Different size service components

Merry Christmas

bwat

EE
Location
NC
Occupation
EE
What determines the size of a service? Or can "service size" actually be an ambiguous term?

For example, per Art 230, service conductors must be at least sized on load and 220 calcs, and similar with the service disconnect rating. And the service OCPD per 230.90 specifies that it must protect the service conductors.

So what if I have this (all 480/277V):
Utility transformer is 1000kVA, so roughly 1200A
Service conductors with ampacity of 1400A
Service disconnect with bus rating of 1600A
Service OCPD is 1600A breaker, but is adjustable to be set to 1400A

What size is this service? 1200A? 1400A, 1600A?


I don't actually have this situation, but I realized I don't know the answer to this. I just picked these numbers randomly. And perhaps this isn't even a valid possibility that this could occur.
 

Beaches EE

Senior Member
Location
NE Florida
Occupation
Electrical Engineer / Facilities Manager
You generally start with the load calculation, then size the conductors to serve that load then set the OCPD to protect the conductors. The PoCo equipment is sized according to their guidelines, not to the NEC. I would say you have a 1400 A service based on the conductors, in simple terms based on the hypothetical situation you presented.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
If your service conductors are only rated for an ampacity of 1400 amps then your single OCPD cannot be larger than that. In that case I agree with Beaches then you would have a 1400 amp service. I would be inclined to increase the conductor size slightly and go with the full 1600 amps.
 

Grouch1980

Senior Member
Location
New York, NY
Hi,
If I may please be able to continue this thread, and continue where Bwat left off. This may also help shed some light.

Sometimes I hear engineers give a service size differently from others on a particular design.

For example, in Pic A below, the advisory board engineer called it a 4000 amp service on their drawings. Not sure if it was an oversight, but I would call this a 3000 amp service, since we're limited by the 3000 amp fuses in the service switch, even though the service switch itself is rated 4000 amps. Would that be correct, that this is only a 3000 amp service? Even the 8 sets of 750 MCM aluminum service entrance conductors is only giving you 3080 amps.

And for Pic B below, the service size for this would be the ampacity of the common service entrance conductors... the 350 MCM conductors... so the service size for this setup would be 310 amps. Would that be correct?

Pic A:
IMG_6352.jpg

Pic B:
IMG_6344.jpg
 

Carultch

Senior Member
Location
Massachusetts
For example, in Pic A below, the advisory board engineer called it a 4000 amp service on their drawings. Not sure if it was an oversight, but I would call this a 3000 amp service, since we're limited by the 3000 amp fuses in the service switch, even though the service switch itself is rated 4000 amps. Would that be correct, that this is only a 3000 amp service?

Given that the OCPD is 3000A, I see the service rating as 3000A, and that 3000A would govern the service conductor sizing, and would be the number governed by the calculated load. The blade on the disconnect is rated in excess of the ampacity of the fuses, for reasons not entirely clear to me. It is possible that 4000A is a rating that is more standard, and therefore more readily available. Had this been an example of 500A fuses in a 600A disconnect, I would see precisely why it was built that way, since there are only 8 standard disconnect amp ratings from 30A to 1200A, and a lot more standard fuse ratings that can fit within them.

This reasoning also applies when you have a frame rating and a trip rating of a breaker, in that it is the trip rating that governs what the circuit really is. Provided that it is locked so it is only accessible to qualified personnel, which most people would do anyhow.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Hi,
If I may please be able to continue this thread, and continue where Bwat left off. This may also help shed some light.

Sometimes I hear engineers give a service size differently from others on a particular design.

For example, in Pic A below, the advisory board engineer called it a 4000 amp service on their drawings. Not sure if it was an oversight, but I would call this a 3000 amp service, since we're limited by the 3000 amp fuses in the service switch, even though the service switch itself is rated 4000 amps. Would that be correct, that this is only a 3000 amp service? Even the 8 sets of 750 MCM aluminum service entrance conductors is only giving you 3080 amps.

And for Pic B below, the service size for this would be the ampacity of the common service entrance conductors... the 350 MCM conductors... so the service size for this setup would be 310 amps. Would that be correct?

Pic A:
View attachment 2558056

Pic B:
View attachment 2558054
I agree pic A is a 3000 amp service.
 

Grouch1980

Senior Member
Location
New York, NY
Thanks guys. Also agreed that it’s a 3000 amp service.

Regarding pic b, would you also agree that is a 310 amp service? In that instance you wouldn’t go by the switch OCPD sizes.
 

Carultch

Senior Member
Location
Massachusetts
Regarding pic b, would you also agree that is a 310 amp service? In that instance you wouldn’t go by the switch OCPD sizes.

Without further context, it is difficult to determine. I would add up the OCPD sizes, unless there is another one of a higher rank, that is closer to the service point feeding the two that are indicated. Judging by the wire sizes specified, and the industry norm of 75C terminations, it would probably be a 300A service, based on a probably 100A fused disconnect on the left, and a probably 200A fused disconnect on the right.
 

Grouch1980

Senior Member
Location
New York, NY
Without further context, it is difficult to determine. I would add up the OCPD sizes, unless there is another one of a higher rank, that is closer to the service point feeding the two that are indicated. Judging by the wire sizes specified, and the industry norm of 75C terminations, it would probably be a 300A service, based on a probably 100A fused disconnect on the left, and a probably 200A fused disconnect on the right.
if the common service entrance conductors were sized for 300 MCM instead of 350, then I'm assuming the service size is now 285 amps. (assuming the OCPD's are 100 amps and 200 amps)
 
In one of the jurisdictions I work in, they have a plan review requirement for services 400A and over. I installed a single riser with two sets of 250 AL (208y/120) feeding 2 200A MB panels. I am thinking it's a 368 amp service thus no plan review....guess what the city thought? 😡
 

Grouch1980

Senior Member
Location
New York, NY
In one of the jurisdictions I work in, they have a plan review requirement for services 400A and over. I installed a single riser with two sets of 250 AL (208y/120) feeding 2 200A MB panels. I am thinking it's a 368 amp service thus no plan review....guess what the city thought? 😡
400 and over :)
 
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