Size of GEC

[M. D.]

This is what I have been taught ,.

By Mike Holt for EC&M Magazine

Part I. General

Author?s Comment: Understanding where the service begins and where it ends is critical in the proper application of many Code rules. To understand how to apply these rules, we need to review the following definitions from Article 100.

Service point - The point of connection between the facilities of the serving utility and the premises wiring.

Service conductors - The conductors from the service point to the service disconnecting means (service equipment, not meter). Service conductors would include service-entrance conductors for both overhead (service drop) and underground (service lateral).

 

[Dennis Alwon]

This is what I have been taught ,.

So where is it allowed to size these wires for the calculated load rather then for the service OCPD?

 

[iwire]

They wouldn't have to be sized to the main OCPD?

where is it allowed to size these wires for the calculated load rather then for the service OCPD?

Service conductors supplying a single service disconnect are required to have an ampacity at least as high as the overcurrent device. See 230.90(A) {You could actually apply the rules of 240.4(B) as well, see 230.90(A) Exception 2}


Service conductors supplying multiple service disconnects are required to have an ampacity only as high as the calculated load. See 230.90(A) Exception 3.

:smile:

 

[iwire]

Here is a picture I have posted many times as I think it is a good example of applying 230.90(A) Exception 3.

This service has a single riser with a rating around 200-250 amps but it supplies six 100 amp breakers.

There is no chance at all that the tenants in this multifamily could load the service to 600 amps. The building is a small 3 floor multifamily with two units on each floor. The heat and appliances are natural gas.

 

[Dennis Alwon]

Thanks Bob-- the article is what I was looking for.

 

[iwire]

No problem Dennis.

I realized I made a mistake in my description.

The building is a small 3 floor multifamily with two units on each floor. The heat and appliances are natural gas.

It's 5 units and a landlord panel.

 

[peter d]

There is no chance at all that the tenants in this multifamily could load the service to 600 amps. The building is a small 3 floor multifamily with two units on each floor. The heat and appliances are natural gas.

Is that deadly exposed SEU cable again????

 

[Dennis Alwon]

Here is a picture I have posted many times as I think it is a good example of applying 230.90(A) Exception 3.

This service has a single riser with a rating around 200-250 amps but it supplies six 100 amp breakers.

.

And the GEC is based on which conductors? I bet not the riser conductors.

Is that deadly exposed SEU cable again????

Are you starting trouble again

 

[peter d]

And the GEC is based on which conductors? I bet not the riser conductors.

I bet it is.

 

[Dennis Alwon]

I bet it is.

I believe T 250.66 says otherwise-- based on the largest se conductors-- esp. Note 1

 

[iwire]

I don't know how the GEC was sized in the pictured service.

I do agree about the requirements of Note 1.

 

[peter d]

I believe T 250.66 says otherwise-- based on the largest se conductors-- esp. Note 1

I'm having trouble following this discussion, but I know that we always size the GEC based on the size of the SE conductors when we do a service like that here. I didn't know there was another way to do it.

 

[Dennis Alwon]

I'm having trouble following this discussion, but I know that we always size the GEC based on the size of the SE conductors when we do a service like that here. I didn't know there was another way to do it.

Peter the question is based on the fact that the riser se conductors are smaller than the equivalent size of the se conductors that are tapped from the riser in the gutter.

 

[peter d]

Peter the question is based on the fact that the riser se conductors are smaller than the equivalent size of the se conductors that are tapped from the riser in the gutter.

Thanks, I see.

 

[iwire]

I'm having trouble following this discussion, but I know that we always size the GEC based on the size of the SE conductors when we do a service like that here. I didn't know there was another way to do it.

Did you read note 1 of T250.66?

With a service like that Fall River one in the picture we can not size the GEC on the single riser, we must combine the six smaller risers for the GEC size.

 

[Pierre C Belarge]

230.42 Minimum Size and Rating.
This section is for sizing the service entrance conductors.

230.90 Where Required.
Each ungrounded service conductor shall have overload protection.

230.42 is for sizing the conductors, while 230.90 is for sizing the overcurrent protection, which really provides overload protection. (of the supply side conductors)

230.90(A) Tells us that the conductors are to be protected by an overcurrent protective device set at the rating of the service entrance conductors. So the overcurrent device for a single disconnecting means is really sized based on the calculations performed as per 230.42. The conductor size will have to be rated at least for the size of the overcurrent protective device.


If there are multiple service disconnects then 230.90Exception #3 is referenced. This permits the conductor to be calculated per the load and not the size of the overcurrent device(s). Which means the overcurrent devices can, when added together, be sized larger than the conductor ampere rating.

 

[infinity]

230.42 Minimum Size and Rating.
This section is for sizing the service entrance conductors.

230.90 Where Required.
Each ungrounded service conductor shall have overload protection.

230.42 is for sizing the conductors, while 230.90 is for sizing the overcurrent protection, which really provides overload protection. (of the supply side conductors)

230.90(A) Tells us that the conductors are to be protected by an overcurrent protective device set at the rating of the service entrance conductors. So the overcurrent device for a single disconnecting means is really sized based on the calculations performed as per 230.42. The conductor size will have to be rated at least for the size of the overcurrent protective device.


If there are multiple service disconnects then 230.90Exception #3 is referenced. This permits the conductor to be calculated per the load and not the size of the overcurrent device(s). Which means the overcurrent devices can, when added together, be sized larger than the conductor ampere rating.

OK, but what about the GEC as referenced in the OP? Sized on the one set of 350's or the two sets of #4/0?

 

[peter d]

Did you read note 1 of T250.66?

With a service like that Fall River one in the picture we can not size the GEC on the single riser, we must combine the six smaller risers for the GEC size.

Got it. I just never thought of that before....always the service conductors on the "load" side of the meter, never the line side. Oooops.

 

[Dennis Alwon]

Got it. I just never thought of that before....always the service conductors on the "load" side of the meter, never the line side. Oooops.

No "oops" there because generally the conductors on the load side of the meter are the larger conductors. :smile:

 

[iwire]

OK, but what about the GEC as referenced in the OP? Sized on the one set of 350's or the two sets of #4/0?

Note 1 of T.250.66 requires the combined size of the two 4/0s.

I will go on to point out in this particular case the 350s do not exist to the NEC.

I think the power Co. will install 350 mcm alu conductors