Question No. 1
What is the largest kVA rated dry-type transformer that can be installed in the space over a hung ceiling
used for environmental air-handling purposes?
Answer: Section 450-13(b) permits transformers over a hung ceiling when they do not exceed 50 kVA
not over 600 volts. A note in the 1999 NEC Handbook published by the NFPA (page 523) states that Section
300-22(c)(2) permits dry-type transformers in the air space.
Question No. 2
We have been placing telephone service entrance cable in the same trench with the utility power cables
underground to serve our residential telephone customers. This is in accordance with our telephone
engineering specifications. A question has come up as to what the NEC guidelines are in regards to
"joint trench applications.”
Answer: The installation of communications equipment under the exclusive control of communications
utilities located outdoors is not covered by the National Electrical Code [90-2(b)(4)]. Therefore,
it is not a violation of the NEC to install telephone cables in the same trench with service entrance
cable. However, if the telephone wiring is not under the exclusive control of the communication utilities,
then the requirements of Article 800 apply.
Unfortunately, there is no specific rule covering this subject! Section 800-11(a) requires separation
by brick, concrete, tile partitions, or a suitable barrier for underground circuits entering buildings
in a "raceway, handhole, or manhole.” Section 800-12(b) requires communication cables “on buildings”
to be located at least 4 in. from electric light or power conductors not in a raceway or cable.
Note: Article 810 -- Radio and Television Equipment, Section 810-18 does required underground “radio
and television conductors” to be separated at least 12 in. from power circuit conductors.
Question No. 3
The local inspector just turned down my job for using THHN outside the weather head for service entrance
conductors. He said that the conductors had to be marked "sunlight resistant." I have been
doing this for over 15 years and this is a first for me! Is he right?
Answer: Yes he’s right. This new requirement as added to the 1999 NEC in Section 310-8(d). The exact
text is as follows "Insulated conductors and cables used where exposed to direct rays of the
sun shall be of a type listed or marked "sunlight resistant."
According to UL White book, page 114, THHN, THHW, and THWN conductors that comply with an artificial
weathering test are marked, "sunlight resistant."
Question No. 4
I supplied the bathroom receptacles from one circuit of No. 12/3 nonmetallic sheath cable. The other
circuit (common neutral) was used to supply other loads? Does this meet the NEC requirements?
Answer: Yes, your installation is NEC compliant. A multiwire branch circuit can be use to supply the
20-ampere branch circuit for bathroom receptacles as required in Section 210-11(c)(3). The answer
is contained in Section 210-4(a) which states that a multiwire branch circuit can be used to supply
any of the circuits required by article 210, which would be the bathroom receptacle circuit [210-11(c)(3)],
and that a multiwire branch circuit is considered as two multiple circuits, Figure
Note: Figure are not posted on the Internet.
Question No. 5
What are the NEC requirements for the installation of fire and/or smoke detectors in a dwelling unit?
Answer: None. This subject is not covered by the National Electrical Code, but it is covered by the
Life Safety Code NFPA –101.
Question No. 6
Can a GFCI receptacle be installed in the back splash area behind the sink in the kitchen?
Answer: Yes. However, Section 210-52(c)(5) indicates that the receptacle outlets shall be located above,
but not more than 18 in. above the countertop and that the receptacle outlets shall not be installed
in a face-up position in the work surfaces or countertops, Figure 2.
Note: The fact that the receptacle was a GFCI receptacle had no bearing on the answer.
Question No. 7
How many exit signs can be installed on a branch circuit and does Article 600 – Signs apply to exit
Answer: Article 600 does not apply to exit signs.
The number of exit signs on a circuit can be determined by the following:
Number of Fixtures = Circuit Rating x 80%1/ampere rating of fixture
1 The maximum load on a branch circuit is limited to 80% of the rating of the circuit; see
The number of exit fixtures containing two 25-watt lamps on a 20 ampere circuit would be:
(a) 11 (b) 23 (c) 31
Answer: (d) 38
Number of Fixtures = (20 amperes x 80%)/0.42 amperes* = 38 fixtures
*2 lamps x 25 watts = 50 watts/120 volts = .42 amperes
Question No. 8
I am installing a 480/120-208 volt three-phase transformer on the roof of a standard five-story office
building with no building steel. I was told to ground the transformer to the local water piping up
on the roof. I thought I needed to ground the transformer within five feet from were the water pipe
enters the building.
Answer: In accordance with Section 250-30(a)(3) the grounding electrode for a separately derived system
(transformer) needs to be as near as practicable to the neutral-to-ground bond for the transformer.
The grounding electrode shall be effectively grounded structural metal member of the structure (which
you don’t have) or effectively grounded metal water pipe within 5 ft from the point of entrance into
the building (which is on the first floor). If effectively grounded structural steel or water pipe
is not available then you must use the service equipment grounding electrode.
Note: Section 250-104(a)(4) also requires that the grounded (neutral) conductor of the separately derived
system to be bonded to the nearest available point of the interior metal water piping system in the
area served by the separately derived system. This means that you must bond the neutral at the location
where the neutral-to-ground bond is for the transformer to the metal water pipe up on the roof.
Author’s Comment: I don’t have the space to explain the reasons why two bonding connections
are required, and to be honest with you I’m not quite sure I understand the logic of these two requirements
Question No. 9
The NEC requires metal parts of the electrical system to be grounded to the earth to limit high voltage
on the metal parts from lightning, line surges or unintentional contact with higher voltage systems
[250-2(a)]. To accomplish this a conductive path from the metal parts of the electrical system to
the earth must be installed. What is the largest size copper grounding electrode conductor required
to bond the metal parts of the electrical system to a "made electrode" such as a ground
rod? How does the size of the service affect the sizing of the grounding electrode conductor to a
Answer: The size of the service has no affect on the sizing of the grounding electrode conductor to
a made electrode, because this conductor is not sized to carry fault current (lightning doesn’t know
the size of the service). As a result the grounding electrode conductor from the grounded conductor
to a "made electrodes" such as a ground rod, is not required to be larger than No. 6 copper
wire or No. 4 aluminum. This requirement is contained in Section 250-66(a), Figure
Question No. 10
To quickly remove dangerous voltage resulting from a phase-to-ground fault the interior metal water
pipe system must be bonded to the system grounded conductor. How is this copper bonding conductor
sized? How does the size of the service affect the sizing of this bonding jumper? Is the bonding connection
to the water pipe required to be within 5 feet of entrance into the building?
Answer: Because the bonding jumper must carry fault current (to assist in clearing the fault), the
service conductor size is used to determine the interior metal water pipe bonding conductor. Section
250-104(a) specifies that the bonding jumper shall be sized in accordance with Table 250-66 based
on the size of largest service-entrance conductor. The following provides an example on the use of
Table 250-66 for different size services:
Service Size Service Conductor
200 ampere No. 3/0
400 ampere 0ne - 500 kcmil
800 ampere Two - 500 kcmil
The bonding if the interior metal water pipe system is not required to be within 5 feet of entrance
of the metal water pipe into the building.
Note: The 5 foot water pipe limitations contained in Section 250-50 only applies when the water pipe
is used as a conductor to interconnect electrodes that are part of the grounding electrode system.
The above questions and answers (including graphics) by Mike
Holt are in EC&M magazine. Click
here for more magazine articles.