Sizing Neutral Conductor

[Mark_Mayandi]

Good day,

1. I have a 4160 VAC/ 480 VAC, 2000 KVA, delta- wye with neutral service transformer. Transformer secondary current is 2405 Amp. This transformer feeds a switchgear rated for 3000 Amp. The switchgear main breaker size is 3000 Amp.

2. I'm planning to install 8 - 600 MCM power cable per phase from transformer secondary to switchgear main. So, it will be a total of 24 - 600 MCM power cables from transformer secondary to switchgear main.

3. Question 1 - What is NEC recommended neutral conductor size from transformer secondary to switchgear main ? Would 8 - 600 MCM cables work for neutral ? Can you please confirm which NEC section is applicable for this scenario ?

4. The switchgear will have four 800 Amp breakers. Only one 800 Amp breaker has 1 phase, 277 V load. This 800 Amp breaker in switchgear feeds following two loads/ panels:

a) 3 Phase, 800 Amp, 480 V panel - I'm planning to install 2 sets of 600 MCM power cable from 800 Amp switchgear breaker to panel main breaker (rated for 800 Amp) and

b) 1 phase, 100 Amp, 277 V panel - What is NEC recommended cable size for this 1 phase, 100 Amp, 277 V panel (i.e) what cable size do I have to install from switchgear to this 1 phase, 100 Amp, 277 V panel ? Can you please confirm applicable NEC section for this scenario ? Do we need to run and install 2 sets of 600 MCM for neutral conductor or one set of AWG# 2 is acceptable ?

Thanks !

 

[augie47]

Your profile indicates "Electrical Engineer" so these are question we should be asking you !
Your Code references would be 250.24(C) ,250.102(C), and for your feeder 220.61, 215.2(A)(2).
Take a look at those and let us know what you think !

 

[kwired]

Your grounded service conductor only needs to be sized per the load on it, but in no case can it be smaller than the required GEC.

Beyond service equipment you don't even need to run a grounded conductor if load doesn't require one, but again only needs sized per load on it, but for feeders can never be smaller than required EGC. No minimum size is mentioned for branch circuits (other than it must be able to carry load connected to it which is a general rule for all conductors).

 

[texie]

Your grounded service conductor only needs to be sized per the load on it, but in no case can it be smaller than the required GEC.

It must be sized to 250.102, not the GEC.

 

[kwired]

It must be sized to 250.102, not the GEC.

I knew that:ashamed1:

They are the same at the smaller conductor end of the tables.

 

[texie]

I knew that:ashamed1:

They are the same at the smaller conductor end of the tables.

I knew you did. Just keeping you on your toes.

 

[Dennis Alwon]

I knew you did. Just keeping you on your toes.

I have had to catch myself often on this. It was always based on T250.66 but they changed it to T250.102 and everything is almost the same except the last column. Probably where the op will end up also..

 

[kwired]

I have had to catch myself often on this. It was always based on T250.66 but they changed it to T250.102 and everything is almost the same except the last column. Probably where the op will end up also..

I'd have to go back and look at older codes, but I think you still would have come up with same size back then as you now come up with recent codes. They just thought they needed separate sections on GEC's and SSBJ's at some point.

 

[Dennis Alwon]

Yeah but if you didn't read section 250.102 you wouldn't know that

250.102 Bonding Conductors and Jumpers.
(C) Size — Supply-Side Bonding Jumper.
(1) Size for Supply Conductors in a Single Raceway or
Cable. The supply-side bonding jumper shall not be
smaller than the sizes shown in Table 250.66 for grounding
electrode conductors. Where the ungrounded supply conductors
are larger than 1100 kcmil copper or 1750 kcmil
aluminum, the supply-side bonding jumper shall have an
area not less than 121⁄2 percent of the area of the largest set
of ungrounded supply conductors.

 

[kwired]

Yeah but if you didn't read section 250.102 you wouldn't know that

Happened to take a look in 1993 NEC, 250.66 was 250-94 then, AFAIK table values were still same.

250-79(d) does mention the 12.5% for larger ungrounded conductors than the table covers. Didn't look for specific wording on determining minimum size grounded conductor, kind of assume it was based on same principles as it is now though.

One difference today is you don't get referred to two or three different sections as easily as you used to, I was referred to 250-79(d) from 250-26 which was about grounding of separately derived systems, and then 250-79(d) sends you to 250-94, but throws in that 12.5% if you are dealing with conductors not in the table.

I seem to recall that 12.5% mentioned in the table or as notes to the table at one point.