 700 loads. All the major genset vendors are able to supply multiple breakers as part of the genset instead of a single breaker.
 
code handbook 700.10

code handbook 700.10

If you look in the code handbook under 700.10 you'll see an illustration of nearly exactly what I'm doing. The only difference is that they don't show an OCPD at the generator. Now I know, there is this oddity in my opinion, that I may forgo putting a breaker on the generator. This is very counterintuitive for me.

So I include the breaker on the generator, I make sure it is coordinated to .01 seconds and I think I'm good.

I know that I could put the breakers on the generator. I'm choosing this way for reasons I won't bore you with. I appreciate your input and look forward to additional input.

thanks,

Mike
 
mshields said:
... Bearing in mind that this OCPD (at the generator) coordinates to .01 with the Emergency OCPD at the wireway as well as with Emergency panelboard breakers, I think my design is NEC 2014 code compliant. What do you think?
Click to expand...
Many read the new definition of selective coordination as found in the 2014 NEC as requiring the coordination to time zero.
 
Question about 0 seconds vs 0.01

Question about 0 seconds vs 0.01

Ron,

From a practical standpoint, what is the difference between coordination to 0 seconds vs coordination to 0.01

My design is coordinated using tested combinations from Eaton's published table. I'm utilizing fused devices at the branch level, And these coordinate with the breaker I'm putting at the wireway point which in turn coordinates as per this table with the single breaker I have at generator.

I suppose there could be a concern with that breaker not being coordinated to this level with the Optional Standby,
Do you agree that this is the issue?

I guess the solution is to remove the OCPD at the generator, a very counterintuitive thing for me to do but correct me if I'm wrong, code compliant in accordance with the 445 code section cited in my original posing. Do you agree?

Thanks,
 
The inspector is correct. As long as you have that OCPD at the generator, you are not in compliance with the emergency separation requirements of 700.10(B).

Also, unless this is a hospital, selective coordination applies to the whole range of available overcurrents. See Article 100.

If it's part of the essential electrical system of a hospital, coordination only needs to be to 0.1 second per 517.30(G).
 
mshields said:
Bearing in mind that this OCPD (at the generator) coordinates to .01 with the Emergency OCPD at the wireway as well as with Emergency panelboard breakers, I think my design is NEC 2014 code compliant.
Click to expand...
If the inspector is citing 700.10, then this statement is not relevant. Selective coordination comes into play in 700.28, and you comply with that article (disregarding the question of the .01 seconds versus 0 seconds). I take note that you didn't say whether the generator breaker is coordinated with the standby OCPD and with all other OCPDs downstream of that point. I know that 700.28 does not require that degree of coordination. But you don't want the system to be susceptible to a fault on the standby system that can take out the generator breaker. For that reason, I agree with the inspector.

I have dealt with this same issue before. What 700.10 requires is that the emergency wiring be separated from all other wiring. With a breaker installed at the generator, it can be argued that the emergency wiring begins at that point. Thus, between that point and the next two downstream breakers, the wiring that serves the emergency loads also serves the standby loads. I believe that that is what the inspector is calling a violation. On the other hand, in the absence of a generator breaker, it can be argued that the emergency wiring does not begin until the load side of the emergency OCPD. Thus, the emergency and standby wiring are completely separated from each other.

What is a bit crazy about this situation is that the wires from the generator to the two OCPDs still carries current for both the emergency and standby systems. The presence or absence of a generator breaker does not change that fact. What does change, however, is that the absence of the generator breaker means that nothing that happens to that wire can cause the system to trip.
 
follow up question

follow up question

Damn! and I was so sure of myself :)

Regardless of that, is it really code compliant from a conductor protection standpoint, to NOT have an OCPD at the generator?

It sounds like that is my out as far as Article 700 and 445 are concerned. It is certainly true that if I do that, I'll have designed a system identical to that shown in the Article 700.10 Exhibit. Yet, it is very counterintuitive to me not to have the generator feeder protected in accordance with Article 310.

Thanks,

Mike
 
New Question - Same system

New Question - Same system

As I indicated earlier in this spring, I'm coming in with my single feeder (now with no OCPD ahead of it at the generator) and into a wireway located in the main electrical room (RI does not require a separate room since I'm at less than 150V to ground). From that wireway I tap off to my Emergency breaker and to my Standby breaker which will both be immediately above the wireway.

My question pertains to the 2 hour rating of emergency feeders. My question; does the connection from the wireway to the emergency circuit breaker need to be MI cable. i.e. for the sake of a connection that might be as short as 6 inches?


Thanks,

Mike
 
mshields said:
Damn! and I was so sure of myself :)
Click to expand...

I think you may be correct. 700.10(B) says that emergency wiring shall be kept separate from other wiring, unless otherwise permitted in 700.10(B) (1) through (5).

Unless I'm reading your installation wrong, 700.10(B)(5)(d) seems like it would apply.
 
mshields said:
My question pertains to the 2 hour rating of emergency feeders. My question; does the connection from the wireway to the emergency circuit breaker need to be MI cable. i.e. for the sake of a connection that might be as short as 6 inches?
Click to expand...
Is the room fully sprinklered? Because if it is, I think you don't need MI because it complies with 700.10(D)(1)(1), assuming the building is over 75 feet high or occupancy of 1000 people or more.

Or does RI have the same amendment that MA does that excludes such a circumstance?
 
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