Commercial Service
- Thread starter laketime
- Start date
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Hameedulla-Ekhlas
Senior Member
- Location
- AFG
See NEC-2008 Table-310.16 and also voltage drop, derating and other factors to size your feeder.
- Location
- Massachusetts
Currently there are no voltage drop requirements in the NEC for typical feeders or branch circuits.
- Location
- Massachusetts
As mentioned 310.16 but ...........
How did you come up with 800 amps?
What is the calculated load?
Hameedulla-Ekhlas
Senior Member
- Location
- AFG
bob,
In NEC has already given a voltage drop calculation example and voltage drop is regardless the type of cable or branch or sepcific place.
- Location
- Massachusetts
The NEC does not require us to consider voltage drop in our calculations for most circuits.
The NEC is not a design specification.
- Location
- Massachusetts
Yes unless you have to derate then you could use the 90 column.
However for 800 amps we would normally run two sets of 500 or 600 copper.
Hameedulla-Ekhlas
Senior Member
- Location
- AFG
Ok, thanks for this but I suggested him for sizing and designing feeder, he should also consider voltage drop, derating and other factors. Was I wrong?
- Location
- Massachusetts
No I don't think you are wrong, we should consider everything we can think of ........ including the costs.
I just wanted to make clear that voltage drop considerations are not required by the NEC except for a few specific instances, fire pumps are one example.
- Location
- Massachusetts
Wrong column, back up to 75C
Two 500 coppers in separate raceways will get you 760 amps of capacity and is allowed with an 800 amp breaker but should never be loaded beyond 760 amps.
Two 600 coppers in separate raceways will get you 840 amps of capacity.
If you decided to run two sets of say 4/0 copper in two raceways (four 4/0 per phase total) then the 90 C column would be useful.
laketime
Senior Member
The electrical engineer is calling for parallel 500's from the load side of the meter to the panel board. Why is that?
- Location
- Massachusetts
It is legal and common to use two sets of 500s copper for 800 amps. Even though the ampacity is only 760 amps section 240.4(B) allows us to use an 800 amp breaker.
However the calculated load must never exceed 760 amps. This means if a year from now someone wants to add load they are supposed to do a full load calculation to make sure the added load does not bring the total over 760.
However the calculated load must never exceed 760 amps. This means if a year from now someone wants to add load they are supposed to do a full load calculation to make sure the added load does not bring the total over 760.
laketime
Senior Member
So since I am bidding against a group of guys that probably won't check any of this out, I think I will go with the 500's to not price myself out of the job.
- Location
- Massachusetts
If the engineer is showing 500s I certainly would not bid 600s without it being an alternate.
laketime
Senior Member
So I just talked to my supplier and they are suggesting I go with aluminum conductors for the feed. Some electrician there was trying to say that I could use parallel 500mcm aluminum to feed an 800amp section. The way I read 310.16 is I would need at least 700mcm aluminum to feed 800amps. Am I right or is the other idiot right? 
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
You are correct. You would need parallel 700 kcm alum. for a 800 amp service as long as the calculated load is not greater than 750 amps. If not you may need parallel 900kcm alum. depending on the calculated load.So I just talked to my supplier and they are suggesting I go with aluminum conductors for the feed. Some electrician there was trying to say that I could use parallel 500mcm aluminum to feed an 800amp section. The way I read 310.16 is I would need at least 700mcm aluminum to feed 800amps. Am I right or is the other idiot right?
bradleyelectric
Senior Member
- Location
- forest hill, md
actually they make a 750 breaker so you can't just upsize to an additional larger size breaker unless I'm mistaken. The other fine gentleman is badly mistaken. You need 750MCM Al.
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