Service drop sizing / 400 amp meter main combo...?...

[riverstrat]

... I don't have a load design, the customer has an existing
3 or 4 KVA irrigation pump on 5 acres.

An old house was removed from the property
last year.

So this is for future expansion convenience so to speak.
One of my older books mentioned sizing one size below given ampacity
indicating I could use 400 mcm cu.

My latter books don't mention under sizing, so it looks like I need to
use 500 mcm cu as per 310-16.

The can will be hung today and a 3" wheatherhead will be installed
in the next few days.
I will be sizing my grd according to my entrance cable so either way
it will probably be 1/0 CU, to an 3/4" 8' grd rod.
Opinions as to the safety advantage of adding a second grd
rod 6' away from the main grd rod would also be apprecitated....

 

[jtester]

500 MCM copper is good for 380 amps. It's OK to use a 400 amp switch but the load has to be 380 amps or less.

Refer to 250.66(A) for wire to a ground rod, and 250.56 for the need for a second rod.

Jim T

 

[riverstrat]

500 MCM copper is good for 380 amps. It's OK to use a 400 amp switch but the load has to be 380 amps or less.

Refer to 250.66(A) for wire to a ground rod, and 250.56 for the need for a second rod.

Jim T

Thanks ,
I thought 500 thhn mcm cu was good through 430 amps, or are you saying
the lugs won't be rated for 90c ?

 

[riverstrat]

500 MCM copper is good for 380 amps. It's OK to use a 400 amp switch but the load has to be 380 amps or less.

Refer to 250.66(A) for wire to a ground rod, and 250.56 for the need for a second rod.

Jim T

Thanks,
why do you say 380 , I thought 500 thhn mcm cu was good for 430 amps.

 

[celtic]

Thanks,
why do you say 380 , I thought 500 thhn mcm cu was good for 430 amps.

Which column in 310.16 are you reading 430A from ... the 90?C column.
What is the rating of the termination of this 500MCM...75?C.
You have to use the 75? column ~ 380A


Below copied from NECH:

Section 110.14(C)(1)(a) requires that conductor terminations, as well as conductors, be rated for the operating temperature of the circuit. For example, the load on an 8 AWG THHN, 90?C copper wire is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60?C. This same 8 AWG THHN, 90?C wire is limited to 50 amperes where connected to a fusible switch with terminals rated at 75?C. The conductor ampacities were selected from Table 310.16. Not only does this requirement apply to conductor terminations of breakers and fusible switches, but the equipment enclosure must also permit terminations above 60?C.

The example above is from 110.14(C)(1)(a) ~ which is for circuits rated 100 amperes or less.

You have a 400A situation so we will read on to 110.14(C)(1)(b)
From the NEC:

110.14(C)(1)(b)
Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for one of the following:
(1) Conductors rated 75?C (167?F)
(2) Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75?C (167?F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors

(Both quoted passages from '02 edition)

Have you read 240.4?

 

[riverstrat]

Best book I have left from the when I was contracting is the old 93 handbook,
the parts house has to order my new book, didnt find the section on second
grd rod requirements , I was asking for opinions,
I don't mind driving a second one if your opinions suggest it
to be a big safety + back up, regardless of minimum requirements.

If it's required, I'll get gigged and add one....

:grin:

 

[jack horner]

If this is a 120/240 single-phase dwelling why don't you use this instead.


(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders For individual dwelling units of one family, two-family, and multifamily dwellings, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboards(s). The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.61, and 230.42 are met.

Table 310.15(B)(6) Conductor Types and Sizes for 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. Conductor Types RHH, RHW, RHW-2, THHN, THHW, THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE, USE, USE-2
Conductor (AWG or kcmil) Service or Feeder Rating (Amperes)
Copper Aluminum or Copper-Clad Aluminum

4 2 100
3 1 110
2 1/0 125
1 2/0 150
1/0 3/0 175
2/0 4/0 200
3/0 250 225
4/0 300 250
250 350 300
350 500 350
400 600 400

 

[infinity]

I would add that if your only using ground rods forget the #1/0, you don't need more than a #6 AWG GEC. Also You might want to consider 2 rods 6' or more apart.